Lock table in DROP STATISTICS

The DROP STATISTICS code failed to properly lock the table, leading to

  ERROR:  tuple concurrently deleted

when executed concurrently with ANALYZE.

Fixed by modifying RemoveStatisticsById() to acquire the same lock as
ANALYZE. This function is called only by DROP STATISTICS, as ANALYZE
calls RemoveStatisticsDataById() directly.

Reported by Justin Pryzby, fix by me. Backpatch through 12. The code was
like this since it was introduced in 10, but older releases are EOL.

Reported-by: Justin Pryzby
Reviewed-by: Tom Lane
Backpatch-through: 12

Discussion: https://postgr.es/m/ZUuk-8CfbYeq6g_u@pryzbyj2023

src/backend/commands/statscmds.c

@@ -722,20 +722,14 @@ AlterStatistics(AlterStatsStmt *stmt)
 }
 
 /*
- * Guts of statistics object deletion.
- */
-void
-RemoveStatisticsById(Oid statsOid)
+ * Delete entry in pg_statistic_ext_data catalog.
+*/
+static void
+RemoveStatisticsDataById(Oid statsOid)
 {
 	Relation	relation;
 	HeapTuple	tup;
-	Form_pg_statistic_ext statext;
-	Oid			relid;
 
-	/*
-	 * First delete the pg_statistic_ext_data tuple holding the actual
-	 * statistical data.
-	 */
 	relation = table_open(StatisticExtDataRelationId, RowExclusiveLock);
 
 	tup = SearchSysCache1(STATEXTDATASTXOID, ObjectIdGetDatum(statsOid));
@@ -748,6 +742,19 @@ RemoveStatisticsById(Oid statsOid)
 	ReleaseSysCache(tup);
 
 	table_close(relation, RowExclusiveLock);
+}
+
+/*
+ * Guts of statistics object deletion.
+ */
+void
+RemoveStatisticsById(Oid statsOid)
+{
+	Relation	relation;
+	Relation	rel;
+	HeapTuple	tup;
+	Form_pg_statistic_ext statext;
+	Oid			relid;
 
 	/*
 	 * Delete the pg_statistic_ext tuple.  Also send out a cache inval on the
@@ -763,12 +770,24 @@ RemoveStatisticsById(Oid statsOid)
 	statext = (Form_pg_statistic_ext) GETSTRUCT(tup);
 	relid = statext->stxrelid;
 
+	/*
+	 * Delete the pg_statistic_ext_data tuple holding the actual statistical
+	 * data. We lock the user table first, to prevent other processes (e.g.
+	 * DROP STATISTICS) from removing the row concurrently.
+	 */
+	rel = table_open(relid, ShareUpdateExclusiveLock);
+
+	RemoveStatisticsDataById(statsOid);
+
 	CacheInvalidateRelcacheByRelid(relid);
 
 	CatalogTupleDelete(relation, &tup->t_self);
 
 	ReleaseSysCache(tup);
 
+	/* Keep lock until the end of the transaction. */
+	table_close(rel, NoLock);
+
 	table_close(relation, RowExclusiveLock);
 }