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

1 files changed, 4 insertions, 7 deletions

diff --git a/src/backend/access/tablesample/system.c b/src/backend/access/tablesample/system.c
index 26f7de3e45d..b1fb9b32015 100644
--- a/src/backend/access/tablesample/system.c
+++ b/src/backend/access/tablesample/system.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"
@@ -56,7 +55,7 @@ static void system_beginsamplescan(SampleScanState *node,
 					   Datum *params,
 					   int nparams,
 					   uint32 seed);
-static BlockNumber system_nextsampleblock(SampleScanState *node);
+static BlockNumber system_nextsampleblock(SampleScanState *node, BlockNumber nblocks);
 static OffsetNumber system_nextsampletuple(SampleScanState *node,
 					   BlockNumber blockno,
 					   OffsetNumber maxoffset);
@@ -177,11 +176,9 @@ system_beginsamplescan(SampleScanState *node,
  * Select next block to sample.
  */
 static BlockNumber
-system_nextsampleblock(SampleScanState *node)
+system_nextsampleblock(SampleScanState *node, BlockNumber nblocks)
 {
 	SystemSamplerData *sampler = (SystemSamplerData *) node->tsm_state;
-	TableScanDesc scan = node->ss.ss_currentScanDesc;
-	HeapScanDesc hscan = (HeapScanDesc) scan;
 	BlockNumber nextblock = sampler->nextblock;
 	uint32		hashinput[2];
 
@@ -200,7 +197,7 @@ system_nextsampleblock(SampleScanState *node)
 	 * Loop over block numbers until finding suitable block or reaching end of
 	 * relation.
 	 */
-	for (; nextblock < hscan->rs_nblocks; nextblock++)
+	for (; nextblock < nblocks; nextblock++)
 	{
 		uint32		hash;
 
@@ -212,7 +209,7 @@ system_nextsampleblock(SampleScanState *node)
 			break;
 	}
 
-	if (nextblock < hscan->rs_nblocks)
+	if (nextblock < nblocks)
 	{
 		/* Found a suitable block; remember where we should start next time */
 		sampler->nextblock = nextblock + 1;