bernoulli.c

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/*-------------------------------------------------------------------------
 *
 * bernoulli.c
 *    support routines for BERNOULLI tablesample method
 *
 * To ensure repeatability of samples, it is necessary that selection of a
 * given tuple be history-independent; otherwise syncscanning would break
 * repeatability, to say nothing of logically-irrelevant maintenance such
 * as physical extension or shortening of the relation.
 *
 * To achieve that, we proceed by hashing each candidate TID together with
 * the active seed, and then selecting it if the hash is less than the
 * cutoff value computed from the selection probability by BeginSampleScan.
 *
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/tablesample/bernoulli.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include <math.h>
 
#include "access/tsmapi.h"
#include "catalog/pg_type.h"
#include "common/hashfn.h"
#include "optimizer/optimizer.h"
#include "utils/fmgrprotos.h"
 
 
/* Private state */
typedef struct
{
    uint64      cutoff;         /* select tuples with hash less than this */
    uint32      seed;           /* random seed */
    OffsetNumber lt;            /* last tuple returned from current block */
} BernoulliSamplerData;
 
 
static void bernoulli_samplescangetsamplesize(PlannerInfo *root,
                                              RelOptInfo *baserel,
                                              List *paramexprs,
                                              BlockNumber *pages,
                                              double *tuples);
static void bernoulli_initsamplescan(SampleScanState *node,
                                     int eflags);
static void bernoulli_beginsamplescan(SampleScanState *node,
                                      Datum *params,
                                      int nparams,
                                      uint32 seed);
static OffsetNumber bernoulli_nextsampletuple(SampleScanState *node,
                                              BlockNumber blockno,
                                              OffsetNumber maxoffset);
 
 
/*
 * Create a TsmRoutine descriptor for the BERNOULLI method.
 */
Datum
tsm_bernoulli_handler(PG_FUNCTION_ARGS)
{
    TsmRoutine *tsm = makeNode(TsmRoutine);
 
    tsm->parameterTypes = list_make1_oid(FLOAT4OID);
    tsm->repeatable_across_queries = true;
    tsm->repeatable_across_scans = true;
    tsm->SampleScanGetSampleSize = bernoulli_samplescangetsamplesize;
    tsm->InitSampleScan = bernoulli_initsamplescan;
    tsm->BeginSampleScan = bernoulli_beginsamplescan;
    tsm->NextSampleBlock = NULL;
    tsm->NextSampleTuple = bernoulli_nextsampletuple;
    tsm->EndSampleScan = NULL;
 
    PG_RETURN_POINTER(tsm);
}
 
/*
 * Sample size estimation.
 */
static void
bernoulli_samplescangetsamplesize(PlannerInfo *root,
                                  RelOptInfo *baserel,
                                  List *paramexprs,
                                  BlockNumber *pages,
                                  double *tuples)
{
    Node       *pctnode;
    float4      samplefract;
 
    /* Try to extract an estimate for the sample percentage */
    pctnode = (Node *) linitial(paramexprs);
    pctnode = estimate_expression_value(root, pctnode);
 
    if (IsA(pctnode, Const) &&
        !((Const *) pctnode)->constisnull)
    {
        samplefract = DatumGetFloat4(((Const *) pctnode)->constvalue);
        if (samplefract >= 0 && samplefract <= 100 && !isnan(samplefract))
            samplefract /= 100.0f;
        else
        {
            /* Default samplefract if the value is bogus */
            samplefract = 0.1f;
        }
    }
    else
    {
        /* Default samplefract if we didn't obtain a non-null Const */
        samplefract = 0.1f;
    }
 
    /* We'll visit all pages of the baserel */
    *pages = baserel->pages;
 
    *tuples = clamp_row_est(baserel->tuples * samplefract);
}
 
/*
 * Initialize during executor setup.
 */
static void
bernoulli_initsamplescan(SampleScanState *node, int eflags)
{
    node->tsm_state = palloc0(sizeof(BernoulliSamplerData));
}
 
/*
 * Examine parameters and prepare for a sample scan.
 */
static void
bernoulli_beginsamplescan(SampleScanState *node,
                          Datum *params,
                          int nparams,
                          uint32 seed)
{
    BernoulliSamplerData *sampler = (BernoulliSamplerData *) node->tsm_state;
    double      percent = DatumGetFloat4(params[0]);
    double      dcutoff;
 
    if (percent < 0 || percent > 100 || isnan(percent))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
                 errmsg("sample percentage must be between 0 and 100")));
 
    /*
     * The cutoff is sample probability times (PG_UINT32_MAX + 1); we have to
     * store that as a uint64, of course.  Note that this gives strictly
     * correct behavior at the limits of zero or one probability.
     */
    dcutoff = rint(((double) PG_UINT32_MAX + 1) * percent / 100);
    sampler->cutoff = (uint64) dcutoff;
    sampler->seed = seed;
    sampler->lt = InvalidOffsetNumber;
 
    /*
     * Use bulkread, since we're scanning all pages.  But pagemode visibility
     * checking is a win only at larger sampling fractions.  The 25% cutoff
     * here is based on very limited experimentation.
     */
    node->use_bulkread = true;
    node->use_pagemode = (percent >= 25);
}
 
/*
 * Select next sampled tuple in current block.
 *
 * It is OK here to return an offset without knowing if the tuple is visible
 * (or even exists).  The reason is that we do the coinflip for every tuple
 * offset in the table.  Since all tuples have the same probability of being
 * returned, it doesn't matter if we do extra coinflips for invisible tuples.
 *
 * When we reach end of the block, return InvalidOffsetNumber which tells
 * SampleScan to go to next block.
 */
static OffsetNumber
bernoulli_nextsampletuple(SampleScanState *node,
                          BlockNumber blockno,
                          OffsetNumber maxoffset)
{
    BernoulliSamplerData *sampler = (BernoulliSamplerData *) node->tsm_state;
    OffsetNumber tupoffset = sampler->lt;
    uint32      hashinput[3];
 
    /* Advance to first/next tuple in block */
    if (tupoffset == InvalidOffsetNumber)
        tupoffset = FirstOffsetNumber;
    else
        tupoffset++;
 
    /*
     * We compute the hash by applying hash_any to an array of 3 uint32's
     * containing the block, offset, and seed.  This is efficient to set up,
     * and with the current implementation of hash_any, it gives
     * machine-independent results, which is a nice property for regression
     * testing.
     *
     * These words in the hash input are the same throughout the block:
     */
    hashinput[0] = blockno;
    hashinput[2] = sampler->seed;
 
    /*
     * Loop over tuple offsets until finding suitable TID or reaching end of
     * block.
     */
    for (; tupoffset <= maxoffset; tupoffset++)
    {
        uint32      hash;
 
        hashinput[1] = tupoffset;
 
        hash = DatumGetUInt32(hash_any((const unsigned char *) hashinput,
                                       (int) sizeof(hashinput)));
        if (hash < sampler->cutoff)
            break;
    }
 
    if (tupoffset > maxoffset)
        tupoffset = InvalidOffsetNumber;
 
    sampler->lt = tupoffset;
 
    return tupoffset;
}