From: Xiaoxuan Li <xioxuan@amazon.com>
Date: Fri, 13 Nov 2026 15:34:12 -0800
Subject: [PATCH v1] Fix auto_explain crash for EXPLAIN without ANALYZE on index scans

Commit f026fbf059f turned the IndexScanInstrumentation of index scan,
index-only scan and bitmap index scan nodes into a pointer that is
allocated only when the node is going to be run.  For a plan started
with EXEC_FLAG_EXPLAIN_ONLY it stays NULL, but show_indexscan_info()
dereferenced it whenever es->analyze was set.

EXPLAIN never sets es->analyze for such a plan, but auto_explain does.
EXPLAIN (BUFFERS) without ANALYZE puts INSTRUMENT_BUFFERS into the
query's instrument options, and with auto_explain.log_analyze enabled
auto_explain then prints the plan with es->analyze set, which crashed
the backend.

Fix by treating a missing IndexScanInstrumentation as zero counts.
That is what was shown before f026fbf059f, and what is shown for any
other node that was never executed.

Author: Xiaoxuan Li <xioxuan@amazon.com>
Backpatch-through: 19
---
 contrib/auto_explain/t/001_auto_explain.pl |   51 ++++++++++++++++++++++++++++
 src/backend/commands/explain.c             |   23 ++++++++++---
 2 files changed, 69 insertions(+), 5 deletions(-)

diff --git a/contrib/auto_explain/t/001_auto_explain.pl b/contrib/auto_explain/t/001_auto_explain.pl
index f32a5e84f65..0df5202a484 100644
--- a/contrib/auto_explain/t/001_auto_explain.pl
+++ b/contrib/auto_explain/t/001_auto_explain.pl
@@ -135,6 +135,57 @@ unlike(
 	qr/Query Identifier:/,
 	"query identifier not logged with compute_query_id=regress, text mode");
 
+# EXPLAIN (BUFFERS) without ANALYZE.  The plan is started but never run, yet
+# with log_analyze enabled it is logged as if it had been analyzed (BUFFERS
+# is what requests instrumentation; a plain EXPLAIN is logged bare).  Check
+# that each kind of index scan copes with that.
+$log_contents = query_log($node,
+	q{EXPLAIN (BUFFERS) SELECT * FROM pg_proc WHERE proname = 'int4pl';});
+
+like(
+	$log_contents,
+	qr/Index Scan using pg_proc_proname_args_nsp_index on pg_proc .*\(never executed\)/,
+	"index scan logged as never executed, text mode");
+
+like(
+	$log_contents,
+	qr/Index Searches: 0/,
+	"index searches logged for index scan, text mode");
+
+$log_contents = query_log($node,
+	q{EXPLAIN (BUFFERS) SELECT proname FROM pg_proc WHERE proname = 'int4pl';}
+);
+
+like(
+	$log_contents,
+	qr/Index Only Scan using pg_proc_proname_args_nsp_index on pg_proc .*\(never executed\)/,
+	"index-only scan logged as never executed, text mode");
+
+like(
+	$log_contents,
+	qr/Heap Fetches: 0/,
+	"heap fetches logged for index-only scan, text mode");
+
+like(
+	$log_contents,
+	qr/Index Searches: 0/,
+	"index searches logged for index-only scan, text mode");
+
+$log_contents = query_log(
+	$node,
+	q{EXPLAIN (BUFFERS) SELECT * FROM pg_proc WHERE proname = 'int4pl';},
+	{ "enable_indexscan" => "off" });
+
+like(
+	$log_contents,
+	qr/Bitmap Index Scan on pg_proc_proname_args_nsp_index .*\(never executed\)/,
+	"bitmap index scan logged as never executed, text mode");
+
+like(
+	$log_contents,
+	qr/Index Searches: 0/,
+	"index searches logged for bitmap index scan, text mode");
+
 # JSON format.
 $log_contents = query_log(
 	$node,
diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index b8d1d0ec751..52e631ca96a 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -3982,14 +3982,23 @@ show_indexscan_info(PlanState *planstate, ExplainState *es)
 	if (!es->analyze)
 		return;
 
-	/* Initialize counters with stats from the local process first */
+	/*
+	 * Initialize counters with stats from the local process first.
+	 *
+	 * The node's instrumentation is not allocated when the plan was started
+	 * with EXEC_FLAG_EXPLAIN_ONLY.  We can still get here in that case: for
+	 * EXPLAIN (BUFFERS) without ANALYZE, auto_explain prints the plan with
+	 * es->analyze set.  Report zero counts then, like for any other node that
+	 * was never executed.
+	 */
 	switch (nodeTag(plan))
 	{
 		case T_IndexScan:
 			{
 				IndexScanState *indexstate = ((IndexScanState *) planstate);
 
-				nsearches = indexstate->iss_Instrument->nsearches;
+				if (indexstate->iss_Instrument)
+					nsearches = indexstate->iss_Instrument->nsearches;
 				SharedInfo = indexstate->iss_SharedInfo;
 				break;
 			}
@@ -3997,8 +4006,11 @@ show_indexscan_info(PlanState *planstate, ExplainState *es)
 			{
 				IndexOnlyScanState *indexstate = ((IndexOnlyScanState *) planstate);
 
-				nsearches = indexstate->ioss_Instrument->nsearches;
-				ntabletuplefetches = indexstate->ioss_Instrument->ntabletuplefetches;
+				if (indexstate->ioss_Instrument)
+				{
+					nsearches = indexstate->ioss_Instrument->nsearches;
+					ntabletuplefetches = indexstate->ioss_Instrument->ntabletuplefetches;
+				}
 				SharedInfo = indexstate->ioss_SharedInfo;
 				break;
 			}
@@ -4006,7 +4018,8 @@ show_indexscan_info(PlanState *planstate, ExplainState *es)
 			{
 				BitmapIndexScanState *indexstate = ((BitmapIndexScanState *) planstate);
 
-				nsearches = indexstate->biss_Instrument->nsearches;
+				if (indexstate->biss_Instrument)
+					nsearches = indexstate->biss_Instrument->nsearches;
 				SharedInfo = indexstate->biss_SharedInfo;
 				break;
 			}
-- 
2.54.0 (Apple Git-157)