0003-Lower-the-lock-level-for-ALTER-TABLE-.-ATTACH-DETACH.patch

From 3b28c098a89100e18c55832378d39e4ecd5a03ba Mon Sep 17 00:00:00 2001
From: Robert Haas <rhaas@postgresql.org>
Date: Thu, 20 Dec 2018 12:59:21 -0500
Subject: [PATCH 3/3] Lower the lock level for ALTER TABLE .. ATTACH/DETACH
 PARTITION.

---
 src/backend/commands/tablecmds.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index ce0c7b3153..42862d9ef5 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -3627,7 +3627,29 @@ AlterTableGetLockLevel(List *cmds)
 
 			case AT_AttachPartition:
 			case AT_DetachPartition:
-				cmd_lockmode = AccessExclusiveLock;
+				/*
+				 * We can attach or detach a partition with only
+				 * ShareUpdateExclusiveLock on the partitioned table, but at
+				 * least in the case of an ATTACH PARTITION operation, we need
+				 * a stronger lock on the partition itself and on any default
+				 * partition of the partitioned table.  If we didn't do this,
+				 * we could be in the middle of routing a tuple to a table and
+				 * at the same time its partition constraint could be changing
+				 * under us, which would possibly result in inserting a tuple
+				 * that does not satisfy the partition constraint.  Or, we
+				 * could decide to prune the table from the query while the
+				 * partition constraint is changing in such a way that the
+				 * table should no longer be pruned.
+				 *
+				 * Note that attaching or detaching a partition becomes visible
+				 * to other sessions as soon as the transaction which performed
+				 * the operation commits. We can't use the query snapshot,
+				 * which might be older, to determine which partitions are
+				 * visible to a particular query, because the tables that were
+				 * visible at that time might no longer exist, might no longer
+				 * have a matching tuple descriptor, etc.
+				 */
+				cmd_lockmode = ShareUpdateExclusiveLock;
 				break;
 
 			default:			/* oops */
-- 
2.14.3 (Apple Git-98)