Re: Can this query go faster???

Hi,

> -----Ursprüngliche Nachricht-----
> Von: pgsql-performance-owner(at)postgresql(dot)org
> [mailto:pgsql-performance-owner(at)postgresql(dot)org] Im Auftrag
> von Joost Kraaijeveld
> Gesendet: Dienstag, 6. Dezember 2005 10:44
> An: Pgsql-Performance
> Betreff: [PERFORM] Can this query go faster???

> SELECT customers.objectid FROM prototype.customers,
> prototype.addresses WHERE customers.contactaddress =
> addresses.objectid ORDER BY zipCode asc, housenumber asc
> LIMIT 1 OFFSET 283745
>
> Explain:
>
> Limit (cost=90956.71..90956.71 rows=1 width=55)
> -> Sort (cost=90247.34..91169.63 rows=368915 width=55)
> Sort Key: addresses.zipcode, addresses.housenumber
> -> Hash Join (cost=14598.44..56135.75 rows=368915 width=55)
> Hash Cond: ("outer".contactaddress = "inner".objectid)
> -> Seq Scan on customers (cost=0.00..31392.15
> rows=368915 width=80)
> -> Hash (cost=13675.15..13675.15 rows=369315 width=55)
> -> Seq Scan on addresses (cost=0.00..13675.15
> rows=369315 width=55)
>
> The customers table has an index on contactaddress and objectid.
> The addresses table has an index on zipcode+housenumber and objectid.

The planner chooses sequential scans on customers.contactaddress and addresses.objectid instead of using the indices. In order to determine whether this is a sane decision, you should run EXPLAIN ANALYZE on this query, once with SET ENABLE_SEQSCAN = on; and once with SET ENABLE_SEQSCAN = off;. If the query is significantly faster with SEQSCAN off, then something is amiss - either you haven't run analyze often enough so the stats are out of date or you have random_page_cost set too high (look for the setting in postgresql.conf) - these two are the "usual suspects".

Kind regards

Markus