Re: bogus: logical replication rows/cols combinations

On 2022-Apr-28, Tomas Vondra wrote:

> Attached is a patch doing the same thing in tablesync. The overall idea
> is to generate copy statement with CASE expressions, applying filters to
> individual columns. For Alvaro's example, this generates something like
>
> SELECT
> (CASE WHEN (a < 0) OR (a > 0) THEN a ELSE NULL END) AS a,
> (CASE WHEN (a > 0) THEN b ELSE NULL END) AS b,
> (CASE WHEN (a < 0) THEN c ELSE NULL END) AS c
> FROM uno WHERE (a < 0) OR (a > 0)

I've been reading the tablesync.c code you propose and the idea seems
correct. (I was distracted by wondering if a different data structure
would be more appropriate, because what's there looks slightly
uncomfortable to work with. But after playing around I can't find
anything that feels better in an obvious way.)

(I confess I'm a bit bothered by the fact that there are now three
different data structures in our code called PublicationInfo).

I propose some comment changes in the attached patch, and my
interpretation (untested) of the idea of optimizing for a single
publication. (In there I also rename logicalrep_relmap_free_entry
because it's confusing. That should be a separate patch but I didn't
split it before posting, apologies.)

> There's a couple options how we might optimize this for common cases.
> For example if there's just a single publication, there's no need to
> generate the CASE expressions - the WHERE filter will do the trick.

Right.

--
Álvaro Herrera Breisgau, Deutschland — https://www.EnterpriseDB.com/