tableam: sample scan.

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

Author: anarazel

contrib/tsm_system_rows/tsm_system_rows.c

@@ -28,7 +28,6 @@
 
 #include "postgres.h"
 
-#include "access/heapam.h"
 #include "access/relscan.h"
 #include "access/tsmapi.h"
 #include "catalog/pg_type.h"
@@ -46,7 +45,6 @@ typedef struct
 {
 	uint32		seed;			/* random seed */
 	int64		ntuples;		/* number of tuples to return */
-	int64		donetuples;		/* number of tuples already returned */
 	OffsetNumber lt;			/* last tuple returned from current block */
 	BlockNumber doneblocks;		/* number of already-scanned blocks */
 	BlockNumber lb;				/* last block visited */
@@ -67,11 +65,10 @@ static void system_rows_beginsamplescan(SampleScanState *node,
 							Datum *params,
 							int nparams,
 							uint32 seed);
-static BlockNumber system_rows_nextsampleblock(SampleScanState *node);
+static BlockNumber system_rows_nextsampleblock(SampleScanState *node, BlockNumber nblocks);
 static OffsetNumber system_rows_nextsampletuple(SampleScanState *node,
 							BlockNumber blockno,
 							OffsetNumber maxoffset);
-static bool SampleOffsetVisible(OffsetNumber tupoffset, HeapScanDesc scan);
 static uint32 random_relative_prime(uint32 n, SamplerRandomState randstate);
 
 
@@ -187,7 +184,6 @@ system_rows_beginsamplescan(SampleScanState *node,
 
 	sampler->seed = seed;
 	sampler->ntuples = ntuples;
-	sampler->donetuples = 0;
 	sampler->lt = InvalidOffsetNumber;
 	sampler->doneblocks = 0;
 	/* lb will be initialized during first NextSampleBlock call */
@@ -206,11 +202,9 @@ system_rows_beginsamplescan(SampleScanState *node,
  * Uses linear probing algorithm for picking next block.
  */
 static BlockNumber
-system_rows_nextsampleblock(SampleScanState *node)
+system_rows_nextsampleblock(SampleScanState *node, BlockNumber nblocks)
 {
 	SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
-	TableScanDesc scan = node->ss.ss_currentScanDesc;
-	HeapScanDesc hscan = (HeapScanDesc) scan;
 
 	/* First call within scan? */
 	if (sampler->doneblocks == 0)
@@ -222,14 +216,14 @@ system_rows_nextsampleblock(SampleScanState *node)
 			SamplerRandomState randstate;
 
 			/* If relation is empty, there's nothing to scan */
-			if (hscan->rs_nblocks == 0)
+			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 = hscan->rs_nblocks;
+			sampler->nblocks = nblocks;
 
 			/* Choose random starting block within the relation */
 			/* (Actually this is the predecessor of the first block visited) */
@@ -246,7 +240,7 @@ system_rows_nextsampleblock(SampleScanState *node)
 
 	/* If we've read all blocks or returned all needed tuples, we're done */
 	if (++sampler->doneblocks > sampler->nblocks ||
-		sampler->donetuples >= sampler->ntuples)
+		node->donetuples >= sampler->ntuples)
 		return InvalidBlockNumber;
 
 	/*
@@ -259,7 +253,7 @@ system_rows_nextsampleblock(SampleScanState *node)
 	{
 		/* Advance lb, using uint64 arithmetic to forestall overflow */
 		sampler->lb = ((uint64) sampler->lb + sampler->step) % sampler->nblocks;
-	} while (sampler->lb >= hscan->rs_nblocks);
+	} while (sampler->lb >= nblocks);
 
 	return sampler->lb;
 }
@@ -279,77 +273,27 @@ system_rows_nextsampletuple(SampleScanState *node,
 							OffsetNumber maxoffset)
 {
 	SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
-	TableScanDesc scan = node->ss.ss_currentScanDesc;
-	HeapScanDesc hscan = (HeapScanDesc) scan;
 	OffsetNumber tupoffset = sampler->lt;
 
 	/* Quit if we've returned all needed tuples */
-	if (sampler->donetuples >= sampler->ntuples)
+	if (node->donetuples >= sampler->ntuples)
 		return InvalidOffsetNumber;
 
-	/*
-	 * Because we should only count visible tuples as being returned, we need
-	 * to search for a visible tuple rather than just let the core code do it.
-	 */
-
-	/* We rely on the data accumulated in pagemode access */
-	Assert(scan->rs_pageatatime);
-	for (;;)
-	{
-		/* Advance to next possible offset on page */
-		if (tupoffset == InvalidOffsetNumber)
-			tupoffset = FirstOffsetNumber;
-		else
-			tupoffset++;
-
-		/* Done? */
-		if (tupoffset > maxoffset)
-		{
-			tupoffset = InvalidOffsetNumber;
-			break;
-		}
+	/* Advance to next possible offset on page */
+	if (tupoffset == InvalidOffsetNumber)
+		tupoffset = FirstOffsetNumber;
+	else
+		tupoffset++;
 
