[postgis-users] Help with Bad Query Plan
robe.dnd at cityofboston.gov
Fri Jan 9 05:02:44 PST 2009
> i) PostgreSQL 8.3 has a new costing model which we have seen in some
> cases causes evaluation of the more expensive _st_intersects()
> above the && operator. As yet we don't really have a system that we
> use to verify this properly :(
> However, if you still have your original .sql scripts around, I would
> suggest trying to load them into a PostgreSQL 8.2 installation and
> posting the results back to the list so we can determine if this is
> case or not.
Slight correction. I don't believe this statement to be true.
&& from all my experience in 8.3 still evaluates first before
_ST_Intersects. Where you get bitten is
that _ST_Intersects sometimes evaluates before other things.
I think like if you had something like
WHERE ST_Intersects(...) AND somedate between a and b
Then in 8.2 if you had an index on somedate, ST_Intersects would run
second regardless of the order of your
conditions. In 8.3 if you put ST_Intersects first because we didn't
define a costing for ST_Intersects -- it often evaluates first.
which would be bad since an indexed somedate would be a better choice to
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