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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
56 lines
1.7 KiB
C
56 lines
1.7 KiB
C
/*-------------------------------------------------------------------------
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*
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* sampling.h
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* definitions for sampling functions
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*
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* Portions Copyright (c) 1996-2014, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* src/include/utils/sampling.h
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*
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*-------------------------------------------------------------------------
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*/
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#ifndef SAMPLING_H
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#define SAMPLING_H
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#include "storage/bufmgr.h"
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/* Random generator for sampling code */
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typedef unsigned short SamplerRandomState[3];
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extern void sampler_random_init_state(long seed,
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SamplerRandomState randstate);
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extern double sampler_random_fract(SamplerRandomState randstate);
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/* Block sampling methods */
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/* Data structure for Algorithm S from Knuth 3.4.2 */
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typedef struct
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{
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BlockNumber N; /* number of blocks, known in advance */
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int n; /* desired sample size */
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BlockNumber t; /* current block number */
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int m; /* blocks selected so far */
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SamplerRandomState randstate; /* random generator state */
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} BlockSamplerData;
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typedef BlockSamplerData *BlockSampler;
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extern void BlockSampler_Init(BlockSampler bs, BlockNumber nblocks,
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int samplesize, long randseed);
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extern bool BlockSampler_HasMore(BlockSampler bs);
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extern BlockNumber BlockSampler_Next(BlockSampler bs);
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/* Reservoid sampling methods */
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typedef struct
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{
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double W;
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SamplerRandomState randstate; /* random generator state */
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} ReservoirStateData;
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typedef ReservoirStateData *ReservoirState;
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extern void reservoir_init_selection_state(ReservoirState rs, int n);
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extern double reservoir_get_next_S(ReservoirState rs, double t, int n);
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#endif /* SAMPLING_H */
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