On 05/01/2026 19:01, Álvaro Herrera wrote:
On 2025-Dec-22, Heikki Linnakangas wrote:
@@ -146,7 +146,11 @@ find_inheritance_children_extended(Oid parentrelId, bool
omit_detached,
Snapshot snap;
xmin = HeapTupleHeaderGetXmin(inheritsTuple->t_data);
+
+ /* XXX: what should we do with a non-MVCC
snapshot? */
snap = GetActiveSnapshot();
+ if (snap->snapshot_type != SNAPSHOT_MVCC)
+ elog(ERROR, "cannot look up partition
information with a non-MVCC snapshot");
if (!XidInMVCCSnapshot(xmin, snap))
{
I think this code should be skipped when a non-MVCC snapshot is in use.
I assume you won't hit the case unless you test for it specifically, so
I'm not surprised that just adding an elog(ERROR) does not cause visible
failures; however in production it probably would. And looking up
partition info in that case is, most likely, useless. So changing the
'if' test
if (!XidInMVCCSnapshot(xmin, snap))
to only do this dance if "->snapshot_type == SNAPSHOT_MVCC" seems
reasonable to me.
Yeah, I suppose the most reasonable behavior is to treat a non-MVCC
snapshot the same as no active snapshot at all here. Although I'm not
convinced that this is reachable in any sensible scenario (see below).
I'm not sure what's a good way to verify this, however. We would need a
test that adds partitions pending detach to partitioned tables used by
as many other tests as possible ... (Maybe: have test_setup.sql add an
event trigger that runs on CREATE TABLE, detects PARTITION BY, and
creates and partially detaches a partition. Not sure how feasible this
is.)
None of the built-in code pushes a historic snapshot to the active
stack, so that won't find anything. The only known instance of that is
pglogical's row filters. Is it sane for a row filter, or other
hypothetical expression that is executed during logical decoding, to
access a partitioned table?
We don't allow marking a partitioned table as a "user catalog table":
postgres=# CREATE TABLE parttab (i int) PARTITION BY RANGE (i);
CREATE TABLE
postgres=# ALTER TABLE parttab SET (user_catalog_table = true);
ERROR: cannot specify storage parameters for a partitioned table
HINT: Specify storage parameters for its leaf partitions instead.
And it's not sane to access tables other than catalog tables with a
historic snapshot. So I'm not sure this reachable in any sensible scenario.
- Heikki
