sampling.c

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/*-------------------------------------------------------------------------
 *
 * sampling.c
 *    Relation block sampling routines.
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/utils/misc/sampling.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include <math.h>
 
#include "utils/sampling.h"
 
 
/*
 * BlockSampler_Init -- prepare for random sampling of blocknumbers
 *
 * BlockSampler provides algorithm for block level sampling of a relation
 * as discussed on pgsql-hackers 2004-04-02 (subject "Large DB")
 * It selects a random sample of samplesize blocks out of
 * the nblocks blocks in the table. If the table has less than
 * samplesize blocks, all blocks are selected.
 *
 * Since we know the total number of blocks in advance, we can use the
 * straightforward Algorithm S from Knuth 3.4.2, rather than Vitter's
 * algorithm.
 *
 * Returns the number of blocks that BlockSampler_Next will return.
 */
BlockNumber
BlockSampler_Init(BlockSampler bs, BlockNumber nblocks, int samplesize,
                  uint32 randseed)
{
    bs->N = nblocks;            /* measured table size */
 
    /*
     * If we decide to reduce samplesize for tables that have less or not much
     * more than samplesize blocks, here is the place to do it.
     */
    bs->n = samplesize;
    bs->t = 0;                  /* blocks scanned so far */
    bs->m = 0;                  /* blocks selected so far */
 
    sampler_random_init_state(randseed, &bs->randstate);
 
    return Min(bs->n, bs->N);
}
 
bool
BlockSampler_HasMore(BlockSampler bs)
{
    return (bs->t < bs->N) && (bs->m < bs->n);
}
 
BlockNumber
BlockSampler_Next(BlockSampler bs)
{
    BlockNumber K = bs->N - bs->t;  /* remaining blocks */
    int         k = bs->n - bs->m;  /* blocks still to sample */
    double      p;              /* probability to skip block */
    double      V;              /* random */
 
    Assert(BlockSampler_HasMore(bs));   /* hence K > 0 and k > 0 */
 
    if ((BlockNumber) k >= K)
    {
        /* need all the rest */
        bs->m++;
        return bs->t++;
    }
 
    /*----------
     * It is not obvious that this code matches Knuth's Algorithm S.
     * Knuth says to skip the current block with probability 1 - k/K.
     * If we are to skip, we should advance t (hence decrease K), and
     * repeat the same probabilistic test for the next block.  The naive
     * implementation thus requires a sampler_random_fract() call for each
     * block number.  But we can reduce this to one sampler_random_fract()
     * call per selected block, by noting that each time the while-test
     * succeeds, we can reinterpret V as a uniform random number in the range
     * 0 to p. Therefore, instead of choosing a new V, we just adjust p to be
     * the appropriate fraction of its former value, and our next loop
     * makes the appropriate probabilistic test.
     *
     * We have initially K > k > 0.  If the loop reduces K to equal k,
     * the next while-test must fail since p will become exactly zero
     * (we assume there will not be roundoff error in the division).
     * (Note: Knuth suggests a "<=" loop condition, but we use "<" just
     * to be doubly sure about roundoff error.)  Therefore K cannot become
     * less than k, which means that we cannot fail to select enough blocks.
     *----------
     */
    V = sampler_random_fract(&bs->randstate);
    p = 1.0 - (double) k / (double) K;
    while (V < p)
    {
        /* skip */
        bs->t++;
        K--;                    /* keep K == N - t */
 
        /* adjust p to be new cutoff point in reduced range */
        p *= 1.0 - (double) k / (double) K;
    }
 
    /* select */
    bs->m++;
    return bs->t++;
}
 
/*
 * These two routines embody Algorithm Z from "Random sampling with a
 * reservoir" by Jeffrey S. Vitter, in ACM Trans. Math. Softw. 11, 1
 * (Mar. 1985), Pages 37-57.  Vitter describes his algorithm in terms
 * of the count S of records to skip before processing another record.
 * It is computed primarily based on t, the number of records already read.
 * The only extra state needed between calls is W, a random state variable.
 *
 * reservoir_init_selection_state computes the initial W value.
 *
 * Given that we've already read t records (t >= n), reservoir_get_next_S
 * determines the number of records to skip before the next record is
 * processed.
 */
void
reservoir_init_selection_state(ReservoirState rs, int n)
{
    /*
     * Reservoir sampling is not used anywhere where it would need to return
     * repeatable results so we can initialize it randomly.
     */
    sampler_random_init_state(pg_prng_uint32(&pg_global_prng_state),
                              &rs->randstate);
 
