Обсуждение: Strange behavior of child table.

Hi All,

I have created partition on table Round_Action , which has 3 inherited partition tables.

When I am firing a simple select query with limit on parent table it is taking huge time to execute. But when I am firing this query directly on inherited table it is taking few milliseconds.

EXP.

select * from Round_Action where action_id =50000 limit 100 → execution time 80 sec

select * from Round_Action_CH1 action_id =50000 limit 100 → execution time 0.1 sec

Round_Action is the parent table and has no record in the tables, all the records are lying in child tables.

Table is having index on action_id.

Partition is trigger based.

Postgres Version : (PostgreSQL) 8.4.6

Why there is difference in execution time? What I am doing wrong?

--
Thanks & regards,
JENISH VYAS

On 06/01/2011 02:07 AM, Jenish wrote:
> select * from Round_Action where action_id =50000 limit 100 →
> execution time 80 sec
>
> select * from Round_Action_CH1 action_id =50000 limit 100 → execution
> time 0.1 sec
>
>

First off:  each of the child tables needs to have the index created on
them.  That doesn't get inherited just by putting it on the master.

If you already did that, try running these both with "EXPLAIN". Sharing
the two query plans here would help figure out what's happening.
Showing the definition of the index on one of the child tables would be
helpful too.

There are some optimizer limitations in PostgreSQL versions before 9.0
that prevent it from using an index on the child tables in some
situations where people expect it to, which includes aggregates like
MIN/MIN.  I'm not sure if your LIMIT case is running into the same
issue, the plan will help confirm what's going on.

--
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support  www.2ndQuadrant.us
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