Re: [HACKERS] generated columns

From: Erik Rijkers <er(at)xs4all(dot)nl>
To: Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Sergei Kornilov <sk(at)zsrv(dot)org>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Jaime Casanova <jaime(dot)casanova(at)2ndquadrant(dot)com>
Subject: Re: [HACKERS] generated columns
Date: 2019-04-02 13:36:19
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On 2019-04-02 14:43, Peter Eisentraut wrote:
> On 2019-04-01 10:52, Peter Eisentraut wrote:
>> On 2019-03-31 05:49, Erik Rijkers wrote:
>>> STORED in a
>>> file_fdw foreign table still silently creates the column which then
>>> turns out to be useless on SELECT, with an error like:
>>> "ERROR: column some_column_name is a generated column
>>> DETAIL: Generated columns cannot be used in COPY."
>>> Maybe it'd be possible to get an error earlier, i.e., while trying to
>>> create such a useless column?
>> I'll look into it.
> I've been trying to create a test case for file_fdw for this, but I'm
> not getting your result. Can you send a complete test case?

Ah, I had not noticed before: with an asterisk ('select * from table' )
one gets no error, just empty values.

An actual error seems to occur when one mentions the
generated-column-name explicitly in the select-list.

select "id", "Ratio Log2 GEN" from <file_fdw foreign table>;
ERROR: column "Ratio Log2 GEN" is a generated column
DETAIL: Generated columns cannot be used in COPY.

That's from a quick test here at work; maybe that gives you enough info.

If that doesn't make it repeatable (for you) I'll make a more complete
example this evening (from home).

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