/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.cassandra.db;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.*;
import java.util.stream.Collectors;

import com.google.common.collect.Iterables;
import com.google.common.collect.Sets;

import org.apache.commons.lang3.tuple.Pair;

import org.apache.cassandra.cache.IRowCacheEntry;
import org.apache.cassandra.cache.RowCacheKey;
import org.apache.cassandra.cache.RowCacheSentinel;
import org.apache.cassandra.concurrent.Stage;
import org.apache.cassandra.concurrent.StageManager;
import org.apache.cassandra.config.CFMetaData;
import org.apache.cassandra.config.ColumnDefinition;
import org.apache.cassandra.config.DatabaseDescriptor;
import org.apache.cassandra.db.lifecycle.*;
import org.apache.cassandra.db.filter.*;
import org.apache.cassandra.db.partitions.*;
import org.apache.cassandra.db.rows.*;
import org.apache.cassandra.db.transform.RTBoundValidator;
import org.apache.cassandra.db.transform.Transformation;
import org.apache.cassandra.exceptions.RequestExecutionException;
import org.apache.cassandra.io.sstable.format.SSTableReader;
import org.apache.cassandra.io.sstable.format.SSTableReadsListener;
import org.apache.cassandra.io.util.DataInputPlus;
import org.apache.cassandra.io.util.DataOutputPlus;
import org.apache.cassandra.metrics.TableMetrics;
import org.apache.cassandra.net.MessageOut;
import org.apache.cassandra.net.MessagingService;
import org.apache.cassandra.schema.IndexMetadata;
import org.apache.cassandra.service.CacheService;
import org.apache.cassandra.service.ClientState;
import org.apache.cassandra.service.StorageProxy;
import org.apache.cassandra.service.pager.*;
import org.apache.cassandra.thrift.ThriftResultsMerger;
import org.apache.cassandra.tracing.Tracing;
import org.apache.cassandra.transport.ProtocolVersion;
import org.apache.cassandra.utils.FBUtilities;
import org.apache.cassandra.utils.SearchIterator;
import org.apache.cassandra.utils.btree.BTreeSet;


A read command that selects a (part of a) single partition.
/**
 * A read command that selects a (part of a) single partition.
 */
public class SinglePartitionReadCommand extends ReadCommand
{
    protected static final SelectionDeserializer selectionDeserializer = new Deserializer();

    private final DecoratedKey partitionKey;
    private final ClusteringIndexFilter clusteringIndexFilter;

    private int oldestUnrepairedTombstone = Integer.MAX_VALUE;

    private SinglePartitionReadCommand(boolean isDigest,
                                       int digestVersion,
                                       boolean isForThrift,
                                       CFMetaData metadata,
                                       int nowInSec,
                                       ColumnFilter columnFilter,
                                       RowFilter rowFilter,
                                       DataLimits limits,
                                       DecoratedKey partitionKey,
                                       ClusteringIndexFilter clusteringIndexFilter,
                                       IndexMetadata index)
    {
        super(Kind.SINGLE_PARTITION, isDigest, digestVersion, isForThrift, metadata, nowInSec, columnFilter, rowFilter, limits, index);
        assert partitionKey.getPartitioner() == metadata.partitioner;
        this.partitionKey = partitionKey;
        this.clusteringIndexFilter = clusteringIndexFilter;
    }

    
Creates a new read command on a single partition.
Params:
isForThrift – whether the query is for thrift or not.
metadata – the table to query.
nowInSec – the time in seconds to use are "now" for this query.
columnFilter – the column filter to use for the query.
rowFilter – the row filter to use for the query.
limits – the limits to use for the query.
partitionKey – the partition key for the partition to query.
clusteringIndexFilter – the clustering index filter to use for the query.
indexMetadata – explicitly specified index to use for the query
Returns:
a newly created read command./**
     * Creates a new read command on a single partition.
     *
     * @param isForThrift whether the query is for thrift or not.
     * @param metadata the table to query.
     * @param nowInSec the time in seconds to use are "now" for this query.
     * @param columnFilter the column filter to use for the query.
     * @param rowFilter the row filter to use for the query.
     * @param limits the limits to use for the query.
     * @param partitionKey the partition key for the partition to query.
     * @param clusteringIndexFilter the clustering index filter to use for the query.
     * @param indexMetadata explicitly specified index to use for the query
     *
     * @return a newly created read command.
     */
    public static SinglePartitionReadCommand create(boolean isForThrift,
                                                    CFMetaData metadata,
                                                    int nowInSec,
                                                    ColumnFilter columnFilter,
                                                    RowFilter rowFilter,
                                                    DataLimits limits,
                                                    DecoratedKey partitionKey,
                                                    ClusteringIndexFilter clusteringIndexFilter,
                                                    IndexMetadata indexMetadata)
    {
        return new SinglePartitionReadCommand(false,
                                              0,
                                              isForThrift,
                                              metadata,
                                              nowInSec,
                                              columnFilter,
                                              rowFilter,
                                              limits,
                                              partitionKey,
                                              clusteringIndexFilter,
                                              indexMetadata);
    }

    
Creates a new read command on a single partition.
Params:
metadata – the table to query.
nowInSec – the time in seconds to use are "now" for this query.
columnFilter – the column filter to use for the query.
rowFilter – the row filter to use for the query.
limits – the limits to use for the query.
partitionKey – the partition key for the partition to query.
clusteringIndexFilter – the clustering index filter to use for the query.
Returns:
a newly created read command./**
     * Creates a new read command on a single partition.
     *
     * @param metadata the table to query.
     * @param nowInSec the time in seconds to use are "now" for this query.
     * @param columnFilter the column filter to use for the query.
     * @param rowFilter the row filter to use for the query.
     * @param limits the limits to use for the query.
     * @param partitionKey the partition key for the partition to query.
     * @param clusteringIndexFilter the clustering index filter to use for the query.
     *
     * @return a newly created read command.
     */
    public static SinglePartitionReadCommand create(CFMetaData metadata,
                                                    int nowInSec,
                                                    ColumnFilter columnFilter,
                                                    RowFilter rowFilter,
                                                    DataLimits limits,
                                                    DecoratedKey partitionKey,
                                                    ClusteringIndexFilter clusteringIndexFilter)
    {
        return create(false, metadata, nowInSec, columnFilter, rowFilter, limits, partitionKey, clusteringIndexFilter);
    }

    
Creates a new read command on a single partition.
Params:
isForThrift – whether the query is for thrift or not.
metadata – the table to query.
nowInSec – the time in seconds to use are "now" for this query.
columnFilter – the column filter to use for the query.
rowFilter – the row filter to use for the query.
limits – the limits to use for the query.
partitionKey – the partition key for the partition to query.
clusteringIndexFilter – the clustering index filter to use for the query.
Returns:
a newly created read command./**
     * Creates a new read command on a single partition.
     *
     * @param isForThrift whether the query is for thrift or not.
     * @param metadata the table to query.
     * @param nowInSec the time in seconds to use are "now" for this query.
     * @param columnFilter the column filter to use for the query.
     * @param rowFilter the row filter to use for the query.
     * @param limits the limits to use for the query.
     * @param partitionKey the partition key for the partition to query.
     * @param clusteringIndexFilter the clustering index filter to use for the query.
     *
     * @return a newly created read command.
     */
    public static SinglePartitionReadCommand create(boolean isForThrift,
                                                    CFMetaData metadata,
                                                    int nowInSec,
                                                    ColumnFilter columnFilter,
                                                    RowFilter rowFilter,
                                                    DataLimits limits,
                                                    DecoratedKey partitionKey,
                                                    ClusteringIndexFilter clusteringIndexFilter)
    {
        return create(isForThrift,
                      metadata,
                      nowInSec,
                      columnFilter,
                      rowFilter,
                      limits,
                      partitionKey,
                      clusteringIndexFilter,
                      findIndex(metadata, rowFilter));
    }

