Fix thinko in construction of old_conpfeqop list.

tglsfdc · tglsfdc · commit 67bd6adcb64b · 2019-07-16T18:17:47.000-04:00
This should lappend the OIDs, not lcons them; the existing code produced a list in reversed order. This is harmless for single-key FKs or FKs where all the key columns are of the same type, which probably explains how it went unnoticed. But if those conditions are not met, ATAddForeignKeyConstraint would make the wrong decision about whether an existing FK needs to be revalidated. I think it would almost always err in the safe direction by revalidating a constraint that didn't need it. You could imagine scenarios where the pfeqop check was fooled by swapping the types of two FK columns in one ALTER TABLE, but that case would probably be rejected by other tests, so it might be impossible to get to the worst-case scenario where an FK should be revalidated and isn't. (And even then, it's likely to be fine, unless there are weird inconsistencies in the equality behavior of the replacement types.) However, this is a performance bug at least. Noted while poking around to see whether lcons calls could be converted to lappend. This bug is old, dating to commit cb3a7c2, so back-patch to all supported branches.
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
@@ -6251,7 +6251,6 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,
 							new_castfunc == old_castfunc &&
 							(!IsPolymorphicType(pfeqop_right) ||
 							 new_fktype == old_fktype));
-
 		}
 
 		pfeqoperators[i] = pfeqop;
@@ -8581,7 +8580,7 @@ TryReuseForeignKey(Oid oldId, Constraint *con)
 
 	/* stash a List of the operator Oids in our Constraint node */
 	for (i = 0; i < numkeys; i++)
-		con->old_conpfeqop = lcons_oid(rawarr[i], con->old_conpfeqop);
+		con->old_conpfeqop = lappend_oid(con->old_conpfeqop, rawarr[i]);
 
 	ReleaseSysCache(tup);
 }

Original file line number	Diff line number	Diff line change
`@@ -6251,7 +6251,6 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,`
`6251`	`6251`	`new_castfunc == old_castfunc &&`
`6252`	`6252`	`(!IsPolymorphicType(pfeqop_right) \|\|`
`6253`	`6253`	`new_fktype == old_fktype));`
`6254`		`-`
`6255`	`6254`	`}`
`6256`	`6255`
`6257`	`6256`	`pfeqoperators[i] = pfeqop;`
`@@ -8581,7 +8580,7 @@ TryReuseForeignKey(Oid oldId, Constraint *con)`
`8581`	`8580`
`8582`	`8581`	`/* stash a List of the operator Oids in our Constraint node */`
`8583`	`8582`	`for (i = 0; i < numkeys; i++)`
`8584`		`- con->old_conpfeqop = lcons_oid(rawarr[i], con->old_conpfeqop);`
	`8583`	`+ con->old_conpfeqop = lappend_oid(con->old_conpfeqop, rawarr[i]);`
`8585`	`8584`
`8586`	`8585`	`ReleaseSysCache(tup);`
`8587`	`8586`	`}`