Re: How to read a query plan?

Konstantinos Agouros writes:

> logs=> explain select distinct on (url) url, category, action from websenseflat where webhost='dir.yahoo.com' and datum >='1-26-2001' and datum < '1-27-2001';
> NOTICE: QUERY PLAN:
>
> Unique (cost=0.01..0.01 rows=1 width=36)
> -> Sort (cost=0.01..0.01 rows=1 width=36)
> -> Seq Scan on websenseflat (cost=0.00..0.00 rows=1 width=36)
>
> EXPLAIN
>
> after this I created an index on the field webhost, now the query plan says:
>
> NOTICE: QUERY PLAN:
>
> Unique (cost=19690.08..19690.22 rows=5 width=36)
> -> Sort (cost=19690.08..19690.08 rows=53 width=36)
> -> Index Scan using wsflwebhost on websenseflat (cost=0.00..19688.55 rows=53 width=36)
>
> EXPLAIN
>
> Could someone give me an interpretation (or a URL that explains this)?

http://www.de.postgresql.org/devel-corner/docs/postgres/performance-tips.html

--
Peter Eisentraut peter_e(at)gmx(dot)net http://funkturm.homeip.net/~peter