Re: Planner is ignoring index when view is queried but uses index when running underlying view's sql - Mailing list pgsql-admin
Thanks Tom.
So I ended up with the CTE as one of my optimization attempts. I started off with a regular sub-select so that's not it. However I have stumbled onto something - deleting the index (on last_seen) actually makes it faster!!! Somehow the combination of the index and query in the view was tripping up the query planner into selecting a bad plan? Is that possible?
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Regards,
Max Kremer
On Tue, Jun 7, 2016 at 5:40 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Max Kremer <mkremer@trialfire.com> writes:
> I've encountered a very strange result when using a view in a query VS the
> underlying sql of the view. When accessing the view I get a different query
> plan than running the sql that makes up the view. Using the view is 10x
> slower and generates a strange query plan that seems to ignore
> the existence of an index.
I believe the problem is that the WITH attached to the view's SELECT
prevents the view sub-select from being flattened into the outer query
(ie, this is another way in which WITH acts as an optimization fence).
So the sub-select is planned without awareness of the ORDER BY/LIMIT
that would encourage picking a suitably ordered join plan.
Try writing the CTE as a plain sub-select, instead.
(AFAIR, this behavior just stems from a lack of round tuits and not
any fundamental difficulty: the WITHs could perfectly well be hoisted
up to the outer query. But don't hold your breath waiting for that
to happen.)
(BTW, what PG version is that?)
regards, tom lane
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