Re: Logical replication row filtering and TOAST

Antonin Houska <ah(at)cybertec(dot)at> wrote:

> I spent some time thinking about a special case of evaluation of the row
> filter and wrote a comment that might be useful (see the attachment). However
> now I think that it's not perfect if the code really relies on the fact that
> value of an indexed column cannot be TOASTed due to size restrictions.
>
> I could hit two different error messages when trying activate TOAST on an
> index column (in this case PG was build with 16kB pages), but still I think
> the code is unnecessarily fragile if it relies on such errors:
>
>
> ERROR: index row requires 8224 bytes, maximum size is 8191
>
> ERROR: index row size 8048 exceeds btree version 4 maximum 5432 for index "b_pkey"
> DETAIL: Index row references tuple (0,3) in relation "b".
> HINT: Values larger than 1/3 of a buffer page cannot be indexed.
>
>
> Note that at least in ExtractReplicaIdentity() we do expect that an indexed
> column value can be TOASTed.
>
> /*
> * If the tuple, which by here only contains indexed columns, still has
> * toasted columns, force them to be inlined. This is somewhat unlikely
> * since there's limits on the size of indexed columns, so we don't
> * duplicate toast_flatten_tuple()s functionality in the above loop over
> * the indexed columns, even if it would be more efficient.
> */
> if (HeapTupleHasExternal(key_tuple))
> {
> HeapTuple oldtup = key_tuple;
>
> key_tuple = toast_flatten_tuple(oldtup, desc);
> heap_freetuple(oldtup);
> }
>
> Do I miss anything?

Well, I see now that the point might be that, in heap_update(),
"id_has_external" would be true the indexed value could be TOASTed, so that
the (flattened) old tuple would be WAL logged:

old_key_tuple = ExtractReplicaIdentity(relation, &oldtup,
bms_overlap(modified_attrs, id_attrs) ||
id_has_external,
&old_key_copied);

Nevertheless, a comment in pgoutput_row_filter(), saying that TOASTed values
are not expected if old_slot is NULL, might be useful.

--
Antonin Houska
Web: https://www.cybertec-postgresql.com