tsm_system_rows.c

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/*-------------------------------------------------------------------------
 *
 * tsm_system_rows.c
 *    support routines for SYSTEM_ROWS tablesample method
 *
 * The desire here is to produce a random sample with a given number of rows
 * (or the whole relation, if that is fewer rows).  We use a block-sampling
 * approach.  To ensure that the whole relation will be visited if necessary,
 * we start at a randomly chosen block and then advance with a stride that
 * is randomly chosen but is relatively prime to the relation's nblocks.
 *
 * Because of the dependence on nblocks, this method cannot be repeatable
 * across queries.  (Even if the user hasn't explicitly changed the relation,
 * maintenance activities such as autovacuum might change nblocks.)  However,
 * we can at least make it repeatable across scans, by determining the
 * sampling pattern only once on the first scan.  This means that rescans
 * won't visit blocks added after the first scan, but that is fine since
 * such blocks shouldn't contain any visible tuples anyway.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    contrib/tsm_system_rows/tsm_system_rows.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/tsmapi.h"
#include "catalog/pg_type.h"
#include "miscadmin.h"
#include "optimizer/optimizer.h"
#include "utils/sampling.h"
 
PG_MODULE_MAGIC_EXT(
                    .name = "tsm_system_rows",
                    .version = PG_VERSION
);
 
PG_FUNCTION_INFO_V1(tsm_system_rows_handler);
 
 
/* Private state */
typedef struct
{
    uint32      seed;           /* random seed */
    int64       ntuples;        /* number of tuples to return */
    OffsetNumber lt;            /* last tuple returned from current block */
    BlockNumber doneblocks;     /* number of already-scanned blocks */
    BlockNumber lb;             /* last block visited */
    /* these three values are not changed during a rescan: */
    BlockNumber nblocks;        /* number of blocks in relation */
    BlockNumber firstblock;     /* first block to sample from */
    BlockNumber step;           /* step size, or 0 if not set yet */
} SystemRowsSamplerData;
 
static void system_rows_samplescangetsamplesize(PlannerInfo *root,
                                                RelOptInfo *baserel,
                                                List *paramexprs,
                                                BlockNumber *pages,
                                                double *tuples);
static void system_rows_initsamplescan(SampleScanState *node,
                                       int eflags);
static void system_rows_beginsamplescan(SampleScanState *node,
                                        Datum *params,
                                        int nparams,
                                        uint32 seed);
static BlockNumber system_rows_nextsampleblock(SampleScanState *node, BlockNumber nblocks);
static OffsetNumber system_rows_nextsampletuple(SampleScanState *node,
                                                BlockNumber blockno,
                                                OffsetNumber maxoffset);
static uint32 random_relative_prime(uint32 n, pg_prng_state *randstate);
 
 
/*
 * Create a TsmRoutine descriptor for the SYSTEM_ROWS method.
 */
Datum
tsm_system_rows_handler(PG_FUNCTION_ARGS)
{
    TsmRoutine *tsm = makeNode(TsmRoutine);
 
    tsm->parameterTypes = list_make1_oid(INT8OID);
 
    /* See notes at head of file */
    tsm->repeatable_across_queries = false;
    tsm->repeatable_across_scans = true;
 
    tsm->SampleScanGetSampleSize = system_rows_samplescangetsamplesize;
    tsm->InitSampleScan = system_rows_initsamplescan;
    tsm->BeginSampleScan = system_rows_beginsamplescan;
    tsm->NextSampleBlock = system_rows_nextsampleblock;
    tsm->NextSampleTuple = system_rows_nextsampletuple;
    tsm->EndSampleScan = NULL;
 
    PG_RETURN_POINTER(tsm);
}
 
/*
 * Sample size estimation.
 */
static void
system_rows_samplescangetsamplesize(PlannerInfo *root,
                                    RelOptInfo *baserel,
                                    List *paramexprs,
                                    BlockNumber *pages,
                                    double *tuples)
{
    Node       *limitnode;
    int64       ntuples;
    double      npages;
 
    /* Try to extract an estimate for the limit rowcount */
    limitnode = (Node *) linitial(paramexprs);
    limitnode = estimate_expression_value(root, limitnode);
 
    if (IsA(limitnode, Const) &&
        !((Const *) limitnode)->constisnull)
    {
        ntuples = DatumGetInt64(((Const *) limitnode)->constvalue);
        if (ntuples < 0)
        {
            /* Default ntuples if the value is bogus */
            ntuples = 1000;
        }
    }
    else
    {
        /* Default ntuples if we didn't obtain a non-null Const */
        ntuples = 1000;
    }
 
    /* Clamp to the estimated relation size */
    if (ntuples > baserel->tuples)
        ntuples = (int64) baserel->tuples;
    ntuples = clamp_row_est(ntuples);
 
    if (baserel->tuples > 0 && baserel->pages > 0)
    {
        /* Estimate number of pages visited based on tuple density */
        double      density = baserel->tuples / (double) baserel->pages;
 
        npages = ntuples / density;
    }
    else
    {
        /* For lack of data, assume one tuple per page */
        npages = ntuples;
    }
 
