Re: Slow select performance despite seemingly reasonable query plan

Hi,

Interesting, for one index on one partition:

idx_scan: 329
idx_tup_fetch: 8905730

So maybe a reindex would help?

David.

On Thu, May 7, 2009 at 10:26 AM, Scott Mead
<scott(dot)lists(at)enterprisedb(dot)com> wrote:
> On Thu, May 7, 2009 at 10:14 AM, David Brain <dbrain(at)bandwidth(dot)com> wrote:
>>
>> Hi,
>>
>> Some context, we have a _lot_ of data, > 1TB, mostly in 1 'table' -
>> the 'datatable' in the example below although in order to improve
>> performance this table is partitioned (by date range) into a number of
>> partition tables.  Each partition contains up to 20GB of data (tens of
>> millons of rows), with an additional ~3GB of indexes, all this is
>> served off a fairly high performance server (8 core 32Gb, with FC
>> attached SAN storage).  PostgreSQL version is 8.3.5 (running on 64bit
>> RHEL 5.2)
>>
>> This has been working reasonably well, however in the last few days
>> I've been seeing extremely slow performance on what are essentially
>> fairly simple 'index hitting' selects on this data.
>
>    Have you re-indexed any of your partitioned tables?  If you're index is
> fragmented, you'll be incurring extra I/O's per index access.  Take a look
> at the pgstattuple contrib for some functions to determine index
> fragmentation.  You can also take a look at the pg_stat_all_indexes tables.
> If your number of tup's fetched is 100 x more than your idx_scans, you *may*
> consider reindexing.
>
> --Scott
>
>

--
David Brain
dbrain(at)bandwidth(dot)com
919.297.1078