hashsort.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * hashsort.c
 *      Sort tuples for insertion into a new hash index.
 *
 * When building a very large hash index, we pre-sort the tuples by bucket
 * number to improve locality of access to the index, and thereby avoid
 * thrashing.  We use tuplesort.c to sort the given index tuples into order.
 *
 * Note: if the number of rows in the table has been underestimated,
 * bucket splits may occur during the index build.  In that case we'd
 * be inserting into two or more buckets for each possible masked-off
 * hash code value.  That's no big problem though, since we'll still have
 * plenty of locality of access.
 *
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/hash/hashsort.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/hash.h"
#include "miscadmin.h"
#include "utils/tuplesort.h"


/*
 * Status record for spooling/sorting phase.
 */
struct HSpool
{
    Tuplesortstate *sortstate;  /* state data for tuplesort.c */
    Relation    index;

    /*
     * We sort the hash keys based on the buckets they belong to. Below masks
     * are used in _hash_hashkey2bucket to determine the bucket of given hash
     * key.
     */
    uint32      high_mask;
    uint32      low_mask;
    uint32      max_buckets;
};


/*
 * create and initialize a spool structure
 */
HSpool *
_h_spoolinit(Relation heap, Relation index, uint32 num_buckets)
{
    HSpool     *hspool = (HSpool *) palloc0(sizeof(HSpool));

    hspool->index = index;

    /*
     * Determine the bitmask for hash code values.  Since there are currently
     * num_buckets buckets in the index, the appropriate mask can be computed
     * as follows.
     *
     * NOTE : This hash mask calculation should be in sync with similar
     * calculation in _hash_init_metabuffer.
     */
    hspool->high_mask = (((uint32) 1) << _hash_log2(num_buckets + 1)) - 1;
    hspool->low_mask = (hspool->high_mask >> 1);
    hspool->max_buckets = num_buckets - 1;

    /*
     * We size the sort area as maintenance_work_mem rather than work_mem to
     * speed index creation.  This should be OK since a single backend can't
     * run multiple index creations in parallel.
     */
    hspool->sortstate = tuplesort_begin_index_hash(heap,
                                                   index,
                                                   hspool->high_mask,
                                                   hspool->low_mask,
                                                   hspool->max_buckets,
                                                   maintenance_work_mem,
                                                   false);

    return hspool;
}

/*
 * clean up a spool structure and its substructures.
 */
void
_h_spooldestroy(HSpool *hspool)
{
    tuplesort_end(hspool->sortstate);
    pfree(hspool);
}

/*
 * spool an index entry into the sort file.
 */
void
_h_spool(HSpool *hspool, ItemPointer self, Datum *values, bool *isnull)
{
    tuplesort_putindextuplevalues(hspool->sortstate, hspool->index,
                                  self, values, isnull);
}

/*
 * given a spool loaded by successive calls to _h_spool,
 * create an entire index.
 */
void
_h_indexbuild(HSpool *hspool, Relation heapRel)
{
    IndexTuple  itup;
#ifdef USE_ASSERT_CHECKING
    uint32      hashkey = 0;
#endif

    tuplesort_performsort(hspool->sortstate);

    while ((itup = tuplesort_getindextuple(hspool->sortstate, true)) != NULL)
    {
        /*
         * Technically, it isn't critical that hash keys be found in sorted
         * order, since this sorting is only used to increase locality of
         * access as a performance optimization.  It still seems like a good
         * idea to test tuplesort.c's handling of hash index tuple sorts
         * through an assertion, though.
         */
#ifdef USE_ASSERT_CHECKING
        uint32      lasthashkey = hashkey;

        hashkey = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
                                       hspool->max_buckets, hspool->high_mask,
                                       hspool->low_mask);
        Assert(hashkey >= lasthashkey);
#endif

        _hash_doinsert(hspool->index, itup, heapRel);
    }
}