Re: Proposal: Conflict log history table for Logical Replication

On Thu, Jun 4, 2026 at 4:05 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> I noticed that it is currently possible to acquire explicit locks on a CLT:
>
> --Session locks table and does not commit txn:
> postgres=# BEGIN;
> LOCK TABLE pg_conflict.pg_conflict_log_16481 IN SHARE MODE;
> BEGIN
> LOCK TABLE
>
> Doing so can cause the apply worker to block indefinitely when it
> attempts to modify the CLT:
>
> [247433] LOG: logical replication apply worker for subscription
> "sub1" has started
> [247433] LOG: process 247433 still waiting for RowExclusiveLock on
> relation 16482 of database 5 after 1001.030 ms
> [247433] DETAIL: Process holding the lock: 245584. Wait queue: 247433.
> [247433] CONTEXT: waiting for RowExclusiveLock on relation 16482 of database 5
>
> Toast Table behaviour:
> postgres=*# LOCK TABLE pg_toast.pg_toast_16384 IN SHARE MODE;
> ERROR: cannot lock relation "pg_toast_16384"
> DETAIL: This operation is not supported for TOAST tables.
>
> Should we consider disallowing explicit LOCK TABLE operations on CLTs,
> similar to how PostgreSQL handles TOAST tables? Or does anyone see any
> legitimate use-case (I don't) where we would need to allow explicit
> LOCKs on CLT?

We need to add namespace-based checks here, as the current logic
relies solely on relkind [1], which classifies TOAST tables
separately. In my view, choosing to either allow or disallow this
behavior will not cause significant inconvenience or seem unusual to
anyone. Therefore, I prefer the path that minimizes special-purpose
code. Since explicitly disallowing this requires additional
special-purpose logic (as shown below [1]), allowing it seems to be
the cleaner approach. Thoughts?

[1]
@@ -92,6 +93,14 @@ RangeVarCallbackForLockTable(const RangeVar *rv,
Oid relid, Oid oldrelid,

rv->relname),

errdetail_relkind_not_supported(relkind)));

+ /* Disallow explicit LOCK TABLE on conflict log tables */
+ if (IsConflictLogTableNamespace(get_rel_namespace(relid)))
+ ereport(ERROR,
+ (errcode(ERRCODE_WRONG_OBJECT_TYPE),
+ errmsg("cannot lock relation \"%s\"",
+ rv->relname),
+ errdetail("This operation is not
supported for conflict log tables.")));
+

/*
* Make note if a temporary relation has been accessed in this
* transaction.
@@ -198,6 +207,14 @@ LockViewRecurse_walker(Node *node,
LockViewRecurse_context *context)
relkind != RELKIND_VIEW)
continue;

+ /* Disallow locking conflict log tables even
via views. */
+ if
(IsConflictLogTableNamespace(get_rel_namespace(relid)))
+ ereport(ERROR,
+
(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+ errmsg("cannot lock
relation \"%s\"",
+ relname),
+ errdetail("This
operation is not supported for conflict log tables.")));
+

--
Regards,
Dilip Kumar
Google