    /* Initial value of W (for use when Algorithm Z is first applied) */
    rs->W = exp(-log(sampler_random_fract(&rs->randstate)) / n);
}
 
double
reservoir_get_next_S(ReservoirState rs, double t, int n)
{
    double      S;
 
    /* The magic constant here is T from Vitter's paper */
    if (t <= (22.0 * n))
    {
        /* Process records using Algorithm X until t is large enough */
        double      V,
                    quot;
 
        V = sampler_random_fract(&rs->randstate);   /* Generate V */
        S = 0;
        t += 1;
        /* Note: "num" in Vitter's code is always equal to t - n */
        quot = (t - (double) n) / t;
        /* Find min S satisfying (4.1) */
        while (quot > V)
        {
            S += 1;
            t += 1;
            quot *= (t - (double) n) / t;
        }
    }
    else
    {
        /* Now apply Algorithm Z */
        double      W = rs->W;
        double      term = t - (double) n + 1;
 
        for (;;)
        {
            double      numer,
                        numer_lim,
                        denom;
            double      U,
                        X,
                        lhs,
                        rhs,
                        y,
                        tmp;
 
            /* Generate U and X */
            U = sampler_random_fract(&rs->randstate);
            X = t * (W - 1.0);
            S = floor(X);       /* S is tentatively set to floor(X) */
            /* Test if U <= h(S)/cg(X) in the manner of (6.3) */
            tmp = (t + 1) / term;
            lhs = exp(log(((U * tmp * tmp) * (term + S)) / (t + X)) / n);
            rhs = (((t + X) / (term + S)) * term) / t;
            if (lhs <= rhs)
            {
                W = rhs / lhs;
                break;
            }
            /* Test if U <= f(S)/cg(X) */
            y = (((U * (t + 1)) / term) * (t + S + 1)) / (t + X);
            if ((double) n < S)
            {
                denom = t;
                numer_lim = term + S;
            }
            else
            {
                denom = t - (double) n + S;
                numer_lim = t + 1;
            }
            for (numer = t + S; numer >= numer_lim; numer -= 1)
            {
                y *= numer / denom;
                denom -= 1;
            }
            W = exp(-log(sampler_random_fract(&rs->randstate)) / n);    /* Generate W in advance */
            if (exp(log(y) / n) <= (t + X) / t)
                break;
        }
        rs->W = W;
    }
    return S;
}
 
 
/*----------
 * Random number generator used by sampling
 *----------
 */
void
sampler_random_init_state(uint32 seed, pg_prng_state *randstate)
{
    pg_prng_seed(randstate, (uint64) seed);
}
 
/* Select a random value R uniformly distributed in (0 - 1) */
double
sampler_random_fract(pg_prng_state *randstate)
{
    double      res;
 
    /* pg_prng_double returns a value in [0.0 - 1.0), so we must reject 0.0 */
    do
    {
        res = pg_prng_double(randstate);
    } while (unlikely(res == 0.0));
    return res;
}
 
 
/*
 * Backwards-compatible API for block sampling
 *
 * This code is now deprecated, but since it's still in use by many FDWs,
 * we should keep it for awhile at least.  The functionality is the same as
 * sampler_random_fract/reservoir_init_selection_state/reservoir_get_next_S,
 * except that a common random state is used across all callers.
 */
static ReservoirStateData oldrs;
static bool oldrs_initialized = false;
 
double
anl_random_fract(void)
{
    /* initialize if first time through */
    if (unlikely(!oldrs_initialized))
    {
        sampler_random_init_state(pg_prng_uint32(&pg_global_prng_state),
                                  &oldrs.randstate);
        oldrs_initialized = true;
    }
 
    /* and compute a random fraction */
    return sampler_random_fract(&oldrs.randstate);
}
 
double
anl_init_selection_state(int n)
{
    /* initialize if first time through */
    if (unlikely(!oldrs_initialized))
    {
        sampler_random_init_state(pg_prng_uint32(&pg_global_prng_state),
                                  &oldrs.randstate);
        oldrs_initialized = true;
    }
 
    /* Initial value of W (for use when Algorithm Z is first applied) */
    return exp(-log(sampler_random_fract(&oldrs.randstate)) / n);
}
 
double
anl_get_next_S(double t, int n, double *stateptr)
{
    double      result;
 
    oldrs.W = *stateptr;
    result = reservoir_get_next_S(&oldrs, t, n);
    *stateptr = oldrs.W;
    return result;
}