v1-01-fix-pgstat-sfc.patch

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index b125802b21..4eddecaa77 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -103,7 +103,7 @@
 #include "storage/lwlock.h"
 #include "storage/pg_shmem.h"
 #include "storage/shmem.h"
-#include "utils/guc.h"
+#include "utils/guc_hooks.h"
 #include "utils/memutils.h"
 #include "utils/pgstat_internal.h"
 #include "utils/timestamp.h"
@@ -1671,3 +1671,17 @@ pgstat_reset_after_failure(void)
 	/* and drop variable-numbered ones */
 	pgstat_drop_all_entries();
 }
+
+/*
+ * GUC assign_hook for stats_fetch_consistency
+ */
+void
+assign_stats_fetch_consistency(int newval, void *extra)
+{
+	/*
+	 * Must clear the snapshot when changing the consistency mode, to avoid
+	 * discrepancies between pgstat_fetch_consistency and the snapshot state.
+	 */
+	if (pgstat_fetch_consistency != newval)
+		pgstat_clear_snapshot();
+}
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 2f42cebaf6..5f90aecd47 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -4821,7 +4821,7 @@ struct config_enum ConfigureNamesEnum[] =
 		},
 		&pgstat_fetch_consistency,
 		PGSTAT_FETCH_CONSISTENCY_CACHE, stats_fetch_consistency,
-		NULL, NULL, NULL
+		NULL, assign_stats_fetch_consistency, NULL
 	},
 
 	{
diff --git a/src/include/utils/guc_hooks.h b/src/include/utils/guc_hooks.h
index a82a85c940..2ecb9fc086 100644
--- a/src/include/utils/guc_hooks.h
+++ b/src/include/utils/guc_hooks.h
@@ -121,6 +121,7 @@ extern void assign_search_path(const char *newval, void *extra);
 extern bool check_session_authorization(char **newval, void **extra, GucSource source);
 extern void assign_session_authorization(const char *newval, void *extra);
 extern void assign_session_replication_role(int newval, void *extra);
+extern void assign_stats_fetch_consistency(int newval, void *extra);
 extern bool check_ssl(bool *newval, void **extra, GucSource source);
 extern bool check_stage_log_stats(bool *newval, void **extra, GucSource source);
 extern bool check_synchronous_standby_names(char **newval, void **extra,
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 813d6d39ea..bb743ae15e 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -985,6 +985,35 @@ SELECT pg_stat_get_snapshot_timestamp();
 
 COMMIT;
 ----
+-- Changing fetch consistency mode inside transaction
+----
+BEGIN;
+SET LOCAL stats_fetch_consistency = cache;
+-- filling stats for the cache mode
+SELECT pg_stat_get_function_calls(0);
+ pg_stat_get_function_calls 
+----------------------------
+                           
+(1 row)
+
+SET LOCAL stats_fetch_consistency = snapshot;
+-- verify that there is no failure on accessing pre-existing snapshot
+SELECT pg_stat_get_function_calls(0);
+ pg_stat_get_function_calls 
+----------------------------
+                           
+(1 row)
+
+SET LOCAL stats_fetch_consistency = none;
+-- verify that snapshot is cleared on changing stats_fetch_consistency
+SELECT pg_stat_get_snapshot_timestamp();
+ pg_stat_get_snapshot_timestamp 
+--------------------------------
+ 
+(1 row)
+
+ROLLBACK;
+----
 -- pg_stat_have_stats behavior
 ----
 -- fixed-numbered stats exist
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 99a28bb79c..502d35e6eb 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -476,6 +476,21 @@ SELECT pg_stat_clear_snapshot();
 SELECT pg_stat_get_snapshot_timestamp();
 COMMIT;
 
+----
+-- Changing fetch consistency mode inside transaction
+----
+BEGIN;
+SET LOCAL stats_fetch_consistency = cache;
+-- filling stats for the cache mode
+SELECT pg_stat_get_function_calls(0);
+SET LOCAL stats_fetch_consistency = snapshot;
+-- verify that there is no failure on accessing pre-existing snapshot
+SELECT pg_stat_get_function_calls(0);
+SET LOCAL stats_fetch_consistency = none;
+-- verify that snapshot is cleared on changing stats_fetch_consistency
+SELECT pg_stat_get_snapshot_timestamp();
+ROLLBACK;
+
 ----
 -- pg_stat_have_stats behavior
 ----