From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Zeugswetter Andreas SB <ZeugswetterA(at)wien(dot)spardat(dot)at> |
Cc: | "'Alex Pilosov'" <alex(at)pilosoft(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: AW: selecting from cursor |
Date: | 2001-07-03 14:12:53 |
Message-ID: | 18210.994169573@sss.pgh.pa.us |
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Zeugswetter Andreas SB <ZeugswetterA(at)wien(dot)spardat(dot)at> writes:
> this. Given that cursors (are supposed to) support FETCH BACKWARDS,
> I really don't see why they shouldn't be expected to handle ReScan...
>> I thought only scrollable cursors can do that. What if cursor isn't
>> scrollable? Should it error during the execution?
> In PostgreSQL, all cursors are scrollable. The allowed grammar keyword is
> simply ignored. I am actually not sure that this is optimal, since there
> are a few very effective optimizations, that you can do if you know, that
> ReScan is not needed (like e.g. not storing the result temporarily).
It's worse than that: we don't distinguish plans for cursors from plans
for any other query, hence *all* query plans are supposed to be able to
run backwards. (In practice, a lot of them don't work :-(.) Someday
that needs to be improved. It would be good if the system understood
whether a particular plan node would ever be asked to rescan itself or
run backwards, and could optimize things on that basis.
regards, tom lane
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