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Replace random(), pg_erand48(), etc with a better PRNG API and algorithm.

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Standardize on xoroshiro128** as our basic PRNG algorithm, eliminating
a bunch of platform dependencies as well as fundamentally-obsolete PRNG
code.  In addition, this API replacement will ease replacing the
algorithm again in future, should that become necessary.

xoroshiro128** is a few percent slower than the drand48 family,
but it can produce full-width 64-bit random values not only 48-bit,
and it should be much more trustworthy.  It's likely to be noticeably
faster than the platform's random(), depending on which platform you
are thinking about; and we can have non-global state vectors easily,
unlike with random().  It is not cryptographically strong, but neither
are the functions it replaces.

Fabien Coelho, reviewed by Dean Rasheed, Aleksander Alekseev, and myself

Discussion: https://postgr.es/m/alpine.DEB.2.22.394.2105241211230.165418@pseudo
This commit is contained in:
Tom Lane
2021-11-28 21:32:36 -05:00
parent f44ceb46ec
commit 3804539e48
50 changed files with 543 additions and 480 deletions
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12
contrib/tsm_system_rows/tsm_system_rows.c
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@ -69,7 +69,7 @@ static BlockNumber system_rows_nextsampleblock(SampleScanState *node, BlockNumbe
static OffsetNumber system_rows_nextsampletuple(SampleScanState *node,
BlockNumber blockno,
OffsetNumber maxoffset);
static uint32 random_relative_prime(uint32 n, SamplerRandomState randstate);
static uint32 random_relative_prime(uint32 n, pg_prng_state *randstate);
/*
@ -213,25 +213,25 @@ system_rows_nextsampleblock(SampleScanState *node, BlockNumber nblocks)
if (sampler->step == 0)
{
/* Initialize now that we have scan descriptor */
SamplerRandomState randstate;
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);
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->firstblock = sampler_random_fract(&randstate) *
sampler->nblocks;
/* Find relative prime as step size for linear probing */
sampler->step = random_relative_prime(sampler->nblocks, randstate);
sampler->step = random_relative_prime(sampler->nblocks, &randstate);
}
/* Reinitialize lb */
@ -317,7 +317,7 @@ gcd(uint32 a, uint32 b)
* (else return 1).
*/
static uint32
random_relative_prime(uint32 n, SamplerRandomState randstate)
random_relative_prime(uint32 n, pg_prng_state *randstate)
{
uint32 r;