On Wed, Sep 1, 2021 at 8:29 PM Euler Taveira <euler@eulerto.com> wrote:
>
> On Wed, Sep 1, 2021, at 9:36 AM, Amit Kapila wrote:
>
> I think this or any other similar solution for row filters (on
> updates) won't work till we solve the problem reported by Hou-San [1].
> The main reason is that we don't have data for unchanged toast columns
> in WAL. For that, we have discussed some probable solutions in email
> [2], however, that also required us to solve one of the existing
> bugs[3].
>
> I didn't mention but I'm working on it in parallel.
>
> I agree with you that including TOAST values in the WAL is a possible solution
> for this issue. This is a popular request for wal2json [1][2][3] and I think
> other output plugins have the same request too. It is useful for CDC solutions.
>
> I'm experimenting 2 approaches: (i) always include unchanged TOAST values to
> new tuple if a GUC is set and (ii) include unchanged TOAST values to new tuple
> iif it wasn't include in the old tuple.
>
In the second approach, we will always end up having unchanged toast
columns for non-key columns in the WAL which will be a significant
overhead, so not sure if that can be acceptable if we want to do it by
default.
> The advantage of the first option is
> that you fix the problem adjusting a parameter in your configuration file.
> However, the disadvantage is that, depending on your setup -- REPLICA IDENTITY
> FULL, you might have the same TOAST value for a single change twice in the WAL.
> The second option solves the disadvantage of (i) but it only works if you have
> REPLICA IDENTITY FULL and Dilip's patch applied [4] (I expect to review it
> soon).
>
Thanks for offering the review of that patch. I think it will be good
to get it committed.
> In the output plugin, (i) requires a simple modification (remove
> restriction for unchanged TOAST values) but (ii) needs a more complex surgery.
>
I think if get Dilip's patch then we can have a rule for filter
columns such that it can contain only replica identity key columns.
This rule is anyway required for Deletes and we can have it for
Updates. At this stage, I haven't checked what it takes to implement
such a solution but it would be worth investigating it.
--
With Regards,
Amit Kapila.