v16-0006-Use-ExecSetTupleBound-hint-during-index-scans.patch

From 91bddbb2c246bce7de9191cb9eb8d9b35217ac9b Mon Sep 17 00:00:00 2001
From: Peter Geoghegan <pg@bowt.ie>
Date: Thu, 22 Jan 2026 13:07:13 -0500
Subject: [PATCH v16 06/17] Use ExecSetTupleBound hint during index scans.

This gives index scans a way to avoid using a read stream during certain
kinds of queries that are very unlikely to benefit from prefetching:
queries whose plan involves a LIMIT node that is consumes tuples from an
index scan (or index-only scan) node.

Testing has shown this to be particularly important with nested loop
joins with a LIMIT on an inner index scan.  This is typical of nested
loop anti-joins, and nested loop semi-joins.

XXX This is still very much a WIP.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Tomas Vondra <tomas@vondra.me>
---
 src/include/access/relscan.h             |  2 +
 src/include/nodes/execnodes.h            |  4 ++
 src/backend/access/heap/heapam_handler.c |  5 +++
 src/backend/access/index/genam.c         |  1 +
 src/backend/executor/execProcnode.c      | 50 ++++++++++++++++++++++++
 src/backend/executor/nodeIndexonlyscan.c | 10 +++++
 src/backend/executor/nodeIndexscan.c     | 13 ++++++
 7 files changed, 85 insertions(+)

diff --git a/src/include/access/relscan.h b/src/include/access/relscan.h
index 3eb41e707..4ac218921 100644
--- a/src/include/access/relscan.h
+++ b/src/include/access/relscan.h
@@ -386,6 +386,8 @@ typedef struct IndexScanDescData
 	/* parallel index scan information, in shared memory */
 	struct ParallelIndexScanDescData *parallel_scan;
 
+	int64		tuples_needed;
+
 	/*
 	 * Limit on distinct heap pages visited before giving up (0 = no limit).
 	 * Used by selfuncs.c to bound the cost of get_actual_variable_endpoint().
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 593a77108..25fda210f 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1706,6 +1706,7 @@ typedef struct
  *		ScanDesc		   index scan descriptor
  *		Instrument		   local index scan instrumentation
  *		SharedInfo		   parallel worker instrumentation (no leader entry)
+ *		TuplesNeeded	   tuple bound, see ExecSetTupleBound
  *
  *		ReorderQueue	   tuples that need reordering due to re-check
  *		ReachedEnd		   have we fetched all tuples from index already?
@@ -1734,6 +1735,7 @@ typedef struct IndexScanState
 	struct IndexScanDescData *iss_ScanDesc;
 	IndexScanInstrumentation *iss_Instrument;
 	SharedIndexScanInstrumentation *iss_SharedInfo;
+	int64		iss_TuplesNeeded;
 
 	/* These are needed for re-checking ORDER BY expr ordering */
 	pairingheap *iss_ReorderQueue;
@@ -1762,6 +1764,7 @@ typedef struct IndexScanState
  *		ScanDesc		   index scan descriptor
  *		Instrument		   local index scan instrumentation
  *		SharedInfo		   parallel worker instrumentation (no leader entry)
+ *		TuplesNeeded	   tuple bound, see ExecSetTupleBound
  *		TableSlot		   slot for holding tuples fetched from the table
  *		PscanLen		   size of parallel index-only scan descriptor
  *		NameCStringAttNums attnums of name typed columns to pad to NAMEDATALEN
@@ -1784,6 +1787,7 @@ typedef struct IndexOnlyScanState
 	struct IndexScanDescData *ioss_ScanDesc;
 	IndexScanInstrumentation *ioss_Instrument;
 	SharedIndexScanInstrumentation *ioss_SharedInfo;
+	int64		ioss_TuplesNeeded;
 	TupleTableSlot *ioss_TableSlot;
 	Size		ioss_PscanLen;
 	AttrNumber *ioss_NameCStringAttNums;
diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index 28a144d1c..bbc02b296 100644
--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -626,9 +626,14 @@ heapam_batch_getnext(IndexScanDesc scan, ScanDirection direction,
 		 * haven't done any heap fetches yet.  We don't want to waste any
 		 * cycles on allocating a read stream until we have a demonstrated
 		 * need for perform heap fetches.
+		 *
+		 * Also avoiding prefetching when the core executor passes the scan a
+		 * tuples_needed hint that indicates that the scan is likely to end
+		 * before long.
 		 */
 		if (!hscan->xs_read_stream && priorBatch && scan->MVCCScan &&
 			hscan->xs_blk != InvalidBlockNumber &&	/* for index-only scans */
+			(scan->tuples_needed == -1 || scan->tuples_needed > 20) &&
 			enable_indexscan_prefetch)
 		{
 			Assert(!batchringbuf->prefetchPos.valid);
diff --git a/src/backend/access/index/genam.c b/src/backend/access/index/genam.c
index d0b68f38e..dede16cf3 100644
--- a/src/backend/access/index/genam.c
+++ b/src/backend/access/index/genam.c
@@ -126,6 +126,7 @@ RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
 	scan->xs_itupdesc = NULL;
 	scan->xs_hitup = NULL;
 	scan->xs_hitupdesc = NULL;
+	scan->tuples_needed = -1;	/* no limit */
 	scan->xs_visited_pages_limit = 0;
 
