postgresql/contrib/tsm_system_rows/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) 1996-2022, 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/relscan.h"
#include "access/tsmapi.h"
#include "catalog/pg_type.h"
#include "miscadmin.h"
#include "optimizer/optimizer.h"
#include "utils/sampling.h"
PG_MODULE_MAGIC;
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;
}