    
Creates a new read command on a single partition.
Params:
metadata – the table to query.
nowInSec – the time in seconds to use are "now" for this query.
key – the partition key for the partition to query.
columnFilter – the column filter to use for the query.
filter – the clustering index filter to use for the query.
Returns:
a newly created read command. The returned command will use no row filter and have no limits./**
     * Creates a new read command on a single partition.
     *
     * @param metadata the table to query.
     * @param nowInSec the time in seconds to use are "now" for this query.
     * @param key the partition key for the partition to query.
     * @param columnFilter the column filter to use for the query.
     * @param filter the clustering index filter to use for the query.
     *
     * @return a newly created read command. The returned command will use no row filter and have no limits.
     */
    public static SinglePartitionReadCommand create(CFMetaData metadata,
                                                    int nowInSec,
                                                    DecoratedKey key,
                                                    ColumnFilter columnFilter,
                                                    ClusteringIndexFilter filter)
    {
        return create(metadata, nowInSec, columnFilter, RowFilter.NONE, DataLimits.NONE, key, filter);
    }

    
Creates a new read command that queries a single partition in its entirety.
Params:
metadata – the table to query.
nowInSec – the time in seconds to use are "now" for this query.
key – the partition key for the partition to query.
Returns:
a newly created read command that queries all the rows of key./**
     * Creates a new read command that queries a single partition in its entirety.
     *
     * @param metadata the table to query.
     * @param nowInSec the time in seconds to use are "now" for this query.
     * @param key the partition key for the partition to query.
     *
     * @return a newly created read command that queries all the rows of {@code key}.
     */
    public static SinglePartitionReadCommand fullPartitionRead(CFMetaData metadata, int nowInSec, DecoratedKey key)
    {
        return create(metadata, nowInSec, key, Slices.ALL);
    }

    
Creates a new read command that queries a single partition in its entirety.
Params:
metadata – the table to query.
nowInSec – the time in seconds to use are "now" for this query.
key – the partition key for the partition to query.
Returns:
a newly created read command that queries all the rows of key./**
     * Creates a new read command that queries a single partition in its entirety.
     *
     * @param metadata the table to query.
     * @param nowInSec the time in seconds to use are "now" for this query.
     * @param key the partition key for the partition to query.
     *
     * @return a newly created read command that queries all the rows of {@code key}.
     */
    public static SinglePartitionReadCommand fullPartitionRead(CFMetaData metadata, int nowInSec, ByteBuffer key)
    {
        return create(metadata, nowInSec, metadata.decorateKey(key), Slices.ALL);
    }

    
Creates a new single partition slice command for the provided single slice.
Params:
metadata – the table to query.
nowInSec – the time in seconds to use are "now" for this query.
key – the partition key for the partition to query.
slice – the slice of rows to query.
Returns:
a newly created read command that queries slice in key. The returned query will query every columns for the table (without limit or row filtering) and be in forward order./**
     * Creates a new single partition slice command for the provided single slice.
     *
     * @param metadata the table to query.
     * @param nowInSec the time in seconds to use are "now" for this query.
     * @param key the partition key for the partition to query.
     * @param slice the slice of rows to query.
     *
     * @return a newly created read command that queries {@code slice} in {@code key}. The returned query will
     * query every columns for the table (without limit or row filtering) and be in forward order.
     */
    public static SinglePartitionReadCommand create(CFMetaData metadata, int nowInSec, DecoratedKey key, Slice slice)
    {
        return create(metadata, nowInSec, key, Slices.with(metadata.comparator, slice));
    }

    
Creates a new single partition slice command for the provided slices.
Params:
metadata – the table to query.
nowInSec – the time in seconds to use are "now" for this query.
key – the partition key for the partition to query.
slices – the slices of rows to query.
Returns:
a newly created read command that queries the slices in key. The returned query will query every columns for the table (without limit or row filtering) and be in forward order./**
     * Creates a new single partition slice command for the provided slices.
     *
     * @param metadata the table to query.
     * @param nowInSec the time in seconds to use are "now" for this query.
     * @param key the partition key for the partition to query.
     * @param slices the slices of rows to query.
     *
     * @return a newly created read command that queries the {@code slices} in {@code key}. The returned query will
     * query every columns for the table (without limit or row filtering) and be in forward order.
     */
    public static SinglePartitionReadCommand create(CFMetaData metadata, int nowInSec, DecoratedKey key, Slices slices)
    {
        ClusteringIndexSliceFilter filter = new ClusteringIndexSliceFilter(slices, false);
        return create(metadata, nowInSec, ColumnFilter.all(metadata), RowFilter.NONE, DataLimits.NONE, key, filter);
    }

    
Creates a new single partition slice command for the provided slices.
Params:
metadata – the table to query.
nowInSec – the time in seconds to use are "now" for this query.
key – the partition key for the partition to query.
slices – the slices of rows to query.
Returns:
a newly created read command that queries the slices in key. The returned query will query every columns for the table (without limit or row filtering) and be in forward order./**
     * Creates a new single partition slice command for the provided slices.
     *
     * @param metadata the table to query.
     * @param nowInSec the time in seconds to use are "now" for this query.
     * @param key the partition key for the partition to query.
     * @param slices the slices of rows to query.
     *
     * @return a newly created read command that queries the {@code slices} in {@code key}. The returned query will
     * query every columns for the table (without limit or row filtering) and be in forward order.
     */
    public static SinglePartitionReadCommand create(CFMetaData metadata, int nowInSec, ByteBuffer key, Slices slices)
    {
        return create(metadata, nowInSec, metadata.decorateKey(key), slices);
    }

    
Creates a new single partition name command for the provided rows.
Params:
metadata – the table to query.
nowInSec – the time in seconds to use are "now" for this query.
key – the partition key for the partition to query.
names – the clustering for the rows to query.
Returns:
a newly created read command that queries the names in key. The returned query will query every columns (without limit or row filtering) and be in forward order./**
     * Creates a new single partition name command for the provided rows.
     *
     * @param metadata the table to query.
     * @param nowInSec the time in seconds to use are "now" for this query.
     * @param key the partition key for the partition to query.
     * @param names the clustering for the rows to query.
     *
     * @return a newly created read command that queries the {@code names} in {@code key}. The returned query will
     * query every columns (without limit or row filtering) and be in forward order.
     */
    public static SinglePartitionReadCommand create(CFMetaData metadata, int nowInSec, DecoratedKey key, NavigableSet<Clustering> names)
    {
        ClusteringIndexNamesFilter filter = new ClusteringIndexNamesFilter(names, false);
        return create(metadata, nowInSec, ColumnFilter.all(metadata), RowFilter.NONE, DataLimits.NONE, key, filter);
    }

