From: | Erik Jones <ejones(at)engineyard(dot)com> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Keresztury Balázs <balazs(at)gaslightmusic(dot)hu>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: forced sequential scan when condition has current_user |
Date: | 2010-01-04 23:13:43 |
Message-ID: | E56D1346-6E42-40D5-9CF1-239F5104F3A5@engineyard.com |
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Lists: | pgsql-performance |
On Jan 4, 2010, at 1:59 PM, Robert Haas wrote:
> The thing is, PostgreSQL doesn't know at planning time what the value of
> current_user() will be, so the plan can't depend on that; the planner
> just takes its best shot.
current_user() is a stable function and the manual is explicit that the result of stable function can be used in an index scan:
"A STABLE function cannot modify the database and is guaranteed to return the same results given the same arguments for all rows within a single statement. This category allows the optimizer to optimize multiple calls of the function to a single call. In particular, it is safe to use an expression containing such a function in an index scan condition. (Since an index scan will evaluate the comparison value only once, not once at each row, it is not valid to use a VOLATILE function in an index scan condition.)"
postgres=# select provolatile from pg_proc where proname = 'current_user';
provolatile
-------------
s
So, I think the OP's question is still valid.
Erik Jones, Database Administrator
Engine Yard
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