    /* Clamp to sane value */
    npages = clamp_row_est(Min((double) baserel->pages, npages));
 
    *pages = npages;
    *tuples = ntuples;
}
 
/*
 * Initialize during executor setup.
 */
static void
system_rows_initsamplescan(SampleScanState *node, int eflags)
{
    node->tsm_state = palloc0(sizeof(SystemRowsSamplerData));
    /* Note the above leaves tsm_state->step equal to zero */
}
 
/*
 * Examine parameters and prepare for a sample scan.
 */
static void
system_rows_beginsamplescan(SampleScanState *node,
                            Datum *params,
                            int nparams,
                            uint32 seed)
{
    SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
    int64       ntuples = DatumGetInt64(params[0]);
 
    if (ntuples < 0)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
                 errmsg("sample size must not be negative")));
 
    sampler->seed = seed;
    sampler->ntuples = ntuples;
    sampler->lt = InvalidOffsetNumber;
    sampler->doneblocks = 0;
    /* lb will be initialized during first NextSampleBlock call */
    /* we intentionally do not change nblocks/firstblock/step here */
 
    /*
     * We *must* use pagemode visibility checking in this module, so force
     * that even though it's currently default.
     */
    node->use_pagemode = true;
}
 
/*
 * Select next block to sample.
 *
 * Uses linear probing algorithm for picking next block.
 */
static BlockNumber
system_rows_nextsampleblock(SampleScanState *node, BlockNumber nblocks)
{
    SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
 
    /* First call within scan? */
    if (sampler->doneblocks == 0)
    {
        /* First scan within query? */
        if (sampler->step == 0)
        {
            /* Initialize now that we have scan descriptor */
            pg_prng_state randstate;
 
            /* If relation is empty, there's nothing to scan */
            if (nblocks == 0)
                return InvalidBlockNumber;
 
            /* We only need an RNG during this setup step */
            sampler_random_init_state(sampler->seed, &randstate);
 
            /* Compute nblocks/firstblock/step only once per query */
            sampler->nblocks = nblocks;
 
            /* Choose random starting block within the relation */
            /* (Actually this is the predecessor of the first block visited) */
            sampler->firstblock = sampler_random_fract(&randstate) *
                sampler->nblocks;
 
            /* Find relative prime as step size for linear probing */
            sampler->step = random_relative_prime(sampler->nblocks, &randstate);
        }
 
        /* Reinitialize lb */
        sampler->lb = sampler->firstblock;
    }
 
    /* If we've read all blocks or returned all needed tuples, we're done */
    if (++sampler->doneblocks > sampler->nblocks ||
        node->donetuples >= sampler->ntuples)
        return InvalidBlockNumber;
 
    /*
     * It's probably impossible for scan->rs_nblocks to decrease between scans
     * within a query; but just in case, loop until we select a block number
     * less than scan->rs_nblocks.  We don't care if scan->rs_nblocks has
     * increased since the first scan.
     */
    do
    {
        /* Advance lb, using uint64 arithmetic to forestall overflow */
        sampler->lb = ((uint64) sampler->lb + sampler->step) % sampler->nblocks;
    } while (sampler->lb >= nblocks);
 
    return sampler->lb;
}
 
/*
 * Select next sampled tuple in current block.
 *
 * In block sampling, we just want to sample all the tuples in each selected
 * block.
 *
 * When we reach end of the block, return InvalidOffsetNumber which tells
 * SampleScan to go to next block.
 */
static OffsetNumber
system_rows_nextsampletuple(SampleScanState *node,
                            BlockNumber blockno,
                            OffsetNumber maxoffset)
{
    SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
    OffsetNumber tupoffset = sampler->lt;
 
    /* Quit if we've returned all needed tuples */
    if (node->donetuples >= sampler->ntuples)
        return InvalidOffsetNumber;
 
    /* 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;
}
 
/*
 * Compute greatest common divisor of two uint32's.
 */
static uint32
gcd(uint32 a, uint32 b)
{
    uint32      c;
 
    while (a != 0)
    {
        c = a;
        a = b % a;
        b = c;
    }
 
    return b;
}
 
/*
 * Pick a random value less than and relatively prime to n, if possible
 * (else return 1).
 */
static uint32
random_relative_prime(uint32 n, pg_prng_state *randstate)
{
    uint32      r;
 
    /* Safety check to avoid infinite loop or zero result for small n. */
    if (n <= 1)
        return 1;
 
    /*
     * This should only take 2 or 3 iterations as the probability of 2 numbers
     * being relatively prime is ~61%; but just in case, we'll include a
     * CHECK_FOR_INTERRUPTS in the loop.
     */
    do
    {
        CHECK_FOR_INTERRUPTS();
        r = (uint32) (sampler_random_fract(randstate) * n);
    } while (r == 0 || gcd(r, n) > 1);
 
    return r;
}