Separate block sampling functions

Refactoring ahead of tablesample patch

Requested and reviewed by Michael Paquier

Petr Jelinek

src/include/utils/sampling.h

@@ -0,0 +1,44 @@
+/*-------------------------------------------------------------------------
+ *
+ * sampling.h
+ *	  definitions for sampling functions
+ *
+ * Portions Copyright (c) 1996-2014, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/utils/sampling.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef SAMPLING_H
+#define SAMPLING_H
+
+#include "storage/bufmgr.h"
+
+extern double sampler_random_fract(void);
+
+/* Block sampling methods */
+/* Data structure for Algorithm S from Knuth 3.4.2 */
+typedef struct
+{
+	BlockNumber N;				/* number of blocks, known in advance */
+	int			n;				/* desired sample size */
+	BlockNumber t;				/* current block number */
+	int			m;				/* blocks selected so far */
+} BlockSamplerData;
+
+typedef BlockSamplerData *BlockSampler;
+
+extern void BlockSampler_Init(BlockSampler bs, BlockNumber nblocks,
+				  int samplesize, long randseed);
+extern bool BlockSampler_HasMore(BlockSampler bs);
+extern BlockNumber BlockSampler_Next(BlockSampler bs);
+
+/* Reservoid sampling methods */
+typedef double ReservoirStateData;
+typedef ReservoirStateData *ReservoirState;
+
+extern void reservoir_init_selection_state(ReservoirState rs, int n);
+extern double reservoir_get_next_S(ReservoirState rs, double t, int n);
+
+#endif /* SAMPLING_H */