'alternatives'

Hi Christophe,

On 2012-11-28 13:07:12 -0800, Christophe Pettus wrote:

In a query plan, I noticed the following:

Join Filter: (((all_permissions.role_recursive AND
(alternatives: SubPlan 5 or hashed SubPlan 6)) OR
(permitted_e.id = deployed_e.id)) AND (NOT (SubPlan 13)))

Check the first item of
http://www.postgresql.org/docs/current/interactive/release-9-2.html#AEN110503

Greetings,

Andres Freund

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Hi, Andres,

Thanks!

On Nov 28, 2012, at 1:58 PM, Andres Freund wrote:

http://www.postgresql.org/docs/current/interactive/release-9-2.html#AEN110503

Does that apply to views as well? (This particular plan was not from a prepared or PL/pgSQL statement, but did include views.)

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Hi,

On 2012-11-28 14:16:18 -0800, Christophe Pettus wrote:

Thanks!

Not much to thank for, the answer was actually wrong...

Does that apply to views as well? (This particular plan was not from a prepared or PL/pgSQL statement, but did include views.)

Its not really relevant for views no.

The real answer for this is that this actually a 8.4 feature not the
aforementioned 9.2 feature.

The commit introducing this is:
http://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=bd3daddaf232d95b0c9ba6f99b0170a0147dd8af

What that does to add hashing support for IN(). But hashing can be
pessimal in comparison to a explicit check if only a few values come in,
so this can be checked at runtime after the above commit...

Greetings,

Andres Freund

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Andres Freund <andres@anarazel.de> writes:

The commit introducing this is:
http://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=bd3daddaf232d95b0c9ba6f99b0170a0147dd8af
What that does to add hashing support for IN(). But hashing can be
pessimal in comparison to a explicit check if only a few values come in,
so this can be checked at runtime after the above commit...

Yeah. If you look at the subplans, one is designed for retail probes
and the other is designed for sucking up the entire subquery result into
a hashtable. EXPLAIN ANALYZE will show you that only one gets used at
runtime. (The idea of dynamic switchover hasn't gotten anywhere yet.)

regards, tom lane

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