On Wed, 2016-03-09 at 10:58 +0100, Shulgin, Oleksandr wrote:
> On Tue, Mar 8, 2016 at 9:10 PM, Joel Jacobson <j...@trustly.com>
> wrote:
>         On Wed, Mar 9, 2016 at 1:25 AM, Shulgin, Oleksandr
>         <oleksandr.shul...@zalando.de> wrote:
>         > Thank you for spending your time to run these :-)
>         n/p, it took like 30 seconds :-)
> Great!  I'm glad to hear it was as easy to use as I hoped for :-)
>         > I don't want to be asking for too much here, but is there a
>         chance you could
>         > try the effects of the proposed patch on an offline copy of
>         your database?
>         Yes, I think that should be possible.
>         > Do you envision or maybe have experienced problems with
>         query plans
>         > referring to the columns that are near the top of the above
>         hist_ratio
>         > report?  In other words: what are the practical implications
>         for you with
>         > the values being duplicated rather badly throughout the
>         histogram like in
>         > the example you shown?
>         I don't know much about the internals of query planner,
>         I just read the "57.1. Row Estimation Examples" to get a basic
>         understanding.
>         If I understand it correctly, if the histogram_bounds contains
>         a lot
>         of duplicated values,
>         then the row estimation will be inaccurate, which in turn will
>         trick
>         the query planner
>         into a sub-optimal plan?
> Yes, basically it should matter the most for the equality comparison
> operator, such that a MCV entry would provide more accurate
> selectivity estimate (and the histogram is not used at all in this
> case anyway).  For the "less/greater-than" comparison both MCV list
> and histogram are used, so the drawback of having repeated values in
> the histogram, in my understanding is the same: less accurate
> selectivity estimates for the values that could fall precisely into a
> bin which didn't make it into the histogram.
>         We've had some problems lately with the query planner, or
>         actually we've always
>         had them but never noticed them nor cared about them, but now
>         during peak times
>         we've had short periods where we haven't been able to fully
>         cope up
>         with the traffic.
>         I tracked down the most self_time-consuming functions and
>         quickly saw
>         how to optimize them.
>         Many of them where on the form:
>         SELECT .. FROM SomeBigTable WHERE Col1 = [some dynamic value]
>         AND Col2
>         = [some constant value] AND Col3 = [some other constant value]
>         The number of rows matching the WHERE clause were very tiny,
>         perfect
>         match for a partial index:
>         CREATE INDEX .. ON SomeBigTable USING btree (Col1) WHERE Col2
>         = [some
>         constant value] AND Col3 = [some other constant value];
>         Even though the new partial index matched the query perfectly,
>         the
>         query planner didn't want to use it. Instead it continued to
>         use some
>         other sub-optimal index.
>         The only way to force it to use the correct index was to use
>         the
>         "+0"-trick which I recently learned from one of my colleagues:
>         SELECT .. FROM SomeBigTable WHERE Col1 = [some dynamic value]
>         AND
>         Col2+0 = [some constant value] AND Col3+0 = [some other
>         constant
>         value]
>         CREATE INDEX .. ON SomeBigTable USING btree (Col1) WHERE Col2
>         +0 =
>         [some constant value] AND Col3+0 = [some other constant
>         value];
>         By adding +0 to the columns, the query planner will as I
>         understand it
>         be extremely motivated to use the correct index, as otherwise
>         it would
>         have to do a seq scan on the entire big table, which would be
>         very
>         costly.
>         I'm glad the trick worked, now the system is fast again.
>         We're still on 9.1, so maybe these problems will go away once
>         we upgrade to 9.5.
> Hm... sounds like a planner bug to me.  I'm not exceptionally aware of
> the changes in partial index handling that were made after 9.1, though
> grepping the commit log for "partial index" produces a number of hits
> after the date of 9.1 release.

My first guess would be this is related to the costing bug addressed in 


I.e. the planner is not accounting for the index predicate correctly,
and ends up choosing the full index. It'd be interesting to see if the
patch makes the optimizer to choose the right index in your example.

The fact that +0 fixes the issue however seems a bit contradictory,
though. That forces the planner to use default selectivity estimates
(~5% for equality expressions, IIRC), not the per-column statistics.

Combined with the independence assumption for multiple AND clauses, this
may easily confuse the planner. I'd expect that would affect both
indexes equally, but perhaps not.

Would be useful to see explain analyze - it's not clear what you mean by
"the number of rows matching WHERE were tiny", whether an estimate or
the actual number.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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