Re: Column Filtering in Logical Replication

From: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
To: Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>
Cc: Rahila Syed <rahilasyed90(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(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: 2021-12-14 19:35:23
Message-ID: 202112141935.3j2lva3je7o6@alvherre.pgsql
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On 2021-Dec-14, Tomas Vondra wrote:

> Yeah. I think it's not clear if this should behave more like an index or a
> view. When an indexed column gets dropped we simply drop the index. But if
> you drop a column referenced by a view, we fail with an error. I think we
> should handle this more like a view, because publications are externally
> visible objects too (while indexes are pretty much just an implementation
> detail).

I agree -- I think it's more like a view than like an index. (The
original proposal was that if you dropped a column that was part of the
column list of a relation in a publication, the entire relation is
dropped from the view, but that doesn't seem very friendly behavior --
you break the replication stream immediately if you do that, and the
only way to fix it is to send a fresh copy of the remaining subset of

> But why would it be easier not to add new object type? We still need to
> check there is no publication referencing the column - either you do that
> automatically through a dependency, or you do that by custom code. Using a
> dependency seems better to me, but I don't know what are the complications
> you mentioned.

The problem is that we need a way to represent the object "column of a
table in a publication". I found myself adding a lot of additional code
to support OBJECT_PUBLICATION_REL_COLUMN and that seemed like too much.

Álvaro Herrera PostgreSQL Developer —

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