    
Creates a new single partition name command for the provided row.
Params:
metadata – the table to query.
nowInSec – the time in seconds to use are "now" for this query.
key – the partition key for the partition to query.
name – the clustering for the row to query.
Returns:
a newly created read command that queries name in key. The returned query will query every columns (without limit or row filtering)./**
     * Creates a new single partition name command for the provided row.
     *
     * @param metadata the table to query.
     * @param nowInSec the time in seconds to use are "now" for this query.
     * @param key the partition key for the partition to query.
     * @param name the clustering for the row to query.
     *
     * @return a newly created read command that queries {@code name} in {@code key}. The returned query will
     * query every columns (without limit or row filtering).
     */
    public static SinglePartitionReadCommand create(CFMetaData metadata, int nowInSec, DecoratedKey key, Clustering name)
    {
        return create(metadata, nowInSec, key, FBUtilities.singleton(name, metadata.comparator));
    }

    public SinglePartitionReadCommand copy()
    {
        return new SinglePartitionReadCommand(isDigestQuery(),
                                              digestVersion(),
                                              isForThrift(),
                                              metadata(),
                                              nowInSec(),
                                              columnFilter(),
                                              rowFilter(),
                                              limits(),
                                              partitionKey(),
                                              clusteringIndexFilter(),
                                              indexMetadata());
    }

    public SinglePartitionReadCommand copyAsDigestQuery()
    {
        return new SinglePartitionReadCommand(true,
                                              digestVersion(),
                                              isForThrift(),
                                              metadata(),
                                              nowInSec(),
                                              columnFilter(),
                                              rowFilter(),
                                              limits(),
                                              partitionKey(),
                                              clusteringIndexFilter(),
                                              indexMetadata());
    }

    public SinglePartitionReadCommand withUpdatedLimit(DataLimits newLimits)
    {
        return new SinglePartitionReadCommand(isDigestQuery(),
                                              digestVersion(),
                                              isForThrift(),
                                              metadata(),
                                              nowInSec(),
                                              columnFilter(),
                                              rowFilter(),
                                              newLimits,
                                              partitionKey(),
                                              clusteringIndexFilter(),
                                              indexMetadata());
    }

    public SinglePartitionReadCommand withUpdatedClusteringIndexFilter(ClusteringIndexFilter filter)
    {
        return new SinglePartitionReadCommand(isDigestQuery(),
                                              digestVersion(),
                                              isForThrift(),
                                              metadata(),
                                              nowInSec(),
                                              columnFilter(),
                                              rowFilter(),
                                              limits(),
                                              partitionKey(),
                                              filter,
                                              indexMetadata());
    }

    static SinglePartitionReadCommand legacySliceCommand(boolean isDigest,
                                                         int digestVersion,
                                                         CFMetaData metadata,
                                                         int nowInSec,
                                                         ColumnFilter columnFilter,
                                                         DataLimits limits,
                                                         DecoratedKey partitionKey,
                                                         ClusteringIndexSliceFilter filter)
    {
        // messages from old nodes will expect the thrift format, so always use 'true' for isForThrift
        return new SinglePartitionReadCommand(isDigest,
                                              digestVersion,
                                              true,
                                              metadata,
                                              nowInSec,
                                              columnFilter,
                                              RowFilter.NONE,
                                              limits,
                                              partitionKey,
                                              filter,
                                              null);
    }

    static SinglePartitionReadCommand legacyNamesCommand(boolean isDigest,
                                                         int digestVersion,
                                                         CFMetaData metadata,
                                                         int nowInSec,
                                                         ColumnFilter columnFilter,
                                                         DecoratedKey partitionKey,
                                                         ClusteringIndexNamesFilter filter)
    {
        // messages from old nodes will expect the thrift format, so always use 'true' for isForThrift
        return new SinglePartitionReadCommand(isDigest, digestVersion, true, metadata, nowInSec, columnFilter, RowFilter.NONE, DataLimits.NONE, partitionKey, filter,null);
    }

    public DecoratedKey partitionKey()
    {
        return partitionKey;
    }

    public ClusteringIndexFilter clusteringIndexFilter()
    {
        return clusteringIndexFilter;
    }

    public ClusteringIndexFilter clusteringIndexFilter(DecoratedKey key)
    {
        return clusteringIndexFilter;
    }

    public long getTimeout()
    {
        return DatabaseDescriptor.getReadRpcTimeout();
    }

    public boolean selectsKey(DecoratedKey key)
    {
        if (!this.partitionKey().equals(key))
            return false;

        return rowFilter().partitionKeyRestrictionsAreSatisfiedBy(key, metadata().getKeyValidator());
    }

    public boolean selectsClustering(DecoratedKey key, Clustering clustering)
    {
        if (clustering == Clustering.STATIC_CLUSTERING)
            return !columnFilter().fetchedColumns().statics.isEmpty();

        if (!clusteringIndexFilter().selects(clustering))
            return false;

        return rowFilter().clusteringKeyRestrictionsAreSatisfiedBy(clustering);
    }

    
Returns a new command suitable to paging from the last returned row.
Params:
lastReturned – the last row returned by the previous page. The newly created command will only query row that comes after this (in query order). This can be null if this is the first page.
limits – the limits to use for the page to query.
Returns:
the newly create command./**
     * Returns a new command suitable to paging from the last returned row.
     *
     * @param lastReturned the last row returned by the previous page. The newly created command
     * will only query row that comes after this (in query order). This can be {@code null} if this
     * is the first page.
     * @param limits the limits to use for the page to query.
     *
     * @return the newly create command.
     */
    public SinglePartitionReadCommand forPaging(Clustering lastReturned, DataLimits limits)
    {
        // We shouldn't have set digest yet when reaching that point
        assert !isDigestQuery();
        return create(isForThrift(),
                      metadata(),
                      nowInSec(),
                      columnFilter(),
                      rowFilter(),
                      limits,
                      partitionKey(),
                      lastReturned == null ? clusteringIndexFilter() : clusteringIndexFilter.forPaging(metadata().comparator, lastReturned, false));
    }

    public PartitionIterator execute(ConsistencyLevel consistency, ClientState clientState, long queryStartNanoTime) throws RequestExecutionException
    {
        return StorageProxy.read(Group.one(this), consistency, clientState, queryStartNanoTime);
    }

    public SinglePartitionPager getPager(PagingState pagingState, ProtocolVersion protocolVersion)
    {
        return getPager(this, pagingState, protocolVersion);
    }

    private static SinglePartitionPager getPager(SinglePartitionReadCommand command, PagingState pagingState, ProtocolVersion protocolVersion)
    {
        return new SinglePartitionPager(command, pagingState, protocolVersion);
    }

    protected void recordLatency(TableMetrics metric, long latencyNanos)
    {
        metric.readLatency.addNano(latencyNanos);
    }

    @SuppressWarnings("resource") // we close the created iterator through closing the result of this method (and SingletonUnfilteredPartitionIterator ctor cannot fail)
    protected UnfilteredPartitionIterator queryStorage(final ColumnFamilyStore cfs, ReadExecutionController executionController)
    {
        UnfilteredRowIterator partition = cfs.isRowCacheEnabled()
                                        ? getThroughCache(cfs, executionController)
                                        : queryMemtableAndDisk(cfs, executionController);
        return new SingletonUnfilteredPartitionIterator(partition, isForThrift());
    }

    
Fetch the rows requested if in cache; if not, read it from disk and cache it.

