Hi Merlin,
So should I interpret this as: there is a potential gain from choosing subqueries over with WITHs ?
On Tue, Feb 19, 2013 at 3:33 PM, Merlin Moncure <mmoncure@xxxxxxxxx> wrote:
Unfortunately, the mailing list archives. Rightly or wrongly,On Tue, Feb 19, 2013 at 9:22 AM, Joe Van Dyk <joe@xxxxxxxxx> wrote:
> On Tue, Feb 19, 2013 at 1:02 AM, Albe Laurenz <laurenz.albe@xxxxxxxxxx>
> wrote:
>>
>> Joe Van Dyk wrote:
>> > My assumption was that WITH acted just like subselects, but apparently
>> > they don't? Using WITH doesn't
>> > use the expected index.
>>
>> Currently WITH acts as an "optimization fence", that means
>> that means that the planner won't move conditions into or
>> out of the WITH query.
>
>
> Where's the best place to read up on this?
postgresql docs are exceptionally light in terms of performance
aspects of various SQL mechanisms.
(non-data modifying) WITH is basically formalization of technique: A
extract to temp table B query that table. Not the optimization fence
characteristic is an implementation detail and not future proofed but
is nevertheless widely replied upon.
merlin
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