system.c

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/*-------------------------------------------------------------------------
 *
 * system.c
 *    support routines for SYSTEM 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 block number 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) 1996-2023, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/tablesample/system.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include <math.h>
 
#include "access/relscan.h"
#include "access/tsmapi.h"
#include "catalog/pg_type.h"
#include "common/hashfn.h"
#include "optimizer/optimizer.h"
#include "utils/builtins.h"
 
 
/* Private state */
typedef struct
{
    uint64      cutoff;         /* select blocks with hash less than this */
    uint32      seed;           /* random seed */
    BlockNumber nextblock;      /* next block to consider sampling */
    OffsetNumber lt;            /* last tuple returned from current block */
} SystemSamplerData;
 
 
static void system_samplescangetsamplesize(PlannerInfo *root,
                                           RelOptInfo *baserel,
                                           List *paramexprs,
                                           BlockNumber *pages,
                                           double *tuples);
static void system_initsamplescan(SampleScanState *node,
                                  int eflags);
static void system_beginsamplescan(SampleScanState *node,
                                   Datum *params,
                                   int nparams,
                                   uint32 seed);
static BlockNumber system_nextsampleblock(SampleScanState *node, BlockNumber nblocks);
static OffsetNumber system_nextsampletuple(SampleScanState *node,
                                           BlockNumber blockno,
                                           OffsetNumber maxoffset);
 
 
/*
 * Create a TsmRoutine descriptor for the SYSTEM method.
 */
Datum
tsm_system_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 = system_samplescangetsamplesize;
    tsm->InitSampleScan = system_initsamplescan;
    tsm->BeginSampleScan = system_beginsamplescan;
    tsm->NextSampleBlock = system_nextsampleblock;
    tsm->NextSampleTuple = system_nextsampletuple;
    tsm->EndSampleScan = NULL;
 
    PG_RETURN_POINTER(tsm);
}
 
/*
 * Sample size estimation.
 */
static void
system_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 a sample of the pages ... */
    *pages = clamp_row_est(baserel->pages * samplefract);
 
    /* ... and hopefully get a representative number of tuples from them */
    *tuples = clamp_row_est(baserel->tuples * samplefract);
}
 
/*
 * Initialize during executor setup.
 */
static void
system_initsamplescan(SampleScanState *node, int eflags)
{
    node->tsm_state = palloc0(sizeof(SystemSamplerData));
}
 
/*
 * Examine parameters and prepare for a sample scan.
 */
static void
system_beginsamplescan(SampleScanState *node,
                       Datum *params,
                       int nparams,
                       uint32 seed)
{
    SystemSamplerData *sampler = (SystemSamplerData *) 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->nextblock = 0;
    sampler->lt = InvalidOffsetNumber;
 
    /*
     * Bulkread buffer access strategy probably makes sense unless we're
     * scanning a very small fraction of the table.  The 1% cutoff here is a
     * guess.  We should use pagemode visibility checking, since we scan all
     * tuples on each selected page.
     */
    node->use_bulkread = (percent >= 1);
    node->use_pagemode = true;
}
 
/*
 * Select next block to sample.
 */
static BlockNumber
system_nextsampleblock(SampleScanState *node, BlockNumber nblocks)
{
    SystemSamplerData *sampler = (SystemSamplerData *) node->tsm_state;
    BlockNumber nextblock = sampler->nextblock;
    uint32      hashinput[2];
 
    /*
     * We compute the hash by applying hash_any to an array of 2 uint32's
     * containing the block number 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[1] = sampler->seed;
 
    /*
     * Loop over block numbers until finding suitable block or reaching end of
     * relation.
     */
    for (; nextblock < nblocks; nextblock++)
    {
        uint32      hash;
 
        hashinput[0] = nextblock;
 
        hash = DatumGetUInt32(hash_any((const unsigned char *) hashinput,
                                       (int) sizeof(hashinput)));
        if (hash < sampler->cutoff)
            break;
    }
 
    if (nextblock < nblocks)
    {
        /* Found a suitable block; remember where we should start next time */
        sampler->nextblock = nextblock + 1;
        return nextblock;
    }
 
    /* Done, but let's reset nextblock to 0 for safety. */
    sampler->nextblock = 0;
    return InvalidBlockNumber;
}
 
/*
 * Select next sampled tuple in current block.
 *
 * In block sampling, we just want to sample all the tuples in each selected
 * block.
 *
 * It is OK here to return an offset without knowing if the tuple is visible
 * (or even exists); nodeSamplescan.c will deal with that.
 *
 * When we reach end of the block, return InvalidOffsetNumber which tells
 * SampleScan to go to next block.
 */
static OffsetNumber
system_nextsampletuple(SampleScanState *node,
                       BlockNumber blockno,
                       OffsetNumber maxoffset)
{
    SystemSamplerData *sampler = (SystemSamplerData *) node->tsm_state;
    OffsetNumber tupoffset = sampler->lt;
 
    /* Advance to next possible offset on page */
    if (tupoffset == InvalidOffsetNumber)
        tupoffset = FirstOffsetNumber;
    else
        tupoffset++;
 
    /* Done? */
    if (tupoffset > maxoffset)
        tupoffset = InvalidOffsetNumber;
 
    sampler->lt = tupoffset;
 
    return tupoffset;
}