If the partition is cached, and the filter given is within its bounds, we return
from cache, otherwise from disk.

If the partition is is not cached, we figure out what filter is "biggest", read
that from disk, then filter the result and either cache that or return it.
/**
     * Fetch the rows requested if in cache; if not, read it from disk and cache it.
     * <p>
     * If the partition is cached, and the filter given is within its bounds, we return
     * from cache, otherwise from disk.
     * <p>
     * If the partition is is not cached, we figure out what filter is "biggest", read
     * that from disk, then filter the result and either cache that or return it.
     */
    private UnfilteredRowIterator getThroughCache(ColumnFamilyStore cfs, ReadExecutionController executionController)
    {
        assert !cfs.isIndex(); // CASSANDRA-5732
        assert cfs.isRowCacheEnabled() : String.format("Row cache is not enabled on table [%s]", cfs.name);

        RowCacheKey key = new RowCacheKey(metadata().ksAndCFName, partitionKey());

        // Attempt a sentinel-read-cache sequence.  if a write invalidates our sentinel, we'll return our
        // (now potentially obsolete) data, but won't cache it. see CASSANDRA-3862
        // TODO: don't evict entire partitions on writes (#2864)
        IRowCacheEntry cached = CacheService.instance.rowCache.get(key);
        if (cached != null)
        {
            if (cached instanceof RowCacheSentinel)
            {
                // Some other read is trying to cache the value, just do a normal non-caching read
                Tracing.trace("Row cache miss (race)");
                cfs.metric.rowCacheMiss.inc();
                return queryMemtableAndDisk(cfs, executionController);
            }

            CachedPartition cachedPartition = (CachedPartition)cached;
            if (cfs.isFilterFullyCoveredBy(clusteringIndexFilter(), limits(), cachedPartition, nowInSec()))
            {
                cfs.metric.rowCacheHit.inc();
                Tracing.trace("Row cache hit");
                UnfilteredRowIterator unfilteredRowIterator = clusteringIndexFilter().getUnfilteredRowIterator(columnFilter(), cachedPartition);
                cfs.metric.updateSSTableIterated(0);
                return unfilteredRowIterator;
            }

            cfs.metric.rowCacheHitOutOfRange.inc();
            Tracing.trace("Ignoring row cache as cached value could not satisfy query");
            return queryMemtableAndDisk(cfs, executionController);
        }

        cfs.metric.rowCacheMiss.inc();
        Tracing.trace("Row cache miss");

        // Note that on tables with no clustering keys, any positive value of
        // rowsToCache implies caching the full partition
        boolean cacheFullPartitions = metadata().clusteringColumns().size() > 0 ?
                                      metadata().params.caching.cacheAllRows() :
                                      metadata().params.caching.cacheRows();

        // To be able to cache what we read, what we read must at least covers what the cache holds, that
        // is the 'rowsToCache' first rows of the partition. We could read those 'rowsToCache' first rows
        // systematically, but we'd have to "extend" that to whatever is needed for the user query that the
        // 'rowsToCache' first rows don't cover and it's not trivial with our existing filters. So currently
        // we settle for caching what we read only if the user query does query the head of the partition since
        // that's the common case of when we'll be able to use the cache anyway. One exception is if we cache
        // full partitions, in which case we just always read it all and cache.
        if (cacheFullPartitions || clusteringIndexFilter().isHeadFilter())
        {
            RowCacheSentinel sentinel = new RowCacheSentinel();
            boolean sentinelSuccess = CacheService.instance.rowCache.putIfAbsent(key, sentinel);
            boolean sentinelReplaced = false;

            try
            {
                final int rowsToCache = metadata().params.caching.rowsPerPartitionToCache();
                final boolean enforceStrictLiveness = metadata().enforceStrictLiveness();

                @SuppressWarnings("resource") // we close on exception or upon closing the result of this method
                UnfilteredRowIterator iter = fullPartitionRead(metadata(), nowInSec(), partitionKey()).queryMemtableAndDisk(cfs, executionController);
                try
                {
                    // Use a custom iterator instead of DataLimits to avoid stopping the original iterator
                    UnfilteredRowIterator toCacheIterator = new WrappingUnfilteredRowIterator(iter)
                    {
                        private int rowsCounted = 0;

                        @Override
                        public boolean hasNext()
                        {
                            return rowsCounted < rowsToCache && super.hasNext();
                        }

                        @Override
                        public Unfiltered next()
                        {
                            Unfiltered unfiltered = super.next();
                            if (unfiltered.isRow())
                            {
                                Row row = (Row) unfiltered;
                                if (row.hasLiveData(nowInSec(), enforceStrictLiveness))
                                    rowsCounted++;
                            }
                            return unfiltered;
                        }
                    };

                    // We want to cache only rowsToCache rows
                    CachedPartition toCache = CachedBTreePartition.create(toCacheIterator, nowInSec());

                    if (sentinelSuccess && !toCache.isEmpty())
                    {
                        Tracing.trace("Caching {} rows", toCache.rowCount());
                        CacheService.instance.rowCache.replace(key, sentinel, toCache);
                        // Whether or not the previous replace has worked, our sentinel is not in the cache anymore
                        sentinelReplaced = true;
                    }

                    // We then re-filter out what this query wants.
                    // Note that in the case where we don't cache full partitions, it's possible that the current query is interested in more
                    // than what we've cached, so we can't just use toCache.
                    UnfilteredRowIterator cacheIterator = clusteringIndexFilter().getUnfilteredRowIterator(columnFilter(), toCache);
                    if (cacheFullPartitions)
                    {
                        // Everything is guaranteed to be in 'toCache', we're done with 'iter'
                        assert !iter.hasNext();
                        iter.close();
                        return cacheIterator;
                    }
                    return UnfilteredRowIterators.concat(cacheIterator, clusteringIndexFilter().filterNotIndexed(columnFilter(), iter));
                }
                catch (RuntimeException | Error e)
                {
                    iter.close();
                    throw e;
                }
            }
            finally
            {
                if (sentinelSuccess && !sentinelReplaced)
                    cfs.invalidateCachedPartition(key);
            }
        }

        Tracing.trace("Fetching data but not populating cache as query does not query from the start of the partition");
        return queryMemtableAndDisk(cfs, executionController);
    }

