diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
new file mode 100644
index ecdff1e..49ad323
*** a/src/backend/commands/tablecmds.c
--- b/src/backend/commands/tablecmds.c
*************** ATAddForeignKeyConstraint(AlteredTableIn
*** 6016,6031 ****
  	ListCell   *old_pfeqop_item = list_head(fkconstraint->old_conpfeqop);
  
  	/*
! 	 * Grab an exclusive lock on the pk table, so that someone doesn't delete
! 	 * rows out from under us. (Although a lesser lock would do for that
! 	 * purpose, we'll need exclusive lock anyway to add triggers to the pk
! 	 * table; trying to start with a lesser lock will just create a risk of
! 	 * deadlock.)
  	 */
  	if (OidIsValid(fkconstraint->old_pktable_oid))
! 		pkrel = heap_open(fkconstraint->old_pktable_oid, AccessExclusiveLock);
  	else
! 		pkrel = heap_openrv(fkconstraint->pktable, AccessExclusiveLock);
  
  	/*
  	 * Validity checks (permission checks wait till we have the column
--- 6016,6028 ----
  	ListCell   *old_pfeqop_item = list_head(fkconstraint->old_conpfeqop);
  
  	/*
! 	 * Grab an share lock on the pk table, so that someone doesn't delete
! 	 * rows out from under us.
  	 */
  	if (OidIsValid(fkconstraint->old_pktable_oid))
! 		pkrel = heap_open(fkconstraint->old_pktable_oid, ShareLock);
  	else
! 		pkrel = heap_openrv(fkconstraint->pktable, ShareLock);
  
  	/*
  	 * Validity checks (permission checks wait till we have the column
diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
new file mode 100644
index f4c0ffa..214755d
*** a/src/backend/commands/trigger.c
--- b/src/backend/commands/trigger.c
*************** CreateTrigger(CreateTrigStmt *stmt, cons
*** 157,165 ****
  				referenced;
  
  	if (OidIsValid(relOid))
! 		rel = heap_open(relOid, AccessExclusiveLock);
  	else
! 		rel = heap_openrv(stmt->relation, AccessExclusiveLock);
  
  	/*
  	 * Triggers must be on tables or views, and there are additional
--- 157,165 ----
  				referenced;
  
  	if (OidIsValid(relOid))
! 		rel = heap_open(relOid, ShareLock);
  	else
! 		rel = heap_openrv(stmt->relation, ShareLock);
  
  	/*
  	 * Triggers must be on tables or views, and there are additional
*************** CreateTrigger(CreateTrigStmt *stmt, cons
*** 525,532 ****
  	 * can skip this for internally generated triggers, since the name
  	 * modification above should be sufficient.
  	 *
! 	 * NOTE that this is cool only because we have AccessExclusiveLock on the
! 	 * relation, so the trigger set won't be changing underneath us.
  	 */
  	if (!isInternal)
  	{
--- 525,531 ----
  	 * can skip this for internally generated triggers, since the name
  	 * modification above should be sufficient.
  	 *
! 	 * NOTE that this is cool only because of the unique contraint.
  	 */
  	if (!isInternal)
  	{
*************** RemoveTriggerById(Oid trigOid)
*** 1102,1108 ****
  	 */
  	relid = ((Form_pg_trigger) GETSTRUCT(tup))->tgrelid;
  
! 	rel = heap_open(relid, AccessExclusiveLock);
  
  	if (rel->rd_rel->relkind != RELKIND_RELATION &&
  		rel->rd_rel->relkind != RELKIND_VIEW &&
--- 1101,1107 ----
  	 */
  	relid = ((Form_pg_trigger) GETSTRUCT(tup))->tgrelid;
  
! 	rel = heap_open(relid, ShareLock);
  
  	if (rel->rd_rel->relkind != RELKIND_RELATION &&
  		rel->rd_rel->relkind != RELKIND_VIEW &&
*************** renametrig(RenameStmt *stmt)
*** 1257,1263 ****
  	 * Look up name, check permissions, and acquire lock (which we will NOT
  	 * release until end of transaction).
  	 */
! 	relid = RangeVarGetRelidExtended(stmt->relation, AccessExclusiveLock,
  									 false, false,
  									 RangeVarCallbackForRenameTrigger,
  									 NULL);
--- 1256,1262 ----
  	 * Look up name, check permissions, and acquire lock (which we will NOT
  	 * release until end of transaction).
  	 */
! 	relid = RangeVarGetRelidExtended(stmt->relation, ShareLock,
  									 false, false,
  									 RangeVarCallbackForRenameTrigger,
  									 NULL);
*************** renametrig(RenameStmt *stmt)
*** 1271,1278 ****
  	 * on tgrelid/tgname would complain anyway) and to ensure a trigger does
  	 * exist with oldname.
  	 *
! 	 * NOTE that this is cool only because we have AccessExclusiveLock on the
! 	 * relation, so the trigger set won't be changing underneath us.
  	 */
  	tgrel = heap_open(TriggerRelationId, RowExclusiveLock);
  
--- 1270,1276 ----
  	 * on tgrelid/tgname would complain anyway) and to ensure a trigger does
  	 * exist with oldname.
  	 *
! 	 * NOTE that this is cool only because there is a unique constraint.
  	 */
  	tgrel = heap_open(TriggerRelationId, RowExclusiveLock);
  
diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
new file mode 100644
index 10f45f2..636b951
*** a/src/test/regress/expected/alter_table.out
--- b/src/test/regress/expected/alter_table.out
*************** create trigger ttdummy
*** 1935,1943 ****
  	execute procedure
  	ttdummy (1, 1);
  select * from my_locks order by 1;
!   relname  |    max_lockmode     
! -----------+---------------------
!  alterlock | AccessExclusiveLock
  (1 row)
  
  rollback;
--- 1935,1943 ----
  	execute procedure
  	ttdummy (1, 1);
  select * from my_locks order by 1;
!   relname  | max_lockmode 
! -----------+--------------
!  alterlock | ShareLock
  (1 row)
  
  rollback;
*************** alter table alterlock2 add foreign key (
*** 1951,1957 ****
  select * from my_locks order by 1;
       relname     |    max_lockmode     
  -----------------+---------------------
!  alterlock       | AccessExclusiveLock
   alterlock2      | AccessExclusiveLock
   alterlock2_pkey | AccessShareLock
   alterlock_pkey  | AccessShareLock
--- 1951,1957 ----
  select * from my_locks order by 1;
       relname     |    max_lockmode     
  -----------------+---------------------
!  alterlock       | ShareLock
   alterlock2      | AccessExclusiveLock
   alterlock2_pkey | AccessShareLock
   alterlock_pkey  | AccessShareLock
*************** add constraint alterlock2nv foreign key
*** 1964,1970 ****
  select * from my_locks order by 1;
    relname   |    max_lockmode     
  ------------+---------------------
!  alterlock  | AccessExclusiveLock
   alterlock2 | AccessExclusiveLock
  (2 rows)
  
--- 1964,1970 ----
  select * from my_locks order by 1;
    relname   |    max_lockmode     
  ------------+---------------------
!  alterlock  | ShareLock
   alterlock2 | AccessExclusiveLock
  (2 rows)