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diff --git a/src/include/utils/sampling.h b/src/include/utils/sampling.h
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+++ b/src/include/utils/sampling.h
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+/*-------------------------------------------------------------------------
+ *
+ * sampling.h
+ *	  definitions for sampling functions
+ *
+ * Portions Copyright (c) 1996-2020, 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/block.h"		/* for typedef BlockNumber */
+
+
+/* Random generator for sampling code */
+typedef unsigned short SamplerRandomState[3];
+
+extern void sampler_random_init_state(long seed,
+									  SamplerRandomState randstate);
+extern double sampler_random_fract(SamplerRandomState randstate);
+
+/* 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 */
+	SamplerRandomState randstate;	/* random generator state */
+} BlockSamplerData;
+
+typedef BlockSamplerData *BlockSampler;
+
+extern BlockNumber BlockSampler_Init(BlockSampler bs, BlockNumber nblocks,
+									 int samplesize, long randseed);
+extern bool BlockSampler_HasMore(BlockSampler bs);
+extern BlockNumber BlockSampler_Next(BlockSampler bs);
+
+/* Reservoir sampling methods */
+
+typedef struct
+{
+	double		W;
+	SamplerRandomState randstate;	/* random generator state */
+} 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);
+
+/* Old API, still in use by assorted FDWs */
+/* For backwards compatibility, these declarations are duplicated in vacuum.h */
+
+extern double anl_random_fract(void);
+extern double anl_init_selection_state(int n);
+extern double anl_get_next_S(double t, int n, double *stateptr);
+
+#endif							/* SAMPLING_H */