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:
begin work; create table new_tab AS select * from tab; truncate table tab; insert into tab select * from new_tab; drop table new_tab; commit; analyze tab;
i havenot tried it but plan to do so. but i feel insert would take ages to update the indexes if any.
BTW
is there any way to disable checks and triggers on a table temporarily while loading data (is updating reltriggers in pg_class safe?)
true. but there should not be too many.
The problem is that such phenomenon obscures our judgement used in optimising queries and database.
Lots of phenomenon obscure that ...
If a query runs slow we really cant tell if its a problem with query itself , hardware or dead rows.
I already did vacumm full on the table but it still did not have that effect on performance. In fact the last figures were after doing a vacuum full.
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.
Can there be any more elegent solution to this problem.
As a guess, look into CLUSTER (a Postgres SQL command). CLUSTER will basically recreate the table while ordering rows based on an index. (this might benefit you in other ways as well) Don't forget to analyze after cluster. If the problem is caused by frequent updates/inserts, you may find that re-clustering the table on a certain schedule is worthwhile.
i could consider that option also.
Be warned, this suggestion is based on an educated guess, I make no guarantees that it will help your problem. Read the docs on cluster and come to your own conclusions.
Thanks .
Regds mallah.
Regds Mallah.
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.
SELECT count(*) from rfis where sender_uid > 0;
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.
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.
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