tuning seqscan costs

Katherine Stoovs <ambrosiac@nedsenta.nl> writes:

There must be something
wrong in the planning parameters after all if a plan that is slower by
a factor of tens or hundreds becomes estimated better than the fast
variant.

Instead of handwaving, how about showing us EXPLAIN ANALYZE results for
both cases? You didn't even explain how the index you expect it to use
is defined...

Specifying what PG version you are using is also minimum required
information for this sort of question.

regards, tom lane

On Oct 19, 2005, at 9:51 AM, Katherine Stoovs wrote:

I want to correlate two index rows of different tables to find an
offset so that

table1.value = table2.value AND table1.id = table2.id + offset

is true for a maximum number of rows.

To achieve this, I have the two tables and a table with possible
offset values and execute a query:

SELECT value,(SELECT COUNT(*) FROM table1,table2
WHERE table1.value = table2.value AND
table1.id = table2.id + offset)
AS matches FROM offsets ORDER BY matches;

The query is very inefficient, however, because the planner doesn't
use my indexes and executes seqscans instead. I can get it to execute
fast by setting ENABLE_SEQSCAN to OFF, but I have read this will make
the performance bad on other query types so I want to know how to
tweak the planner costs or possibly other stats so the planner will
plan the query correctly and use index scans. There must be something
wrong in the planning parameters after all if a plan that is slower by
a factor of tens or hundreds becomes estimated better than the fast
variant.

I have already issued ANALYZE commands on the tables.

Thanks for your help,
Katherine Stoovs

Katherine,

If offset is a column in offsets, can you add an index on the
expresion table2.id + offset?

http://www.postgresql.org/docs/8.0/static/indexes-expressional.html

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