Re: allow trigger to get updated columns

From: Daniel Gustafsson <daniel(at)yesql(dot)se>
To: Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: allow trigger to get updated columns
Date: 2020-03-05 12:53:01
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> On 24 Feb 2020, at 10:58, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:
> This is a change to make the bitmap of updated columns available to a trigger in TriggerData. This is the same idea as was recently done to generated columns [0]: Generic triggers such as tsvector_update_trigger can use this information to skip work if the columns they are interested in haven't changed. With the generated columns change, perhaps this isn't so interesting anymore, but I suspect a lot of existing installations still use tsvector_update_trigger. In any case, since I had already written the code, I figured I post it here. Perhaps there are other use cases.

I wouldn't at all be surprised if there are usecases for this in the wild, and
given the very minor impact I absolutely think it's worth doing. The patches
both apply, compile and pass tests without warnings.

The 0001 refactoring patch seems a clear win to me.

In the 0002 patch:

+ For <literal>UPDATE</literal> triggers, a bitmap set indicating the
+ columns that were updated by the triggering command. Generic trigger

Is it worth pointing out that tg_updatedcols will be NULL rather than an empty
Bitmapset for non-UPDATE triggers? bitmapset.c treats NULL as an empty bitmap
but since a Bitmapset can be allocated but empty, maybe it's worth being
explicit to help developers?

There isn't really a test suite that excercises this IIUC, how about adding
something like the attached diff to contrib/lo? It seemed like a lower impact
change than widening test_tsvector.

+1 on the patchset, marking this entry as Ready For Committer.

cheers ./daniel

Attachment Content-Type Size
lo_trig_test.diff application/octet-stream 886 bytes

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