"Travis Bauer" <trbauer@cs.indiana.edu> writes:
> I have a problem with a _very_ slow query. I'm not sure how to understand
> the explain output.
There's some basic info about EXPLAIN in the "understanding performance"
section of the user manual (at least today, it's at
http://www.postgresql.org/docs/postgres/c4962.htm). The thing I usually
try to do first is understand how the estimated row counts (rows=N)
relate to reality. For this you need to know how large the tables are
and how restrictive the WHERE clauses related to each table are, so that
you can figure out the true output row count for each part of the query
plan.
The EXPLAIN output alone is not too useful without seeing both the query
text and the context information about table sizes &etc.
regards, tom lane