Re: Slow queries on big table

Tyrrill, Ed wrote:
> mdsdb=# \d backup_location
> Table "public.backup_location"
> Column | Type | Modifiers
> -----------+---------+-----------
> record_id | bigint | not null
> backup_id | integer | not null
> Indexes:
> "backup_location_pkey" PRIMARY KEY, btree (record_id, backup_id)
> "backup_location_rid" btree (record_id)
> Foreign-key constraints:
> "backup_location_bfk" FOREIGN KEY (backup_id) REFERENCES
> backups(backup_id) ON DELETE CASCADE

[snip]

> mdsdb=# explain analyze select record_id from backup_location where
> backup_id = 1070;
>
> QUERY PLAN
>
> ------------------------------------------------------------------------
> ------------------------------------------------------------------------
> -------------
> Index Scan using backup_location_pkey on backup_location
> (cost=0.00..1475268.53 rows=412394 width=8) (actual
> time=3318.057..1196723.915 rows=2752 loops=1)
> Index Cond: (backup_id = 1070)
> Total runtime: 1196725.617 ms
> (3 rows)

The "backup_location_rid" index on your table is not necessary. The
primary key index on (record_id, backup_id) can be used by Postgres,
even if the query is only constrained by record_id. See
http://www.postgresql.org/docs/8.2/interactive/indexes-multicolumn.html
for details.

The explain plan indicates that your query is filtered on backup_id, but
is using the primary key index on (record_id, backup_id). Based on the
table definition, you do not have any good index for filtering on backup_id.

The explain plan also seems way off, as I would expect a sequential scan
would be used without a good index for backup_id. Did you disable
sequential scans before running this query? Have you altered any other
configuration or planner parameters?

As your "backup_location_rid" is not necessary, I would recommend
dropping that index and creating a new one on just backup_id. This
should be a net wash on space, and the new index should make for a
straight index scan for the query you presented. Don't forget to
analyze after changing the indexes.

Hope this helps.

Andrew