Thread: Logical replication row filtering and TOAST
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?
--
Antonin Houska
Web: https://www.cybertec-postgresql.com
Attachment
Antonin Houska <ah@cybertec.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
On Tue, Apr 5, 2022 at 3:52 PM Antonin Houska <ah@cybertec.at> wrote:
>
> Antonin Houska <ah@cybertec.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:
>
Right.
>
> Nevertheless, a comment in pgoutput_row_filter(), saying that TOASTed values
> are not expected if old_slot is NULL, might be useful.
>
How about something like the attached?
--
With Regards,
Amit Kapila.
Attachment
Amit Kapila <amit.kapila16@gmail.com> wrote: > > Antonin Houska <ah@cybertec.at> wrote: > > > > Nevertheless, a comment in pgoutput_row_filter(), saying that TOASTed values > > are not expected if old_slot is NULL, might be useful. > > > > How about something like the attached? Yes, that'd be sufficient. Thanks. -- Antonin Houska Web: https://www.cybertec-postgresql.com
On Tue, Apr 5, 2022 at 8:32 PM Amit Kapila <amit.kapila16@gmail.com> wrote: > > How about something like the attached? > LGTM. regards, Ajin Cherian Fujitsu Australia
On Wed, Apr 6, 2022 at 7:21 AM Ajin Cherian <itsajin@gmail.com> wrote: > > On Tue, Apr 5, 2022 at 8:32 PM Amit Kapila <amit.kapila16@gmail.com> wrote: > > > > > How about something like the attached? > > > > LGTM. > Pushed. -- With Regards, Amit Kapila.