Re: [ SOLVED ] select count(*) very slow on an already

Bill, if you had alot of updates and deletions and wanted to optimize your
table, can you just issue the cluster command.
Will the cluster command rewrite the table without the obsolete data that a
vacuum flags or do you need to issue a vacuum first?
Dan.

-----Original Message-----
From: Bill Moran [mailto:wmoran(at)potentialtech(dot)com]
Sent: Thursday, April 15, 2004 2:49 PM
To: Rajesh Kumar Mallah
Cc: Postgres Performance
Subject: Re: [PERFORM] [ SOLVED ] select count(*) very slow on an
already

Rajesh Kumar Mallah wrote:
> Bill Moran wrote:
>
>> Rajesh Kumar Mallah wrote:
>>
>>> Hi,
>>>
>>> The problem was solved by reloading the Table.
>>> the query now takes only 3 seconds. But that is
>>> not a solution.
>>
>> If dropping/recreating the table improves things, then we can reasonably
>> assume that the table is pretty active with updates/inserts. Correct?
>
> Yes the table results from an import process and under goes lots
> of inserts and updates , but thats before the vacuum full operation.
> the table is not accessed during vacuum. What i want to know is
> is there any wat to automate the dumping and reload of a table
> individually. will the below be safe and effective:

The CLUSTER command I described is one way of doing this. It
essentially automates the task of copying the table, dropping
the old one, and recreating it.

>> If the data gets too fragmented, a vacuum may not be enough. Also, read
>> up on the recommendations _against_ vacuum full (recommending only using
>> vacuum on databases) With full, vacuum condenses the database, which may
>> actually hurt performance. A regular vacuum just fixes things up, and
>> may leave unused space lying around. However, this should apparently
>> achieve a balance between usage and vacuum. See the docs, they are much
>> better at describing this than I am.
>>
> i understand simultaneous vacuum and usage detoriates performance mostly.
> but this case is different.

Just want to make sure we're on the same page here. I'm not talking about
vacuuming simultaneous with anything. I'm simply saying that "vacuum full"
isn't always the best choice. You should probably only be doing "vacuum".
The reason and details for this are in the admin docs.

--
Bill Moran
Potential Technologies
http://www.potentialtech.com

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