Re: Counterintuitive behavior when toast_tuple_target < TOAST_TUPLE_THRESHOLD

On Thu, 15 Sept 2022 at 04:04, Aleksander Alekseev
<aleksander(at)timescale(dot)com> wrote:
> 1. Forbid setting toast_tuple_target < TOAST_TUPLE_THRESHOLD
> 2. Consider using something like RelationGetToastTupleTarget(rel,
> TOAST_TUPLE_THRESHOLD) in heapam.c:2250, heapam.c:3625 and
> rewriteheap.c:636 and modify the documentation accordingly.
> 3. Add a separate user-defined table setting toast_tuple_threshold
> similar to toast_tuple_target.
>
> Thoughts?

There was some discussion on this problem in [1].

The problem with #2 is that if you look at
heapam_relation_needs_toast_table(), it only decides if the toast
table should be created based on (tuple_length >
TOAST_TUPLE_THRESHOLD). So if you were to change the logic as you
describe for #2 then there might not be a toast table during an
INSERT/UPDATE.

The only way to fix that would be to ensure that we reconsider if we
should create a toast table or not when someone changes the
toast_tuple_target reloption. That can't be done under
ShareUpdateExclusiveLock, so we'd need to obtain an
AccessExclusiveLock instead when changing the toast_tuple_target
reloption. That might upset some people.

The general direction of [1] was to just increase the minimum setting
to TOAST_TUPLE_THRESHOLD, but there were some concerns about breaking
pg_dump as we'd have to error if someone does ALTER TABLE to set the
toast_tuple_target reloption lower than the newly defined minimum
value.

I don't quite follow you on #3. If there's no toast table we can't toast.

David

[1] https://www.postgresql.org/message-id/20190403063759.GF3298@paquier.xyz