diff options
Diffstat (limited to 'src/backend/utils/misc/sampling.c')
| -rw-r--r-- | src/backend/utils/misc/sampling.c | 33 |
1 files changed, 24 insertions, 9 deletions
diff --git a/src/backend/utils/misc/sampling.c b/src/backend/utils/misc/sampling.c index 1eeabaf1588..9becc63bf82 100644 --- a/src/backend/utils/misc/sampling.c +++ b/src/backend/utils/misc/sampling.c @@ -46,6 +46,8 @@ BlockSampler_Init(BlockSampler bs, BlockNumber nblocks, int samplesize, bs->n = samplesize; bs->t = 0; /* blocks scanned so far */ bs->m = 0; /* blocks selected so far */ + + sampler_random_init_state(randseed, bs->randstate); } bool @@ -92,7 +94,7 @@ BlockSampler_Next(BlockSampler bs) * less than k, which means that we cannot fail to select enough blocks. *---------- */ - V = sampler_random_fract(); + V = sampler_random_fract(bs->randstate); p = 1.0 - (double) k / (double) K; while (V < p) { @@ -126,8 +128,14 @@ BlockSampler_Next(BlockSampler bs) 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(random(), rs->randstate); + /* Initial value of W (for use when Algorithm Z is first applied) */ - *rs = exp(-log(sampler_random_fract()) / n); + rs->W = exp(-log(sampler_random_fract(rs->randstate)) / n); } double @@ -142,7 +150,7 @@ reservoir_get_next_S(ReservoirState rs, double t, int n) double V, quot; - V = sampler_random_fract(); /* Generate V */ + V = sampler_random_fract(rs->randstate); /* Generate V */ S = 0; t += 1; /* Note: "num" in Vitter's code is always equal to t - n */ @@ -158,7 +166,7 @@ reservoir_get_next_S(ReservoirState rs, double t, int n) else { /* Now apply Algorithm Z */ - double W = *rs; + double W = rs->W; double term = t - (double) n + 1; for (;;) @@ -174,7 +182,7 @@ reservoir_get_next_S(ReservoirState rs, double t, int n) tmp; /* Generate U and X */ - U = sampler_random_fract(); + 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) */ @@ -203,11 +211,11 @@ reservoir_get_next_S(ReservoirState rs, double t, int n) y *= numer / denom; denom -= 1; } - W = exp(-log(sampler_random_fract()) / n); /* Generate W in advance */ + W = exp(-log(sampler_random_fract(rs->randstate)) / n); /* Generate W in advance */ if (exp(log(y) / n) <= (t + X) / t) break; } - *rs = W; + rs->W = W; } return S; } @@ -217,10 +225,17 @@ reservoir_get_next_S(ReservoirState rs, double t, int n) * Random number generator used by sampling *---------- */ +void +sampler_random_init_state(long seed, SamplerRandomState randstate) +{ + randstate[0] = RAND48_SEED_0; + randstate[1] = (unsigned short) seed; + randstate[2] = (unsigned short) (seed >> 16); +} /* Select a random value R uniformly distributed in (0 - 1) */ double -sampler_random_fract() +sampler_random_fract(SamplerRandomState randstate) { - return ((double) random() + 1) / ((double) MAX_RANDOM_VALUE + 2); + return pg_erand48(randstate); } |