Re: select count(*) very slow on an already vacuumed table.

Richard Huxton wrote:

>On Thursday 15 April 2004 08:10, Rajesh Kumar Mallah wrote:
>
>
>>The problem is that i want to know if i need a Hardware upgrade
>>at the moment.
>>
>>Eg i have another table rfis which contains ~ .6 million records.
>>
>>
>
>
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>>SELECT count(*) from rfis where sender_uid > 0;
>>
>>
>
>
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>>Time: 117560.635 ms
>>
>>Which is approximate 4804 records per second. Is it an acceptable
>>performance  on the hardware below:
>>
>>RAM: 2 GB
>>DISKS: ultra160 , 10 K  , 18 GB
>>Processor: 2* 2.0 Ghz Xeon
>>
>>
>
>Hmm - doesn't seem good, does it? If you run it again, is it much faster
>(since the data should be cached then)? What does "vmstat 10" show while
>you're running the query?
>
>One thing you should have done is read the performance tuning guide at:
>  http://www.varlena.com/varlena/GeneralBits/Tidbits/index.php
>The default values are very conservative, and you will need to change them.
>
>
Hi,

Thanks for the interest . my config are not the default ones.
i was running iostat while running the query. Looks like one
of the disks doesnt' go past a read performance of 20 ,000 KBytes/sec

while the other disk  it goes as high as 40,000 . What i am ding
currently is
loading the table in both the disks and compare the table scan speeds.

The performance is definitely better in the newly loaded table in the other
disk . the load in server is 13 because i am simultaneously re-loading
the data
in other table.


rt2=# SELECT count(*) from rfis where sender_uid > 0;
+--------+
| count  |
+--------+
| 564870 |
+--------+
(1 row)

Time: 10288.359 ms

rt2=#

shall post the comparitive details under normal load soon


regds
mallah.







>
>
>>What kind of upgrades shoud be put on the server for it to become
>>reasonable fast.
>>
>>
>
>If you've only got one disk, then a second disk for OS/logging. Difficult to
>say more without knowing numbers of users/activity etc.
>
>
>