    
Queries both memtable and sstables to fetch the result of this query.
 Please note that this method: 1) does not check the row cache. 2) does not apply the query limit, nor the row filter (and so ignore 2ndary indexes). Those are applied in ReadCommand.executeLocally. 3) does not record some of the read metrics (latency, scanned cells histograms) nor throws TombstoneOverwhelmingException. It is publicly exposed because there is a few places where that is exactly what we want, but it should be used only where you know you don't need thoses things. 
 Also note that one must have created a ReadExecutionController on the queried table and we require it as a parameter to enforce that fact, even though it's not explicitlly used by the method. /**
     * Queries both memtable and sstables to fetch the result of this query.
     * <p>
     * Please note that this method:
     *   1) does not check the row cache.
     *   2) does not apply the query limit, nor the row filter (and so ignore 2ndary indexes).
     *      Those are applied in {@link ReadCommand#executeLocally}.
     *   3) does not record some of the read metrics (latency, scanned cells histograms) nor
     *      throws TombstoneOverwhelmingException.
     * It is publicly exposed because there is a few places where that is exactly what we want,
     * but it should be used only where you know you don't need thoses things.
     * <p>
     * Also note that one must have created a {@code ReadExecutionController} on the queried table and we require it as
     * a parameter to enforce that fact, even though it's not explicitlly used by the method.
     */
    public UnfilteredRowIterator queryMemtableAndDisk(ColumnFamilyStore cfs, ReadExecutionController executionController)
    {
        assert executionController != null && executionController.validForReadOn(cfs);
        Tracing.trace("Executing single-partition query on {}", cfs.name);

        return queryMemtableAndDiskInternal(cfs);
    }

    @Override
    protected int oldestUnrepairedTombstone()
    {
        return oldestUnrepairedTombstone;
    }

    private UnfilteredRowIterator queryMemtableAndDiskInternal(ColumnFamilyStore cfs)
    {
        /*
         * We have 2 main strategies:
         *   1) We query memtables and sstables simulateneously. This is our most generic strategy and the one we use
         *      unless we have a names filter that we know we can optimize futher.
         *   2) If we have a name filter (so we query specific rows), we can make a bet: that all column for all queried row
         *      will have data in the most recent sstable(s), thus saving us from reading older ones. This does imply we
         *      have a way to guarantee we have all the data for what is queried, which is only possible for name queries
         *      and if we have neither non-frozen collections/UDTs nor counters (indeed, for a non-frozen collection or UDT,
         *      we can't guarantee an older sstable won't have some elements that weren't in the most recent sstables,
         *      and counters are intrinsically a collection of shards and so have the same problem).
         */
        if (clusteringIndexFilter() instanceof ClusteringIndexNamesFilter && !queriesMulticellType())
            return queryMemtableAndSSTablesInTimestampOrder(cfs, (ClusteringIndexNamesFilter)clusteringIndexFilter());

        Tracing.trace("Acquiring sstable references");
        ColumnFamilyStore.ViewFragment view = cfs.select(View.select(SSTableSet.LIVE, partitionKey()));
        List<UnfilteredRowIterator> iterators = new ArrayList<>(Iterables.size(view.memtables) + view.sstables.size());
        ClusteringIndexFilter filter = clusteringIndexFilter();
        long minTimestamp = Long.MAX_VALUE;

        try
        {
            for (Memtable memtable : view.memtables)
            {
                Partition partition = memtable.getPartition(partitionKey());
                if (partition == null)
                    continue;

                minTimestamp = Math.min(minTimestamp, memtable.getMinTimestamp());

                @SuppressWarnings("resource") // 'iter' is added to iterators which is closed on exception, or through the closing of the final merged iterator
                UnfilteredRowIterator iter = filter.getUnfilteredRowIterator(columnFilter(), partition);
                if (isForThrift())
                    iter = ThriftResultsMerger.maybeWrap(iter, nowInSec());
                oldestUnrepairedTombstone = Math.min(oldestUnrepairedTombstone, partition.stats().minLocalDeletionTime);
                iterators.add(RTBoundValidator.validate(iter, RTBoundValidator.Stage.MEMTABLE, false));
            }

            /*
             * We can't eliminate full sstables based on the timestamp of what we've already read like
             * in collectTimeOrderedData, but we still want to eliminate sstable whose maxTimestamp < mostRecentTombstone
             * we've read. We still rely on the sstable ordering by maxTimestamp since if
             *   maxTimestamp_s1 > maxTimestamp_s0,
             * we're guaranteed that s1 cannot have a row tombstone such that
             *   timestamp(tombstone) > maxTimestamp_s0
             * since we necessarily have
             *   timestamp(tombstone) <= maxTimestamp_s1
             * In other words, iterating in maxTimestamp order allow to do our mostRecentPartitionTombstone elimination
             * in one pass, and minimize the number of sstables for which we read a partition tombstone.
             */
            Collections.sort(view.sstables, SSTableReader.maxTimestampDescending);
            long mostRecentPartitionTombstone = Long.MIN_VALUE;
            int nonIntersectingSSTables = 0;
            List<SSTableReader> skippedSSTablesWithTombstones = null;
            SSTableReadMetricsCollector metricsCollector = new SSTableReadMetricsCollector();

            for (SSTableReader sstable : view.sstables)
            {
                // if we've already seen a partition tombstone with a timestamp greater
                // than the most recent update to this sstable, we can skip it
                if (sstable.getMaxTimestamp() < mostRecentPartitionTombstone)
                    break;

                if (!shouldInclude(sstable))
                {
                    nonIntersectingSSTables++;
                    if (sstable.mayHaveTombstones())
                    { // if sstable has tombstones we need to check after one pass if it can be safely skipped
                        if (skippedSSTablesWithTombstones == null)
                            skippedSSTablesWithTombstones = new ArrayList<>();
                        skippedSSTablesWithTombstones.add(sstable);

                    }
                    continue;
                }

                minTimestamp = Math.min(minTimestamp, sstable.getMinTimestamp());

                @SuppressWarnings("resource") // 'iter' is added to iterators which is closed on exception,
                                              // or through the closing of the final merged iterator
                UnfilteredRowIteratorWithLowerBound iter = makeIterator(cfs, sstable, true, metricsCollector);
                if (!sstable.isRepaired())
                    oldestUnrepairedTombstone = Math.min(oldestUnrepairedTombstone, sstable.getMinLocalDeletionTime());

                iterators.add(iter);
                mostRecentPartitionTombstone = Math.max(mostRecentPartitionTombstone,
                                                        iter.partitionLevelDeletion().markedForDeleteAt());
            }

            int includedDueToTombstones = 0;
            // Check for sstables with tombstones that are not expired
            if (skippedSSTablesWithTombstones != null)
            {
                for (SSTableReader sstable : skippedSSTablesWithTombstones)
                {
                    if (sstable.getMaxTimestamp() <= minTimestamp)
                        continue;

                    @SuppressWarnings("resource") // 'iter' is added to iterators which is close on exception,
                                                  // or through the closing of the final merged iterator
                    UnfilteredRowIteratorWithLowerBound iter = makeIterator(cfs, sstable, false, metricsCollector);
                    if (!sstable.isRepaired())
                        oldestUnrepairedTombstone = Math.min(oldestUnrepairedTombstone, sstable.getMinLocalDeletionTime());

                    iterators.add(iter);
                    includedDueToTombstones++;
                }
            }
            if (Tracing.isTracing())
                Tracing.trace("Skipped {}/{} non-slice-intersecting sstables, included {} due to tombstones",
                               nonIntersectingSSTables, view.sstables.size(), includedDueToTombstones);

            if (iterators.isEmpty())
                return EmptyIterators.unfilteredRow(cfs.metadata, partitionKey(), filter.isReversed());

