Обсуждение: Discarding the resulting rows

<table border="0" cellpadding="0" cellspacing="0"><tr><td style="font: inherit;" valign="top">Hello Hackers:<br /><br
/>Twoquestions.<br /><br />1.<br /><br />I would like to execute queries such as <br /><br />select * from part and
timethe query. But I want to ignore how much time is taken for printing the result to a file or the screen.<br /><br
/>Basically,I would like to discard the result rows after doing all the work required to execute the query.<br /><br
/>Ilooked at the documentation and I saw something about using the keyword PERFORM rather than SELECT.<br /><br />I
triedPERFORM * from part; <br /><br />But this gave me a syntax error.<br /><br />Please let me know how this can be
done.<br/><br />2.<br /><br />How do I clear the buffer caches between two query runs?<br />I believe this is not
possiblein Postgres. Can someone please confirm this or tell me how it may be done.<br /><br />Thanks,<br /><br
/>Murali.<br/><br /><br /><br />-----------------------------------------------------------------<br />Please visit <a
href="http://NumberFest.com"rel="nofollow" target="_blank">NumberFest.com</a> for educational number puzzles & mind
exercisesfor all ages! And please tell your friends about it. Thank You!<br /></td></tr></table><br /> 

"Murali M. Krishna" <murali1729@yahoo.com> wrote:
> Basically, I would like to discard the result rows after doing all
> the work required to execute the query.
I would use EXPLAIN ANALYZE SELECT ...
> I looked at the documentation and I saw something about using the
> keyword PERFORM rather than SELECT.
I don't remember ever seeing anything like that.  Do you have a URL?
> How do I clear the buffer caches between two query runs?
The easiest way to clear the PostgreSQL cache is to restart the
service.  PostgreSQL goes through the OS cache; so you'll need to
clear that, too.  How you do that is dependent on your OS.  Of
course, in most real use cases, a significant portion of the
database would be cached, so unless you're dealing with a very
unusual situation, it's hard to see what the value would be of such
a benchmark, unless you're trying to create an artificial "worst
case" scenario for bounding purposes.
Oh, and most serious database servers have 256MB or more of battery
backed cache on the RAID controller; don't forget to deal with that
somehow.
-Kevin

On Mon, Apr 26, 2010 at 2:36 PM, Kevin Grittner
<Kevin.Grittner@wicourts.gov> wrote:
> "Murali M. Krishna" <murali1729@yahoo.com> wrote:
>
>> I looked at the documentation and I saw something about using the
>> keyword PERFORM rather than SELECT.
>
> I don't remember ever seeing anything like that.  Do you have a URL?
>

i guess he is refering to the plpgsql's PERFORM statement, which of
course he can't use outside a plpgsql function...
mmm... well, IIRC, in 9.0 he will be able to do DO $$ PERFORM * FROM
tabla; $$ LANGUAGE plpgsql;

but i think DO is not an EXPLAINing statement

--
Atentamente,
Jaime Casanova
Soporte y capacitación de PostgreSQL
Asesoría y desarrollo de sistemas
Guayaquil - Ecuador
Cel. +59387171157

On Mon, Apr 26, 2010 at 3:36 PM, Kevin Grittner
<Kevin.Grittner@wicourts.gov> wrote:
> "Murali M. Krishna" <murali1729@yahoo.com> wrote:
>
>> Basically, I would like to discard the result rows after doing all
>> the work required to execute the query.
>
> I would use EXPLAIN ANALYZE SELECT ...

There's some overhead to that, of course.

....Robert

On Mon, Apr 26, 2010 at 3:03 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Mon, Apr 26, 2010 at 3:36 PM, Kevin Grittner
> <Kevin.Grittner@wicourts.gov> wrote:
>> "Murali M. Krishna" <murali1729@yahoo.com> wrote:
>>
>>> Basically, I would like to discard the result rows after doing all
>>> the work required to execute the query.
>>
>> I would use EXPLAIN ANALYZE SELECT ...
>
> There's some overhead to that, of course.
>

he could see the "actual time" in the very first row of the EXPLAIN
ANALYZE... isn't that a value that is more close to what the OP is
looking for?

--
Atentamente,
Jaime Casanova
Soporte y capacitación de PostgreSQL
Asesoría y desarrollo de sistemas
Guayaquil - Ecuador
Cel. +59387171157

Robert Haas <robertmhaas@gmail.com> wrote:
> On Mon, Apr 26, 2010 at 3:36 PM, Kevin Grittner
> <Kevin.Grittner@wicourts.gov> wrote:
>> "Murali M. Krishna" <murali1729@yahoo.com> wrote:
>>
>>> Basically, I would like to discard the result rows after doing
>>> all the work required to execute the query.
>>
>> I would use EXPLAIN ANALYZE SELECT ...
> 
> There's some overhead to that, of course.
Good point.  At the moment I can't think how to do better, though.
Other suggestions, anyone?
-Kevin

Jaime Casanova <jcasanov@systemguards.com.ec> writes:
> On Mon, Apr 26, 2010 at 3:03 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>> On Mon, Apr 26, 2010 at 3:36 PM, Kevin Grittner
>> <Kevin.Grittner@wicourts.gov> wrote:
>>> I would use EXPLAIN ANALYZE SELECT ...
>> 
>> There's some overhead to that, of course.

> he could see the "actual time" in the very first row of the EXPLAIN
> ANALYZE... isn't that a value that is more close to what the OP is
> looking for?

Well, it will include the instrumentation overhead of EXPLAIN ANALYZE,
which can be nontrivial depending on your hardware and the query plan.

On the other hand, EXPLAIN skips the cost of converting the result data
to text form, not to mention the network overhead of delivering it; so
in another sense it's underestimating the work involved.

I guess the real question is exactly what the OP is hoping to measure
and why.
        regards, tom lane