Re: Slow query performance on large table - Mailing list pgsql-performance

Paul McKay wrote:
> Hopefully you guys can help me with another query I've got that's
> running slow.
>
> This time it's across two tables I have
>
> clearview=# \d panconversation
>       Table "panconversation"
>    Column    |  Type   | Modifiers
> -------------+---------+-----------
>  assessment  | integer | not null
>  interface   | integer |
>  source      | integer |
>  destination | integer |
>  protocol    | integer |
> Indexes: idx_panconversation_destination,
>          idx_panconversation_interface,
>          idx_panconversation_protocol,
>          idx_panconversation_source
> Primary key: panconversation_pkey
> Unique keys: unq_panconversation
> Triggers: RI_ConstraintTrigger_52186648,
>           RI_ConstraintTrigger_52186654,
>           RI_ConstraintTrigger_52186660,
>           RI_ConstraintTrigger_52186666
>
> Primary key is assessment
>
> Along with the table I was dealing with before, with the index I'd
> mislaid put back in
>
> clearview=# \d measurement
>               Table "measurement"
>    Column   |         Type          | Modifiers
> ------------+-----------------------+-----------
>  assessment | integer               |
>  time       | integer               |
>  value      | character varying(50) |
> Indexes: idx_measurement_assessment,
>          idx_measurement_time,
>          ind_measurement_ass_time
>
> The 'explain analyse' of the query I am running is rather evil.
>
> clearview=# explain analyse select source,value
> clearview-#            from measurement, PANConversation
> clearview-#            where PANConversation.assessment =
> measurement.assessment
> clearview-#            and Interface = 11
> clearview-#            and Time > 1046184261 and Time < 1046335461
> clearview-# ;
> NOTICE:  QUERY PLAN:
>
> Hash Join  (cost=1532.83..345460.73 rows=75115 width=23) (actual
> time=1769.84..66687.11 rows=16094 loops=1)
>   ->  Seq Scan on measurement  (cost=0.00..336706.07 rows=418859
> width=15) (actual time=1280.11..59985.47 rows=455788 loops=1)
>   ->  Hash  (cost=1498.21..1498.21 rows=13848 width=8) (actual
> time=253.49..253.49 rows=0 loops=1)
>         ->  Seq Scan on panconversation  (cost=0.00..1498.21 rows=13848
> width=8) (actual time=15.64..223.18 rows=13475 loops=1)
> Total runtime: 66694.82 msec
>
> EXPLAIN
>
> Anybody shed any light on why the indexes I created aren't being used,
> and I have these nasty sequential scans?

Measurement is sequentially scaned, because probably "interface=12"
results in lot of records.

Please, check how many rows you have
- all rows in measurement/panconversation,
- rows in measurement with "Interface"=12
- rows in panconversation between your time.

Regards,
Tomasz Myrta