Re: Column Filtering in Logical Replication

From: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
To: Rahila Syed <rahilasyed90(at)gmail(dot)com>
Cc: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Peter Smith <smithpb2250(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Column Filtering in Logical Replication
Date: 2021-12-13 17:44:11
Message-ID: 202112131744.kw5vn6x4yvzo@alvherre.pgsql
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On 2021-Dec-10, Alvaro Herrera wrote:

> Actually it's not so easy to implement.

So I needed to add "sub object id" support for pg_publication_rel
objects in pg_depend / dependency.c. What I have now is partial (the
describe routines need patched) but it's sufficient to show what's
needed. In essence, we now set these depend entries with column
numbers, so that they can be dropped independently; when the drop comes,
the existing pg_publication_rel row is modified to cover the remaining
columns. As far as I can tell, it works correctly.

There is one policy decision to make: what if ALTER TABLE drops the last
remaining column in the publication? I opted to raise a specific error
in this case, though we could just the same opt to drop the relation
from the publication. Are there opinions on this?

This version incorporates the fixups Peter submitted, plus some other
fixes of my own. Notably, as Peter also mentioned, I changed
pg_publication_rel.prattrs to store int2vector rather than an array of
column names. This makes for better behavior if columns are renamed and
things like that, and also we don't need to be so cautious about
quoting. It does mean we need a slightly more complicated query in a
couple of spots, but that should be okay.

Álvaro Herrera 39°49'30"S 73°17'W —
"Always assume the user will do much worse than the stupidest thing
you can imagine." (Julien PUYDT)

Attachment Content-Type Size
column-filtering-8.patch text/x-diff 17.6 KB

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