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

Author: tvondra

src/backend/commands/statscmds.c

@@ -734,18 +734,11 @@ void
 RemoveStatisticsById(Oid statsOid)
 {
 	Relation	relation;
+	Relation	rel;
 	HeapTuple	tup;
 	Form_pg_statistic_ext statext;
 	Oid			relid;
 
-	/*
-	 * First delete the pg_statistic_ext_data tuples holding the actual
-	 * statistical data. There might be data with/without inheritance, so
-	 * attempt deleting both.
-	 */
-	RemoveStatisticsDataById(statsOid, true);
-	RemoveStatisticsDataById(statsOid, false);
-
 	/*
 	 * Delete the pg_statistic_ext tuple.  Also send out a cache inval on the
 	 * associated table, so that dependent plans will be rebuilt.
@@ -760,12 +753,26 @@ RemoveStatisticsById(Oid statsOid)
 	statext = (Form_pg_statistic_ext) GETSTRUCT(tup);
 	relid = statext->stxrelid;
 
+	/*
+	 * Delete the pg_statistic_ext_data tuples holding the actual statistical
+	 * data. There might be data with/without inheritance, so attempt deleting
+	 * both. 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, true);
+	RemoveStatisticsDataById(statsOid, false);
+
 	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);
 }