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Change FK trigger creation order to better support self-referential FKs.
When a foreign-key constraint references another column of the same table, row updates will queue both the PK's ON UPDATE action and the FK's CHECK action in the same event. The ON UPDATE action must execute first, else the CHECK will check a non-final state of the row and possibly throw an inappropriate error, as seen in bug #6268 from Roman Lytovchenko. Now, the firing order of multiple triggers for the same event is determined by the sort order of their pg_trigger.tgnames, and the auto-generated names we use for FK triggers are "RI_ConstraintTrigger_NNNN" where NNNN is the trigger OID. So most of the time the firing order is the same as creation order, and so rearranging the creation order fixes it. This patch will fail to fix the problem if the OID counter wraps around or adds a decimal digit (eg, from 99999 to 100000) while we are creating the triggers for an FK constraint. Given the small odds of that, and the low usage of self-referential FKs, we'll live with that solution in the back branches. A better fix is to change the auto-generated names for FK triggers, but it seems unwise to do that in stable branches because there may be client code that depends on the naming convention. We'll fix it that way in HEAD in a separate patch. Back-patch to all supported branches, since this bug has existed for a long time.
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‎src/backend/commands/tablecmds.c

Lines changed: 21 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -5495,13 +5495,6 @@ createForeignKeyTriggers(Relation rel, Constraint *fkconstraint,
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/* Make changes-so-far visible */
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CommandCounterIncrement();
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/*
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* Build and execute a CREATE CONSTRAINT TRIGGER statement for the CHECK
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* action for both INSERTs and UPDATEs on the referencing table.
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*/
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CreateFKCheckTrigger(myRel, fkconstraint, constraintOid, indexOid, true);
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CreateFKCheckTrigger(myRel, fkconstraint, constraintOid, indexOid, false);
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/*
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* Build and execute a CREATE CONSTRAINT TRIGGER statement for the ON
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* DELETE action on the referenced table.
@@ -5604,6 +5597,27 @@ createForeignKeyTriggers(Relation rel, Constraint *fkconstraint,
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fk_trigger->args = NIL;
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(void) CreateTrigger(fk_trigger, NULL, constraintOid, indexOid, true);
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/* Make changes-so-far visible */
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CommandCounterIncrement();
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/*
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* Build and execute CREATE CONSTRAINT TRIGGER statements for the CHECK
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* action for both INSERTs and UPDATEs on the referencing table.
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*
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* Note: for a self-referential FK (referencing and referenced tables are
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* the same), it is important that the ON UPDATE action fires before the
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* CHECK action, since both triggers will fire on the same row during an
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* UPDATE event; otherwise the CHECK trigger will be checking a non-final
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* state of the row. Because triggers fire in name order, we are
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* effectively relying on the OIDs of the triggers to sort correctly as
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* text. This will work except when the OID counter wraps around or adds
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* a digit, eg "99999" sorts after "100000". That is infrequent enough,
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* and the use of self-referential FKs is rare enough, that we live with
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* it for now. There will be a real fix in PG 9.2.
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*/
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CreateFKCheckTrigger(myRel, fkconstraint, constraintOid, indexOid, true);
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CreateFKCheckTrigger(myRel, fkconstraint, constraintOid, indexOid, false);
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}
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/*

‎src/test/regress/expected/foreign_key.out

Lines changed: 32 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1287,3 +1287,35 @@ SELECT * FROM tasks;
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(3 rows)
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COMMIT;
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--
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-- Test self-referential FK with CASCADE (bug #6268)
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--
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create temp table selfref (
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a int primary key,
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b int,
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foreign key (b) references selfref (a)
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on update cascade on delete cascade
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);
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NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "selfref_pkey" for table "selfref"
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insert into selfref (a, b)
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values
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(0, 0),
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(1, 1);
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begin;
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update selfref set a = 123 where a = 0;
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select a, b from selfref;
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a | b
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-----+-----
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1 | 1
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123 | 123
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(2 rows)
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update selfref set a = 456 where a = 123;
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select a, b from selfref;
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a | b
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-----+-----
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1 | 1
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456 | 456
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(2 rows)
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commit;

‎src/test/regress/sql/foreign_key.sql

Lines changed: 22 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -921,3 +921,25 @@ SELECT * FROM tasks;
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DELETE FROM users WHERE id = 2;
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SELECT * FROM tasks;
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COMMIT;
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--
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-- Test self-referential FK with CASCADE (bug #6268)
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--
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create temp table selfref (
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a int primary key,
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b int,
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foreign key (b) references selfref (a)
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on update cascade on delete cascade
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);
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insert into selfref (a, b)
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values
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(0, 0),
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(1, 1);
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begin;
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update selfref set a = 123 where a = 0;
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select a, b from selfref;
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update selfref set a = 456 where a = 123;
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select a, b from selfref;
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commit;

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