[Alvaro Herrera <alvherre@2ndquadrant.com>]

As mentioned in https://postgr.es/m/20191231194759.GA24692@alvherre.pgsql
I propose to add a new column to pg_trigger, which allows us to remove a
pg_depend scan when cloning triggers when adding/attaching partitions.
(It's not that I think the scan is a performance problem, but rather
than notionally we try not to depend on pg_depend contents for this kind
of semantic derivation.)

-- 
Álvaro Herrera                            39°49'30"S 73°17'W

Attachments:

  [text/x-diff] 0001-Record-parents-of-triggers.patch (4.8K, ../20200217215641.GA29784@alvherre.pgsql/2-0001-Record-parents-of-triggers.patch)
  download | inline diff:
From 8f1be4b2c24385bbfe2ecba941545ffa84f60b47 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Mon, 17 Feb 2020 17:34:47 -0300
Subject: [PATCH] Record parents of triggers

This let us get rid of a recently introduced ugly hack (commit
1fa846f1c9af).
---
 doc/src/sgml/catalogs.sgml       |  7 +++++
 src/backend/commands/tablecmds.c | 52 ++------------------------------
 src/backend/commands/trigger.c   |  1 +
 src/include/catalog/pg_trigger.h |  1 +
 4 files changed, 11 insertions(+), 50 deletions(-)

diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index a10b66569b..0bba96ee52 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -6951,6 +6951,13 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       <entry>The table this trigger is on</entry>
      </row>
 
+     <row>
+      <entry><structfield>tgparentid</structfield></entry>
+      <entry><type>oid</type></entry>
+      <entry><literal><link linkend="catalog-pg-trigger"><structname>pg_trigger</structname></link>.oid</literal></entry>
+      <entry>Parent trigger of this trigger, if any</entry>
+     </row>
+
      <row>
       <entry><structfield>tgname</structfield></entry>
       <entry><type>name</type></entry>
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index b7c8d663fc..ae39bf1056 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -16447,54 +16447,6 @@ out:
 	MemoryContextDelete(cxt);
 }
 
-/*
- * isPartitionTrigger
- *		Subroutine for CloneRowTriggersToPartition: determine whether
- *		the given trigger has been cloned from another one.
- *
- * We use pg_depend as a proxy for this, since we don't have any direct
- * evidence.  This is an ugly hack to cope with a catalog deficiency.
- * Keep away from children.  Do not stare with naked eyes.  Do not propagate.
- */
-static bool
-isPartitionTrigger(Oid trigger_oid)
-{
-	Relation	pg_depend;
-	ScanKeyData key[2];
-	SysScanDesc scan;
-	HeapTuple	tup;
-	bool		found = false;
-
-	pg_depend = table_open(DependRelationId, AccessShareLock);
-
-	ScanKeyInit(&key[0], Anum_pg_depend_classid,
-				BTEqualStrategyNumber,
-				F_OIDEQ,
-				ObjectIdGetDatum(TriggerRelationId));
-	ScanKeyInit(&key[1], Anum_pg_depend_objid,
-				BTEqualStrategyNumber,
-				F_OIDEQ,
-				ObjectIdGetDatum(trigger_oid));
-
-	scan = systable_beginscan(pg_depend, DependDependerIndexId,
-							  true, NULL, 2, key);
-	while ((tup = systable_getnext(scan)) != NULL)
-	{
-		Form_pg_depend dep = (Form_pg_depend) GETSTRUCT(tup);
-
-		if (dep->refclassid == TriggerRelationId)
-		{
-			found = true;
-			break;
-		}
-	}
-
-	systable_endscan(scan);
-	table_close(pg_depend, AccessShareLock);
-
-	return found;
-}
-
 /*
  * CloneRowTriggersToPartition
  *		subroutine for ATExecAttachPartition/DefineRelation to create row
@@ -16541,7 +16493,7 @@ CloneRowTriggersToPartition(Relation parent, Relation partition)
 		 *
 		 * However, if our parent is a partitioned relation, there might be
 		 * internal triggers that need cloning.  In that case, we must skip
-		 * clone it if the trigger on parent depends on another trigger.
+		 * cloning it if the trigger on parent depends on another trigger.
 		 *
 		 * Note we dare not verify that the other trigger belongs to an
 		 * ancestor relation of our parent, because that creates deadlock
@@ -16549,7 +16501,7 @@ CloneRowTriggersToPartition(Relation parent, Relation partition)
 		 */
 		if (trigForm->tgisinternal &&
 			(!parent->rd_rel->relispartition ||
-			 !isPartitionTrigger(trigForm->oid)))
+			 !OidIsValid(trigForm->tgparentid)))
 			continue;
 
 		/*
diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index b9b1262e30..6e8b7223fe 100644
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -847,6 +847,7 @@ CreateTrigger(CreateTrigStmt *stmt, const char *queryString,
 
 	values[Anum_pg_trigger_oid - 1] = ObjectIdGetDatum(trigoid);
 	values[Anum_pg_trigger_tgrelid - 1] = ObjectIdGetDatum(RelationGetRelid(rel));
+	values[Anum_pg_trigger_tgparentid - 1] = ObjectIdGetDatum(parentTriggerOid);
 	values[Anum_pg_trigger_tgname - 1] = DirectFunctionCall1(namein,
 															 CStringGetDatum(trigname));
 	values[Anum_pg_trigger_tgfoid - 1] = ObjectIdGetDatum(funcoid);
diff --git a/src/include/catalog/pg_trigger.h b/src/include/catalog/pg_trigger.h
index 2f30c7c2f9..9612b9bdd6 100644
--- a/src/include/catalog/pg_trigger.h
+++ b/src/include/catalog/pg_trigger.h
@@ -35,6 +35,7 @@ CATALOG(pg_trigger,2620,TriggerRelationId)
 {
 	Oid			oid;			/* oid */
 	Oid			tgrelid;		/* relation trigger is attached to */
+	Oid			tgparentid;		/* OID of parent trigger, if any */
 	NameData	tgname;			/* trigger's name */
 	Oid			tgfoid;			/* OID of function to be called */
 	int16		tgtype;			/* BEFORE/AFTER/INSTEAD, UPDATE/DELETE/INSERT,
-- 
2.20.1