            StorageHook.instance.reportRead(cfs.metadata.cfId, partitionKey());
            return withSSTablesIterated(iterators, cfs.metric, metricsCollector);
        }
        catch (RuntimeException | Error e)
        {
            try
            {
                FBUtilities.closeAll(iterators);
            }
            catch (Exception suppressed)
            {
                e.addSuppressed(suppressed);
            }
            throw e;
        }
    }

    private boolean shouldInclude(SSTableReader sstable)
    {
        // If some static columns are queried, we should always include the sstable: the clustering values stats of the sstable
        // don't tell us if the sstable contains static values in particular.
        // TODO: we could record if a sstable contains any static value at all.
        if (!columnFilter().fetchedColumns().statics.isEmpty())
            return true;

        return clusteringIndexFilter().shouldInclude(sstable);
    }

    private UnfilteredRowIteratorWithLowerBound makeIterator(ColumnFamilyStore cfs,
                                                             final SSTableReader sstable,
                                                             boolean applyThriftTransformation,
                                                             SSTableReadsListener listener)
    {
        return StorageHook.instance.makeRowIteratorWithLowerBound(cfs,
                                                                  partitionKey(),
                                                                  sstable,
                                                                  clusteringIndexFilter(),
                                                                  columnFilter(),
                                                                  isForThrift(),
                                                                  nowInSec(),
                                                                  applyThriftTransformation,
                                                                  listener);

    }

    
Return a wrapped iterator that when closed will update the sstables iterated and READ sample metrics.
Note that we cannot use the Transformations framework because they greedily get the static row, which
would cause all iterators to be initialized and hence all sstables to be accessed.
/**
     * Return a wrapped iterator that when closed will update the sstables iterated and READ sample metrics.
     * Note that we cannot use the Transformations framework because they greedily get the static row, which
     * would cause all iterators to be initialized and hence all sstables to be accessed.
     */
    private UnfilteredRowIterator withSSTablesIterated(List<UnfilteredRowIterator> iterators,
                                                       TableMetrics metrics,
                                                       SSTableReadMetricsCollector metricsCollector)
    {
        @SuppressWarnings("resource") //  Closed through the closing of the result of the caller method.
        UnfilteredRowIterator merged = UnfilteredRowIterators.merge(iterators, nowInSec());

        if (!merged.isEmpty())
        {
            DecoratedKey key = merged.partitionKey();
            metrics.samplers.get(TableMetrics.Sampler.READS).addSample(key.getKey(), key.hashCode(), 1);
        }

        class UpdateSstablesIterated extends Transformation
        {
           public void onPartitionClose()
           {
               int mergedSSTablesIterated = metricsCollector.getMergedSSTables();
               metrics.updateSSTableIterated(mergedSSTablesIterated);
               Tracing.trace("Merged data from memtables and {} sstables", mergedSSTablesIterated);
           }
        };
        return Transformation.apply(merged, new UpdateSstablesIterated());
    }

    private boolean queriesMulticellType()
    {
        for (ColumnDefinition column : columnFilter().fetchedColumns())
        {
            if (column.type.isMultiCell() || column.type.isCounter())
                return true;
        }
        return false;
    }

    
Do a read by querying the memtable(s) first, and then each relevant sstables sequentially by order of the sstable max timestamp. This is used for names query in the hope of only having to query the 1 or 2 most recent query and then knowing nothing more recent could be in the older sstables (which we can only guarantee if we know exactly which row we queries, and if no collection or counters are included). This method assumes the filter is a ClusteringIndexNamesFilter. /**
     * Do a read by querying the memtable(s) first, and then each relevant sstables sequentially by order of the sstable
     * max timestamp.
     *
     * This is used for names query in the hope of only having to query the 1 or 2 most recent query and then knowing nothing
     * more recent could be in the older sstables (which we can only guarantee if we know exactly which row we queries, and if
     * no collection or counters are included).
     * This method assumes the filter is a {@code ClusteringIndexNamesFilter}.
     */
    private UnfilteredRowIterator queryMemtableAndSSTablesInTimestampOrder(ColumnFamilyStore cfs, ClusteringIndexNamesFilter filter)
    {
        Tracing.trace("Acquiring sstable references");
        ColumnFamilyStore.ViewFragment view = cfs.select(View.select(SSTableSet.LIVE, partitionKey()));

        ImmutableBTreePartition result = null;

        Tracing.trace("Merging memtable contents");
        for (Memtable memtable : view.memtables)
        {
            Partition partition = memtable.getPartition(partitionKey());
            if (partition == null)
                continue;

            try (UnfilteredRowIterator iter = filter.getUnfilteredRowIterator(columnFilter(), partition))
            {
                if (iter.isEmpty())
                    continue;

                result = add(
                    RTBoundValidator.validate(isForThrift() ? ThriftResultsMerger.maybeWrap(iter, nowInSec()) : iter, RTBoundValidator.Stage.MEMTABLE, false),
                    result,
                    filter,
                    false
                );
            }
        }

        /* add the SSTables on disk */
        Collections.sort(view.sstables, SSTableReader.maxTimestampDescending);
        boolean onlyUnrepaired = true;
        // read sorted sstables
        SSTableReadMetricsCollector metricsCollector = new SSTableReadMetricsCollector();
        for (SSTableReader sstable : view.sstables)
        {
            // if we've already seen a partition tombstone with a timestamp greater
            // than the most recent update to this sstable, we're done, since the rest of the sstables
            // will also be older
            if (result != null && sstable.getMaxTimestamp() < result.partitionLevelDeletion().markedForDeleteAt())
                break;

            long currentMaxTs = sstable.getMaxTimestamp();
            filter = reduceFilter(filter, result, currentMaxTs);
            if (filter == null)
                break;

            if (!shouldInclude(sstable))
            {
                // This mean that nothing queried by the filter can be in the sstable. One exception is the top-level partition deletion
                // however: if it is set, it impacts everything and must be included. Getting that top-level partition deletion costs us
                // some seek in general however (unless the partition is indexed and is in the key cache), so we first check if the sstable
                // has any tombstone at all as a shortcut.
                if (!sstable.mayHaveTombstones())
                    continue; // no tombstone at all, we can skip that sstable

