Use SnapshotDirty when checking for conflicting index names.

tglsfdc · tglsfdc · commit fdd82692230a · 2025-06-20T13:41:11.000-04:00
While choosing an autogenerated name for an index, look for pre-existing relations using a SnapshotDirty snapshot, instead of the previous behavior that considered only committed-good pg_class rows. This allows us to detect and avoid conflicts against indexes that are still being built. It's still possible to fail due to a race condition, but the window is now just the amount of time that it takes DefineIndex to validate all its parameters, call smgrcreate(), and enter the index's pg_class row. Formerly the race window covered the entire time needed to create and fill an index, which could be very long if the table is large. Worse, if the conflicting index creation is part of a larger transaction, it wouldn't be visible till COMMIT. So this isn't a complete solution, but it should greatly ameliorate the problem, and the patch is simple enough to be back-patchable. It might at some point be useful to do the same for pg_constraint entries (cf. ChooseConstraintName, ConstraintNameExists, and related functions). However, in the absence of field complaints, I'll leave that alone for now. The relation-name test should be good enough for index-based constraints, while foreign-key constraints seem to be okay since they require exclusive locks to create. Bug: #18959 Reported-by: Maximilian Chrzan <maximilian.chrzan@here.com> Author: Tom Lane <tgl@sss.pgh.pa.us> Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com> Discussion: https://postgr.es/m/18959-f63b53b864bb1417@postgresql.org Backpatch-through: 13
diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
@@ -2461,7 +2461,9 @@ makeObjectName(const char *name1, const char *name2, const char *label)
  * constraint names.)
  *
  * Note: it is theoretically possible to get a collision anyway, if someone
- * else chooses the same name concurrently.  This is fairly unlikely to be
+ * else chooses the same name concurrently.  We shorten the race condition
+ * window by checking for conflicting relations using SnapshotDirty, but
+ * that doesn't close the window entirely.  This is fairly unlikely to be
  * a problem in practice, especially if one is holding an exclusive lock on
  * the relation identified by name1.  However, if choosing multiple names
  * within a single command, you'd better create the new object and do
@@ -2477,15 +2479,45 @@ ChooseRelationName(const char *name1, const char *name2,
 	int			pass = 0;
 	char	   *relname = NULL;
 	char		modlabel[NAMEDATALEN];
+	SnapshotData SnapshotDirty;
+	Relation	pgclassrel;
+
+	/* prepare to search pg_class with a dirty snapshot */
+	InitDirtySnapshot(SnapshotDirty);
+	pgclassrel = table_open(RelationRelationId, AccessShareLock);
 
 	/* try the unmodified label first */
 	strlcpy(modlabel, label, sizeof(modlabel));
 
 	for (;;)
 	{
+		ScanKeyData key[2];
+		SysScanDesc scan;
+		bool		collides;
+
 		relname = makeObjectName(name1, name2, modlabel);
 
-		if (!OidIsValid(get_relname_relid(relname, namespaceid)))
+		/* is there any conflicting relation name? */
+		ScanKeyInit(&key[0],
+					Anum_pg_class_relname,
+					BTEqualStrategyNumber, F_NAMEEQ,
+					CStringGetDatum(relname));
+		ScanKeyInit(&key[1],
+					Anum_pg_class_relnamespace,
+					BTEqualStrategyNumber, F_OIDEQ,
+					ObjectIdGetDatum(namespaceid));
+
+		scan = systable_beginscan(pgclassrel, ClassNameNspIndexId,
+								  true /* indexOK */ ,
+								  &SnapshotDirty,
+								  2, key);
+
+		collides = HeapTupleIsValid(systable_getnext(scan));
+
+		systable_endscan(scan);
+
+		/* break out of loop if no conflict */
+		if (!collides)
 		{
 			if (!isconstraint ||
 				!ConstraintNameExists(relname, namespaceid))
@@ -2497,6 +2529,8 @@ ChooseRelationName(const char *name1, const char *name2,
 		snprintf(modlabel, sizeof(modlabel), "%s%d", label, ++pass);
 	}
 
+	table_close(pgclassrel, AccessShareLock);
+
 	return relname;
 }
 