 	scan->batch_index_opaque_size = 0;
diff --git a/src/backend/executor/execProcnode.c b/src/backend/executor/execProcnode.c
index d35976925..b548e32f7 100644
--- a/src/backend/executor/execProcnode.c
+++ b/src/backend/executor/execProcnode.c
@@ -72,6 +72,7 @@
  */
 #include "postgres.h"
 
+#include "access/relscan.h"
 #include "executor/executor.h"
 #include "executor/instrument.h"
 #include "executor/nodeAgg.h"
@@ -841,6 +842,12 @@ ExecShutdownNode_walker(PlanState *node, void *context)
  * Any negative tuples_needed value means "no limit", which should be the
  * default assumption when this is not called at all for a particular node.
  *
+ * Note: for nodes like Sort, tuples_needed is a hard limit -- the node can
+ * stop after producing exactly that many tuples.  For index scans, however,
+ * tuples_needed is only an approximation, because non-index quals may filter
+ * out some tuples.  The actual number of tuples fetched from the index may
+ * need to exceed tuples_needed to satisfy the caller's requirements.
+ *
  * Note: if this is called repeatedly on a plan tree, the exact same set
  * of nodes must be updated with the new limit each time; be careful that
  * only unchanging conditions are tested here.
@@ -978,6 +985,49 @@ ExecSetTupleBound(int64 tuples_needed, PlanState *child_node)
 
 		ExecSetTupleBound(tuples_needed, outerPlanState(child_node));
 	}
+	else if (IsA(child_node, IndexScanState))
+	{
+		/*
+		 * If it is an IndexScan, save the tuples_needed in the state so it
+		 * can be propagated to the IndexScanDesc when the scan is started.
+		 *
+		 * Note: As with Sort, the index scan node is responsible for reacting
+		 * properly to changes to this parameter.
+		 */
+		IndexScanState *isstate = (IndexScanState *) child_node;
+
+		isstate->iss_TuplesNeeded = tuples_needed;
+
+		/* If scan already started, update the IndexScanDesc too */
+		if (isstate->iss_ScanDesc)
+			isstate->iss_ScanDesc->tuples_needed = tuples_needed;
+	}
+	else if (IsA(child_node, IndexOnlyScanState))
+	{
+		/* Same comments as for IndexScan */
+		IndexOnlyScanState *iosstate = (IndexOnlyScanState *) child_node;
+
+		iosstate->ioss_TuplesNeeded = tuples_needed;
+
+		/* If scan already started, update the IndexScanDesc too */
+		if (iosstate->ioss_ScanDesc)
+			iosstate->ioss_ScanDesc->tuples_needed = tuples_needed;
+	}
+	else if (IsA(child_node, NestLoopState))
+	{
+		/*
+		 * For NestLoop joins where each outer tuple produces at most one
+		 * output tuple, we can propagate the bound to the outer child
+		 */
+		NestLoopState *nlstate = (NestLoopState *) child_node;
+		JoinType	jointype = nlstate->js.jointype;
+
+		if (jointype == JOIN_SEMI || jointype == JOIN_ANTI ||
+			nlstate->js.single_match)
+		{
+			ExecSetTupleBound(tuples_needed, outerPlanState(child_node));
+		}
+	}
 