-		/* Found a candidate? */
-		if (SampleOffsetVisible(tupoffset, hscan))
-		{
-			sampler->donetuples++;
-			break;
-		}
-	}
+	/* Done? */
+	if (tupoffset > maxoffset)
+		tupoffset = InvalidOffsetNumber;
 
 	sampler->lt = tupoffset;
 
 	return tupoffset;
 }
 
-/*
- * Check if tuple offset is visible
- *
- * In pageatatime mode, heapgetpage() already did visibility checks,
- * so just look at the info it left in rs_vistuples[].
- */
-static bool
-SampleOffsetVisible(OffsetNumber tupoffset, HeapScanDesc scan)
-{
-	int			start = 0,
-				end = scan->rs_ntuples - 1;
-
-	while (start <= end)
-	{
-		int			mid = (start + end) / 2;
-		OffsetNumber curoffset = scan->rs_vistuples[mid];
-
-		if (tupoffset == curoffset)
-			return true;
-		else if (tupoffset < curoffset)
-			end = mid - 1;
-		else
-			start = mid + 1;
-	}
-
-	return false;
-}
-
 /*
  * Compute greatest common divisor of two uint32's.
  */

contrib/tsm_system_time/tsm_system_time.c

@@ -26,7 +26,6 @@
 
 #include <math.h>
 
-#include "access/heapam.h"
 #include "access/relscan.h"
 #include "access/tsmapi.h"
 #include "catalog/pg_type.h"
@@ -66,7 +65,7 @@ static void system_time_beginsamplescan(SampleScanState *node,
 							Datum *params,
 							int nparams,
 							uint32 seed);
-static BlockNumber system_time_nextsampleblock(SampleScanState *node);
+static BlockNumber system_time_nextsampleblock(SampleScanState *node, BlockNumber nblocks);
 static OffsetNumber system_time_nextsampletuple(SampleScanState *node,
 							BlockNumber blockno,
 							OffsetNumber maxoffset);
@@ -213,11 +212,9 @@ system_time_beginsamplescan(SampleScanState *node,
  * Uses linear probing algorithm for picking next block.
  */
 static BlockNumber
-system_time_nextsampleblock(SampleScanState *node)
+system_time_nextsampleblock(SampleScanState *node, BlockNumber nblocks)
 {
 	SystemTimeSamplerData *sampler = (SystemTimeSamplerData *) node->tsm_state;
-	TableScanDesc scan = node->ss.ss_currentScanDesc;
-	HeapScanDesc hscan = (HeapScanDesc) scan;
 	instr_time	cur_time;
 
 	/* First call within scan? */
@@ -230,14 +227,14 @@ system_time_nextsampleblock(SampleScanState *node)
 			SamplerRandomState randstate;
 
 			/* If relation is empty, there's nothing to scan */
-			if (hscan->rs_nblocks == 0)
+			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 = hscan->rs_nblocks;
+			sampler->nblocks = nblocks;
 
 			/* Choose random starting block within the relation */
 			/* (Actually this is the predecessor of the first block visited) */
@@ -273,7 +270,7 @@ system_time_nextsampleblock(SampleScanState *node)
 	{
 		/* Advance lb, using uint64 arithmetic to forestall overflow */
 		sampler->lb = ((uint64) sampler->lb + sampler->step) % sampler->nblocks;
-	} while (sampler->lb >= hscan->rs_nblocks);
+	} while (sampler->lb >= nblocks);
 
 	return sampler->lb;
 }

doc/src/sgml/tablesample-method.sgml

@@ -227,7 +227,7 @@ BeginSampleScan (SampleScanState *node,
   <para>
 <programlisting>
 BlockNumber
-NextSampleBlock (SampleScanState *node);
+NextSampleBlock (SampleScanState *node, BlockNumber nblocks);
 </programlisting>
 
    Returns the block number of the next page to be scanned, or
@@ -262,10 +262,9 @@ NextSampleTuple (SampleScanState *node,
     numbers in the range <literal>1 .. maxoffset</literal> actually contain valid
     tuples.  This is not normally a problem since the core code ignores
     requests to sample missing or invisible tuples; that should not result in
-    any bias in the sample.  However, if necessary, the function can
-    examine <literal>node-&gt;ss.ss_currentScanDesc-&gt;rs_vistuples[]</literal>
-    to identify which tuples are valid and visible.  (This
-    requires <literal>node-&gt;use_pagemode</literal> to be <literal>true</literal>.)
+    any bias in the sample.  However, if necessary, the function can use
+    <literal>node-&gt;donetuples</literal> to examine how many of the tuples
+    it returned were vlaid and visible.
    </para>
   </note>