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/*-------------------------------------------------------------------------
*
* bernoulli.c
* interface routines for BERNOULLI tablesample method
*
* Portions Copyright (c) 1996-2014, PostgreSQL Global Development Group
*
* IDENTIFICATION
* src/backend/utils/tablesample/bernoulli.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "fmgr.h"
#include "access/tablesample.h"
#include "access/relscan.h"
#include "nodes/execnodes.h"
#include "nodes/relation.h"
#include "optimizer/clauses.h"
#include "storage/bufmgr.h"
#include "utils/sampling.h"
/* tsdesc */
typedef struct
{
uint32 seed; /* random seed */
BlockNumber startblock; /* starting block, we use ths for syncscan support */
BlockNumber nblocks; /* number of blocks */
BlockNumber blockno; /* current block */
float4 probability; /* probabilty that tuple will be returned (0.0-1.0) */
OffsetNumber lt; /* last tuple returned from current block */
SamplerRandomState randstate; /* random generator tsdesc */
} BernoulliSamplerData;
/*
* Initialize the state.
*/
Datum
tsm_bernoulli_init(PG_FUNCTION_ARGS)
{
TableSampleDesc *tsdesc = (TableSampleDesc *) PG_GETARG_POINTER(0);
uint32 seed = PG_GETARG_UINT32(1);
float4 percent = PG_ARGISNULL(2) ? -1 : PG_GETARG_FLOAT4(2);
HeapScanDesc scan = tsdesc->heapScan;
BernoulliSamplerData *sampler;
if (percent < 0 || percent > 100)
ereport(ERROR,
(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
errmsg("invalid sample size"),
errhint("Sample size must be numeric value between 0 and 100 (inclusive).")));
sampler = palloc0(sizeof(BernoulliSamplerData));
/* Remember initial values for reinit */
sampler->seed = seed;
sampler->startblock = scan->rs_startblock;
sampler->nblocks = scan->rs_nblocks;
sampler->blockno = InvalidBlockNumber;
sampler->probability = percent / 100;
sampler->lt = InvalidOffsetNumber;
sampler_random_init_state(sampler->seed, sampler->randstate);
tsdesc->tsmdata = (void *) sampler;
PG_RETURN_VOID();
}
/*
* Get next block number to read or InvalidBlockNumber if we are at the
* end of the relation.
*/
Datum
tsm_bernoulli_nextblock(PG_FUNCTION_ARGS)
{
TableSampleDesc *tsdesc = (TableSampleDesc *) PG_GETARG_POINTER(0);
BernoulliSamplerData *sampler =
(BernoulliSamplerData *) tsdesc->tsmdata;
/*
* Bernoulli sampling scans all blocks on the table and supports
* syncscan so loop from startblock to startblock instead of
* from 0 to nblocks.
*/
if (sampler->blockno == InvalidBlockNumber)
sampler->blockno = sampler->startblock;
else
{
sampler->blockno++;
if (sampler->blockno >= sampler->nblocks)
sampler->blockno = 0;
if (sampler->blockno == sampler->startblock)
PG_RETURN_UINT32(InvalidBlockNumber);
}
PG_RETURN_UINT32(sampler->blockno);
}
/*
* Get next tuple from current block.
*
* This method implements the main logic in bernoulli sampling.
* The algorithm simply generates new random number (in 0.0-1.0 range) and if
* it falls within user specified probability (in the same range) return the
* tuple offset.
*
* It is ok here to return tuple offset without knowing if tuple is visible
* and not check it via examinetuple. The reason for that is that we do the
* coinflip (random number generation) for every tuple in the table. Since all
* tuples have same probability of being returned the visible and invisible
* tuples will be returned in same ratio as they have in the actual table.
* This means that there is no skew towards either visible or invisible tuples
* and the number returned visible tuples to from the executor node is the
* fraction of visible tuples which was specified in input.
*
* This is faster than doing the coinflip in the examinetuple because we don't
* have to do visibility checks on uninteresting tuples.
*
* If we reach end of the block return InvalidOffsetNumber which tells
* SampleScan to go to next block.
*/
Datum
tsm_bernoulli_nexttuple(PG_FUNCTION_ARGS)
{
TableSampleDesc *tsdesc = (TableSampleDesc *) PG_GETARG_POINTER(0);
OffsetNumber maxoffset = PG_GETARG_UINT16(2);
BernoulliSamplerData *sampler =
(BernoulliSamplerData *) tsdesc->tsmdata;
OffsetNumber tupoffset = sampler->lt;
float4 probability = sampler->probability;
if (tupoffset == InvalidOffsetNumber)
tupoffset = FirstOffsetNumber;
else
tupoffset++;
/*
* Loop over tuple offsets until the random generator returns value that
* is within the probability of returning the tuple or until we reach
* end of the block.
*
* (This is our implementation of bernoulli trial)
*/
while (sampler_random_fract(sampler->randstate) > probability)
{
tupoffset++;
if (tupoffset > maxoffset)
break;
}
if (tupoffset > maxoffset)
/* Tell SampleScan that we want next block. */
tupoffset = InvalidOffsetNumber;
sampler->lt = tupoffset;
PG_RETURN_UINT16(tupoffset);
}
/*
* Cleanup method.
*/
Datum
tsm_bernoulli_end(PG_FUNCTION_ARGS)
{
TableSampleDesc *tsdesc = (TableSampleDesc *) PG_GETARG_POINTER(0);
pfree(tsdesc->tsmdata);
PG_RETURN_VOID();
}
/*
* Reset tsdesc (called by ReScan).
*/
Datum
tsm_bernoulli_reset(PG_FUNCTION_ARGS)
{
TableSampleDesc *tsdesc = (TableSampleDesc *) PG_GETARG_POINTER(0);
BernoulliSamplerData *sampler =
(BernoulliSamplerData *) tsdesc->tsmdata;
sampler->blockno = InvalidBlockNumber;
sampler->lt = InvalidOffsetNumber;
sampler_random_init_state(sampler->seed, sampler->randstate);
PG_RETURN_VOID();
}
/*
* Costing function.
*/
Datum
tsm_bernoulli_cost(PG_FUNCTION_ARGS)
{
PlannerInfo *root = (PlannerInfo *) PG_GETARG_POINTER(0);
Path *path = (Path *) PG_GETARG_POINTER(1);
RelOptInfo *baserel = (RelOptInfo *) PG_GETARG_POINTER(2);
List *args = (List *) PG_GETARG_POINTER(3);
BlockNumber *pages = (BlockNumber *) PG_GETARG_POINTER(4);
double *tuples = (double *) PG_GETARG_POINTER(5);
Node *pctnode;
float4 samplesize;
*pages = baserel->pages;
pctnode = linitial(args);
pctnode = estimate_expression_value(root, pctnode);
if (IsA(pctnode, RelabelType))
pctnode = (Node *) ((RelabelType *) pctnode)->arg;
if (IsA(pctnode, Const))
{
samplesize = DatumGetFloat4(((Const *) pctnode)->constvalue);
samplesize /= 100.0;
}
else
{
/* Default samplesize if the estimation didn't return Const. */
samplesize = 0.1f;
}
*tuples = path->rows * samplesize;
path->rows = *tuples;
PG_RETURN_VOID();
}
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