From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | jks(at)selectacast(dot)net |
Cc: | pgsql-general(at)postgresql(dot)org, Rod Taylor <rbt(at)rbt(dot)ca> |
Subject: | Re: sequence caches |
Date: | 2003-05-20 18:09:14 |
Message-ID: | 460.1053454154@sss.pgh.pa.us |
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Lists: | pgsql-general |
jks(at)selectacast(dot)net writes:
>> Seems that would require predicting the future. How would you know how
>> many times the sequence would get nextval'd in one query?
> Because you parse the query before you take any action, so you know how
> many times the nextval will be called.
No you don't. Consider CASE expressions, SELECTs that will return an
uncertain number of rows, etc.
> Or do you do the locking down in
> the nextval method that knows nothing about the other nextvals?
Exactly.
> Perhaps
> there could be a function call that retrieves X sequence numbers from the
> sequence that overrides the cache settings so I could query like:
> SELECT nextval('seqname',3),nextval('seqname'),nextval('seqname');
Doesn't really seem worth the trouble to me. To take just one obvious
problem, what makes you think that those expressions will be evaluated
left-to-right? There are no guarantees of execution order in SQL.
regards, tom lane
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