From: Tom Lane <tgl@sss.pgh.pa.us>
To: Vitaly Ustinov <vitaly@ustinov.ca>
cc: Peter Eisentraut <peter.eisentraut@enterprisedb.com>,
        pgsql-bugs@lists.postgresql.org,
        "David G. Johnston" <david.g.johnston@gmail.com>
Subject: Re: Generated column is not updated (Postgres 13)
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Comments: In-reply-to Vitaly Ustinov <vitaly@ustinov.ca>
	message dated "Thu, 20 May 2021 15:07:34 -0400"
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Date: Thu, 20 May 2021 17:09:08 -0400
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Vitaly Ustinov <vitaly@ustinov.ca> writes:
> In a BEFORE trigger (PL/pgSQL) I can access not yet computed generated
> columns via the "NEW" whole-row var. I can read from it (NULL), and I
> can write to it, but the assigned value will be ignored.

NEW is not really a whole-row var in the sense that we're discussing
here.  In any case, yes, nodeModifyTable runs computation of generated
columns after firing BEFORE triggers.  I suppose that's to prevent
the triggers from interfering with those values.

> Is it okay with you that the patch 0002 introduces some inconsistency wi=
th that?

In what way would this introduce (new) inconsistency?  I didn't move
the computation of those values from where they were.  Moreover, so
far as I can see, ATRewriteTable doesn't fire BEFORE triggers at all.

BTW, while looking around nodeModifyTable, I noted some other gaps in our
support for "tableoid" in GENERATED expressions: the FDW code paths in
ExecInsert and ExecUpdate fail to fill tts_tableOid till after running
ExecComputeStoredGenerated.  So they have the same bug that 0001 fixes
in ATRewriteTable.  It can be fixed the same way, ie move the code
that fills tts_tableOid; but I remain very concerned about how many
other gaps there are.

			regards, tom lane