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TABLESAMPLE, SQL Standard and extensible

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Add a TABLESAMPLE clause to SELECT statements that allows
user to specify random BERNOULLI sampling or block level
SYSTEM sampling. Implementation allows for extensible
sampling functions to be written, using a standard API.
Basic version follows SQLStandard exactly. Usable
concrete use cases for the sampling API follow in later
commits.

Petr Jelinek

Reviewed by Michael Paquier and Simon Riggs
This commit is contained in:
Simon Riggs
2015-05-15 14:37:10 -04:00
parent 11a83bbedd
commit f6d208d6e5
66 changed files with 2756 additions and 40 deletions
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33
src/backend/utils/misc/sampling.c
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@@ -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);
}