mirror of
https://git.postgresql.org/git/postgresql.git
synced 2024-08-22 13:13:24 +02:00
This moves sample scan support to below tableam. It's not optional as there is, in contrast to e.g. bitmap heap scans, no alternative way to perform tablesample queries. If an AM can't deal with the block based API, it will have to throw an ERROR. The tableam callbacks for this are block based, but given the current TsmRoutine interface, that seems to be required. The new interface doesn't require TsmRoutines to perform visibility checks anymore - that requires the TsmRoutine to know details about the AM, which we want to avoid. To continue to allow taking the returned number of tuples account SampleScanState now has a donetuples field (which previously e.g. existed in SystemRowsSamplerData), which is only incremented after the visibility check succeeds. Author: Andres Freund Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de
341 lines
9.3 KiB
C
341 lines
9.3 KiB
C
/*-------------------------------------------------------------------------
|
|
*
|
|
* 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) 1996-2019, 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, SamplerRandomState 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 */
|
|
SamplerRandomState 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, SamplerRandomState 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;
|
|
}
|