Re: [PERFORM] how to find the sql using most of the i/o in an oltp system

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On Jun 15, 2008, at 10:41 AM, "Uwe Bartels" <uwe.bartels@gmail.com> wrote:

Hi Alexander,

thanks for you answer.
What you wrote in terms of postgres I knew. I just tested to log all statements with statistics. This is a lot of unstructured data in a logfile. But this is the best I found as far.

The database is running on a solaris box. So DTrace is no problem. I couldn't find any dtrace scripts for postgres. Do you know any scripts except this sample script?

Thanks.
Uwe

On Sun, Jun 15, 2008 at 4:03 PM, Alexander Staubo <alex@bengler.no> wrote:

On Sun, Jun 15, 2008 at 3:48 PM, Uwe Bartels <uwe.bartels@gmail.com> wrote:
> Tuning a sql statements I'm familiar with. Finding a sql statement which
> takes too long due to i/o is probably easy as well. But how about statements
> that take about 100 ms, that read a lot and that are executed several times
> per second?

Take a look at the PostgreSQL manual chapter on monitoring and statistics:

 http://www.postgresql.org/docs/8.3/interactive/monitoring.html

If you have access to DTrace (available on Solaris, OS X and possibly
FreeBSD), you could hook the low-level system calls to reads and
writes. If you don't have access to DTrace, the pg_statio_* set of
tables is your main option. In particular, pg_statio_user_tables and
pg_statio_user_indexes. See the documentation for the meaning of the
individual columns.

Unfortunately, the statistics tables are not transaction-specific
(indeed I believe they only update once you commit the transaction,
and then only after a delay), meaning they capture statistics about
everything currently going on in the database. The only way to capture
statistics about a single query, then, is to run it in complete
isolation.

Alexander.