Steve,
It's a notice, not an error, so you don't need to avoid it, merely
understand what it means.
Mark C-A's amazing selectivity magic improvement allowed the planner
to properly evaluate the selectivity of spatial joins between
tables. Things like "a.the_geom && b.the_geom". Note that both
sides of the operator are multiply valued, unlike, for example
"a.the_geom && MakePoint(1,2)". The older selectivity system only
did good estimates for the latter case.
Mark's magic can only work when there are statistics available for
both sides of the operator, and statistics are only gathered for
things that have indexes defined for them. You might have made an
index on a.the_geom, and statistics will be gathered for that.
Statistics will not be gathered for transform(a.the_geom) or expand
(a.the_geom).
Which brings us to the NOTICE. It is warning you that it is
computing join selectivity, but using things for which it doesn't
have proper statistics available. So it might get a bad selectivity
estimate. For simple queries and plans, there won't be a noticeable
difference, because there really is only one efficient plan. For
more complex ones, it's possible you'll get a non-optimal plan.
Paul
On 3-Jan-08, at 7:53 AM, Stephen Woodbridge wrote:
Hi all,
I ran into this last night while playing with some queries:
NOTICE: LWGEOM_gist_joinsel called with arguments that are not
column references
CONTEXT: SQL statement "select a.file, a.name, b.name
from canada_p.pl a, canada_p.pl b
where expand(a.the_geom, 2/69) && b.the_geom
and (strpos(a.name, b.name)>0 or strpos(b.name, a.name)>0)"
This one is new to me. I am trying to find all records with similar
names within about 2 miles of one another in this table.
Thanks,
-Steve
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