                // We need to get the partition deletion and include it if it's live. In any case though, we're done with that sstable.
                try (UnfilteredRowIterator iter = StorageHook.instance.makeRowIterator(cfs,
                                                                                       sstable,
                                                                                       partitionKey(),
                                                                                       filter.getSlices(metadata()),
                                                                                       columnFilter(),
                                                                                       filter.isReversed(),
                                                                                       isForThrift(),
                                                                                       metricsCollector))
                {
                    if (!iter.partitionLevelDeletion().isLive())
                    {
                        result = add(
                            UnfilteredRowIterators.noRowsIterator(iter.metadata(),
                                                                  iter.partitionKey(),
                                                                  Rows.EMPTY_STATIC_ROW,
                                                                  iter.partitionLevelDeletion(),
                                                                  filter.isReversed()),
                            result,
                            filter,
                            sstable.isRepaired()
                        );
                    }
                    else
                    {
                        result = add(
                            RTBoundValidator.validate(iter, RTBoundValidator.Stage.SSTABLE, false),
                            result,
                            filter,
                            sstable.isRepaired()
                        );
                    }
                }

                continue;
            }

            try (UnfilteredRowIterator iter = StorageHook.instance.makeRowIterator(cfs,
                                                                                   sstable,
                                                                                   partitionKey(),
                                                                                   filter.getSlices(metadata()),
                                                                                   columnFilter(),
                                                                                   filter.isReversed(),
                                                                                   isForThrift(),
                                                                                   metricsCollector))
            {
                if (iter.isEmpty())
                    continue;

                if (sstable.isRepaired())
                    onlyUnrepaired = false;

                result = add(
                    RTBoundValidator.validate(isForThrift() ? ThriftResultsMerger.maybeWrap(iter, nowInSec()) : iter, RTBoundValidator.Stage.SSTABLE, false),
                    result,
                    filter,
                    sstable.isRepaired()
                );
            }
        }

        cfs.metric.updateSSTableIterated(metricsCollector.getMergedSSTables());

        if (result == null || result.isEmpty())
            return EmptyIterators.unfilteredRow(metadata(), partitionKey(), false);

        DecoratedKey key = result.partitionKey();
        cfs.metric.samplers.get(TableMetrics.Sampler.READS).addSample(key.getKey(), key.hashCode(), 1);
        StorageHook.instance.reportRead(cfs.metadata.cfId, partitionKey());

        // "hoist up" the requested data into a more recent sstable
        if (metricsCollector.getMergedSSTables() > cfs.getMinimumCompactionThreshold()
            && onlyUnrepaired
            && !cfs.isAutoCompactionDisabled()
            && cfs.getCompactionStrategyManager().shouldDefragment())
        {
            // !!WARNING!!   if we stop copying our data to a heap-managed object,
            //               we will need to track the lifetime of this mutation as well
            Tracing.trace("Defragmenting requested data");

            try (UnfilteredRowIterator iter = result.unfilteredIterator(columnFilter(), Slices.ALL, false))
            {
                final Mutation mutation = new Mutation(PartitionUpdate.fromIterator(iter, columnFilter()));
                StageManager.getStage(Stage.MUTATION).execute(() -> {
                    // skipping commitlog and index updates is fine since we're just de-fragmenting existing data
                    Keyspace.open(mutation.getKeyspaceName()).apply(mutation, false, false);
                });
            }
        }

        return result.unfilteredIterator(columnFilter(), Slices.ALL, clusteringIndexFilter().isReversed());
    }

    private ImmutableBTreePartition add(UnfilteredRowIterator iter, ImmutableBTreePartition result, ClusteringIndexNamesFilter filter, boolean isRepaired)
    {
        if (!isRepaired)
            oldestUnrepairedTombstone = Math.min(oldestUnrepairedTombstone, iter.stats().minLocalDeletionTime);

        int maxRows = Math.max(filter.requestedRows().size(), 1);
        if (result == null)
            return ImmutableBTreePartition.create(iter, maxRows);

        try (UnfilteredRowIterator merged = UnfilteredRowIterators.merge(Arrays.asList(iter, result.unfilteredIterator(columnFilter(), Slices.ALL, filter.isReversed())), nowInSec()))
        {
            return ImmutableBTreePartition.create(merged, maxRows);
        }
    }

    private ClusteringIndexNamesFilter reduceFilter(ClusteringIndexNamesFilter filter, Partition result, long sstableTimestamp)
    {
        if (result == null)
            return filter;

        SearchIterator<Clustering, Row> searchIter = result.searchIterator(columnFilter(), false);

        PartitionColumns columns = columnFilter().fetchedColumns();
        NavigableSet<Clustering> clusterings = filter.requestedRows();

        // We want to remove rows for which we have values for all requested columns. We have to deal with both static and regular rows.
        // TODO: we could also remove a selected column if we've found values for every requested row but we'll leave
        // that for later.

        boolean removeStatic = false;
        if (!columns.statics.isEmpty())
        {
            Row staticRow = searchIter.next(Clustering.STATIC_CLUSTERING);
            removeStatic = staticRow != null && canRemoveRow(staticRow, columns.statics, sstableTimestamp);
        }

        NavigableSet<Clustering> toRemove = null;
        for (Clustering clustering : clusterings)
        {
            Row row = searchIter.next(clustering);
            if (row == null || !canRemoveRow(row, columns.regulars, sstableTimestamp))
                continue;

            if (toRemove == null)
                toRemove = new TreeSet<>(result.metadata().comparator);
            toRemove.add(clustering);
        }

        if (!removeStatic && toRemove == null)
            return filter;

        // Check if we have everything we need
        boolean hasNoMoreStatic = columns.statics.isEmpty() || removeStatic;
        boolean hasNoMoreClusterings = clusterings.isEmpty() || (toRemove != null && toRemove.size() == clusterings.size());
        if (hasNoMoreStatic && hasNoMoreClusterings)
            return null;

        if (toRemove != null)
        {
            BTreeSet.Builder<Clustering> newClusterings = BTreeSet.builder(result.metadata().comparator);
            newClusterings.addAll(Sets.difference(clusterings, toRemove));
            clusterings = newClusterings.build();
        }
        return new ClusteringIndexNamesFilter(clusterings, filter.isReversed());
    }

    private boolean canRemoveRow(Row row, Columns requestedColumns, long sstableTimestamp)
    {
        // We can remove a row if it has data that is more recent that the next sstable to consider for the data that the query
        // cares about. And the data we care about is 1) the row timestamp (since every query cares if the row exists or not)
        // and 2) the requested columns.
        if (row.primaryKeyLivenessInfo().isEmpty() || row.primaryKeyLivenessInfo().timestamp() <= sstableTimestamp)
            return false;

        for (ColumnDefinition column : requestedColumns)
        {
            Cell cell = row.getCell(column);
            if (cell == null || cell.timestamp() <= sstableTimestamp)
                return false;
        }
        return true;
    }

    @Override
    public boolean selectsFullPartition()
    {
        return metadata().isStaticCompactTable() ||
               (clusteringIndexFilter.selectsAllPartition() && !rowFilter().hasExpressionOnClusteringOrRegularColumns());
    }

    @Override
    public String toString()
    {
        return String.format("Read(%s.%s columns=%s rowFilter=%s limits=%s key=%s filter=%s, nowInSec=%d)",
                             metadata().ksName,
                             metadata().cfName,
                             columnFilter(),
                             rowFilter(),
                             limits(),
                             metadata().getKeyValidator().getString(partitionKey().getKey()),
                             clusteringIndexFilter.toString(metadata()),
                             nowInSec());
    }

    public MessageOut<ReadCommand> createMessage(int version)
    {
        return new MessageOut<>(MessagingService.Verb.READ, this, readSerializer);
    }

    protected void appendCQLWhereClause(StringBuilder sb)
    {
        sb.append(" WHERE ");

        sb.append(ColumnDefinition.toCQLString(metadata().partitionKeyColumns())).append(" = ");
        DataRange.appendKeyString(sb, metadata().getKeyValidator(), partitionKey().getKey());

        // We put the row filter first because the clustering index filter can end by "ORDER BY"
        if (!rowFilter().isEmpty())
            sb.append(" AND ").append(rowFilter());

