Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions

On Sun, Sep 29, 2019 at 11:24 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:
> On Sun, Sep 29, 2019 at 12:39 AM Tomas Vondra
> <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> >
>
> Yeah, it is better to deal it separately as I am also not entirely
> convinced at this stage about this parameter. I have mentioned the
> same in the previous email as well.
>
> While glancing through the changes, I noticed a small thing:
> +#logical_decoding_work_mem = 64MB # min 1MB, or -1 to use
maintenance_work_mem
>
> I guess this need to be updated.
>

On further testing, I found that the patch seems to have problems with
toast. Consider below scenario:
Session-1
Create table large_text(t1 text);
INSERT INTO large_text
SELECT (SELECT string_agg('x', ',')
FROM generate_series(1, 1000000)) FROM generate_series(1, 1000);

Session-2
SELECT * FROM pg_create_logical_replication_slot('regression_slot',
'test_decoding');
SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
*--kaboom*

The second statement in Session-2 leads to a crash.

Other than that, I am not sure if the changes related to spill to disk
after logical_decoding_work_mem works for toast table as I couldn't hit
that code for toast table case, but I might be missing something. As
mentioned previously, I feel there should be some way to test whether this
patch works for the cases it claims to work. As of now, I have to check
via debugging. Let me know if there is any way, I can test this.

I am reluctant to say, but I think this patch still needs some more work
(review, test, rework) before we can commit it.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com