 	/*
 	 * In principle we could descend through any plan node type that is
diff --git a/src/backend/executor/nodeIndexonlyscan.c b/src/backend/executor/nodeIndexonlyscan.c
index aeb4bbc3e..94ea92db3 100644
--- a/src/backend/executor/nodeIndexonlyscan.c
+++ b/src/backend/executor/nodeIndexonlyscan.c
@@ -97,6 +97,9 @@ IndexOnlyNext(IndexOnlyScanState *node)
 		node->ioss_ScanDesc = scandesc;
 		Assert(node->ioss_ScanDesc->xs_want_itup);
 
+		/* Pass down any tuple bound */
+		scandesc->tuples_needed = node->ioss_TuplesNeeded;
+
 		/*
 		 * If no run-time keys to calculate or they are ready, go ahead and
 		 * pass the scankeys to the index AM.
@@ -531,6 +534,7 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 	indexstate->ioss_RuntimeKeysReady = false;
 	indexstate->ioss_RuntimeKeys = NULL;
 	indexstate->ioss_NumRuntimeKeys = 0;
+	indexstate->ioss_TuplesNeeded = -1;
 
 	/*
 	 * build the index scan keys from the index qualification
@@ -707,6 +711,9 @@ ExecIndexOnlyScanInitializeDSM(IndexOnlyScanState *node,
 								 piscan);
 	Assert(node->ioss_ScanDesc->xs_want_itup);
 
+	/* Pass down any tuple bound */
+	node->ioss_ScanDesc->tuples_needed = node->ioss_TuplesNeeded;
+
 	/*
 	 * If no run-time keys to calculate or they are ready, go ahead and pass
 	 * the scankeys to the index AM.
@@ -772,6 +779,9 @@ ExecIndexOnlyScanInitializeWorker(IndexOnlyScanState *node,
 								 piscan);
 	Assert(node->ioss_ScanDesc->xs_want_itup);
 
+	/* Pass down any tuple bound */
+	node->ioss_ScanDesc->tuples_needed = node->ioss_TuplesNeeded;
+
 	/*
 	 * If no run-time keys to calculate or they are ready, go ahead and pass
 	 * the scankeys to the index AM.
diff --git a/src/backend/executor/nodeIndexscan.c b/src/backend/executor/nodeIndexscan.c
index 10d85cfd9..468f9a0ce 100644
--- a/src/backend/executor/nodeIndexscan.c
+++ b/src/backend/executor/nodeIndexscan.c
@@ -117,6 +117,9 @@ IndexNext(IndexScanState *node)
 
 		node->iss_ScanDesc = scandesc;
 
+		/* Pass down any tuple bound */
+		scandesc->tuples_needed = node->iss_TuplesNeeded;
+
 		/*
 		 * If no run-time keys to calculate or they are ready, go ahead and
 		 * pass the scankeys to the index AM.
@@ -213,6 +216,9 @@ IndexNextWithReorder(IndexScanState *node)
 
 		node->iss_ScanDesc = scandesc;
 
+		/* Pass down any tuple bound */
+		scandesc->tuples_needed = node->iss_TuplesNeeded;
+
 		/*
 		 * If no run-time keys to calculate or they are ready, go ahead and
 		 * pass the scankeys to the index AM.
@@ -989,6 +995,7 @@ ExecInitIndexScan(IndexScan *node, EState *estate, int eflags)
 	indexstate->iss_RuntimeKeysReady = false;
 	indexstate->iss_RuntimeKeys = NULL;
 	indexstate->iss_NumRuntimeKeys = 0;
+	indexstate->iss_TuplesNeeded = -1;
 
 	/*
 	 * build the index scan keys from the index qualification
@@ -1733,6 +1740,9 @@ ExecIndexScanInitializeDSM(IndexScanState *node,
 								 node->iss_NumOrderByKeys,
 								 piscan);
 
+	/* Pass down any tuple bound */
+	node->iss_ScanDesc->tuples_needed = node->iss_TuplesNeeded;
+
 	/*
 	 * If no run-time keys to calculate or they are ready, go ahead and pass
 	 * the scankeys to the index AM.
@@ -1797,6 +1807,9 @@ ExecIndexScanInitializeWorker(IndexScanState *node,
 								 node->iss_NumOrderByKeys,
 								 piscan);
 
+	/* Pass down any tuple bound */
+	node->iss_ScanDesc->tuples_needed = node->iss_TuplesNeeded;
+
 	/*
 	 * If no run-time keys to calculate or they are ready, go ahead and pass
 	 * the scankeys to the index AM.
-- 
2.53.0