        String filterString = clusteringIndexFilter().toCQLString(metadata());
        if (!filterString.isEmpty())
            sb.append(" AND ").append(filterString);
    }

    protected void serializeSelection(DataOutputPlus out, int version) throws IOException
    {
        metadata().getKeyValidator().writeValue(partitionKey().getKey(), out);
        ClusteringIndexFilter.serializer.serialize(clusteringIndexFilter(), out, version);
    }

    protected long selectionSerializedSize(int version)
    {
        return metadata().getKeyValidator().writtenLength(partitionKey().getKey())
             + ClusteringIndexFilter.serializer.serializedSize(clusteringIndexFilter(), version);
    }

    public boolean isLimitedToOnePartition()
    {
        return true;
    }

    
Groups multiple single partition read commands.
/**
     * Groups multiple single partition read commands.
     */
    public static class Group implements ReadQuery
    {
        public final List<SinglePartitionReadCommand> commands;
        private final DataLimits limits;
        private final int nowInSec;
        private final boolean selectsFullPartitions;

        public Group(List<SinglePartitionReadCommand> commands, DataLimits limits)
        {
            assert !commands.isEmpty();
            this.commands = commands;
            this.limits = limits;
            SinglePartitionReadCommand firstCommand = commands.get(0);
            this.nowInSec = firstCommand.nowInSec();
            this.selectsFullPartitions = firstCommand.selectsFullPartition();
            for (int i = 1; i < commands.size(); i++)
                assert commands.get(i).nowInSec() == nowInSec;
        }

        public static Group one(SinglePartitionReadCommand command)
        {
            return new Group(Collections.singletonList(command), command.limits());
        }

        public PartitionIterator execute(ConsistencyLevel consistency, ClientState clientState, long queryStartNanoTime) throws RequestExecutionException
        {
            return StorageProxy.read(this, consistency, clientState, queryStartNanoTime);
        }

        public int nowInSec()
        {
            return nowInSec;
        }

        public DataLimits limits()
        {
            return limits;
        }

        public CFMetaData metadata()
        {
            return commands.get(0).metadata();
        }

        @Override
        public boolean selectsFullPartition()
        {
            return selectsFullPartitions;
        }

        public ReadExecutionController executionController()
        {
            // Note that the only difference between the command in a group must be the partition key on which
            // they applied. So as far as ReadOrderGroup is concerned, we can use any of the commands to start one.
            return commands.get(0).executionController();
        }

        public PartitionIterator executeInternal(ReadExecutionController controller)
        {
            // Note that the only difference between the command in a group must be the partition key on which
            // they applied.
            boolean enforceStrictLiveness = commands.get(0).metadata().enforceStrictLiveness();
            return limits.filter(UnfilteredPartitionIterators.filter(executeLocally(controller, false), nowInSec),
                                 nowInSec,
                                 selectsFullPartitions,
                                 enforceStrictLiveness);
        }

        public UnfilteredPartitionIterator executeLocally(ReadExecutionController executionController)
        {
            return executeLocally(executionController, true);
        }

        
Implementation of ReadQuery.executeLocally(ReadExecutionController). 
Params:
executionController – - the ReadExecutionController protecting the read.
sort – - whether to sort the inner commands by partition key, required for merging the iterator later on. This will be false when called by ReadQuery.executeInternal(ReadExecutionController) because in this case it is safe to do so as there is no merging involved and we don't want to change the old behavior which was to not sort by partition.
Returns:
- the iterator that can be used to retrieve the query result./**
         * Implementation of {@link ReadQuery#executeLocally(ReadExecutionController)}.
         *
         * @param executionController - the {@code ReadExecutionController} protecting the read.
         * @param sort - whether to sort the inner commands by partition key, required for merging the iterator
         *               later on. This will be false when called by {@link ReadQuery#executeInternal(ReadExecutionController)}
         *               because in this case it is safe to do so as there is no merging involved and we don't want to
         *               change the old behavior which was to not sort by partition.
         *
         * @return - the iterator that can be used to retrieve the query result.
         */
        private UnfilteredPartitionIterator executeLocally(ReadExecutionController executionController, boolean sort)
        {
            List<Pair<DecoratedKey, UnfilteredPartitionIterator>> partitions = new ArrayList<>(commands.size());
            for (SinglePartitionReadCommand cmd : commands)
                partitions.add(Pair.of(cmd.partitionKey, cmd.executeLocally(executionController)));

            if (sort)
                Collections.sort(partitions, (p1, p2) -> p1.getLeft().compareTo(p2.getLeft()));

            return UnfilteredPartitionIterators.concat(partitions.stream().map(p -> p.getRight()).collect(Collectors.toList()));
        }

        public QueryPager getPager(PagingState pagingState, ProtocolVersion protocolVersion)
        {
            if (commands.size() == 1)
                return SinglePartitionReadCommand.getPager(commands.get(0), pagingState, protocolVersion);

            return new MultiPartitionPager(this, pagingState, protocolVersion);
        }

        public boolean selectsKey(DecoratedKey key)
        {
            return Iterables.any(commands, c -> c.selectsKey(key));
        }

        public boolean selectsClustering(DecoratedKey key, Clustering clustering)
        {
            return Iterables.any(commands, c -> c.selectsClustering(key, clustering));
        }

        @Override
        public String toString()
        {
            return commands.toString();
        }
    }

    private static class Deserializer extends SelectionDeserializer
    {
        public ReadCommand deserialize(DataInputPlus in,
                                       int version,
                                       boolean isDigest,
                                       int digestVersion,
                                       boolean isForThrift,
                                       CFMetaData metadata,
                                       int nowInSec,
                                       ColumnFilter columnFilter,
                                       RowFilter rowFilter,
                                       DataLimits limits,
                                       IndexMetadata index)
        throws IOException
        {
            DecoratedKey key = metadata.decorateKey(metadata.getKeyValidator().readValue(in, DatabaseDescriptor.getMaxValueSize()));
            ClusteringIndexFilter filter = ClusteringIndexFilter.serializer.deserialize(in, version, metadata);
            return new SinglePartitionReadCommand(isDigest, digestVersion, isForThrift, metadata, nowInSec, columnFilter, rowFilter, limits, key, filter, index);
        }
    }

    
SSTableReaderListener used to collect metrics about SSTable read access. /**
     * {@code SSTableReaderListener} used to collect metrics about SSTable read access.
     */
    private static final class SSTableReadMetricsCollector implements SSTableReadsListener
    {
        
The number of SSTables that need to be merged. This counter is only updated for single partition queries
since this has been the behavior so far.
/**
         * The number of SSTables that need to be merged. This counter is only updated for single partition queries
         * since this has been the behavior so far.
         */
        private int mergedSSTables;

        @Override
        public void onSSTableSelected(SSTableReader sstable, RowIndexEntry<?> indexEntry, SelectionReason reason)
        {
            sstable.incrementReadCount();
            mergedSSTables++;
        }

        
Returns the number of SSTables that need to be merged.
Returns:
the number of SSTables that need to be merged./**
         * Returns the number of SSTables that need to be merged.
         * @return the number of SSTables that need to be merged.
         */
        public int getMergedSSTables()
        {
            return mergedSSTables;
        }
    }
}