From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Achilleas Mantzios <achill(at)matrix(dot)gatewaynet(dot)com> |
Cc: | pgsql-admin(at)lists(dot)postgresql(dot)org |
Subject: | Re: Index ignored on pkid = curval('some_seq'), used with pkid = (select curval(''some_seq') ) |
Date: | 2019-02-27 15:21:49 |
Message-ID: | 14554.1551280909@sss.pgh.pa.us |
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Lists: | pgsql-admin |
Achilleas Mantzios <achill(at)matrix(dot)gatewaynet(dot)com> writes:
> dynacom=# explain analyze update itemshist set reason='{foo,bar}' where pkid = currval(('public.itemshist_pkid_seq'::text)::regclass);
currval() is marked volatile, so that's not a legal index
qualification.
(Perhaps there's an argument that it'd be more useful to consider it
stable, but certainly if you used it in the same query as a nextval()
on the same sequence, you'd have trouble.)
> -- but if I compare against select currval it uses the index:
> dynacom=# explain analyze update itemshist set reason='{foo,bar}' where pkid = ( SELECT currval(('public.itemshist_pkid_seq'::text)::regclass));
Yeah, the planner does not consider uncorrelated scalar sub-selects
to be volatile; they'll be evaluated only once per query, regardless
of what they contain. So this is sort of a traditional hack for
freezing a volatile function's result. (I have no idea whether other
RDBMSes read the SQL spec the same way on this point.)
regards, tom lane
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