how does pg handle concurrent queries and same queries

[Faludi Gábor <gfaludi(at)fits(dot)hu>]

Hi All,

 

I have taken over the maintenance of a server farm , recently. 2 webserver
on  db server. They are quite powerful 2 processor xeon w/ 6Gig of ram .

 

Couple of days ago we had a serious performance hit and the db server (pg.
v7.4) was overloaded w/ something in a way that operating system was almost
not able to respond or in cases it did not.

 

 

After some analysis i suspect that there is a query that takes up to 1
second and that is the cause. Upon  each page loading this query fires and
takes the one second and blocks the page to load completly . The load was
roughly ~300 connections in one minute .

 

So here are my questions : 

 

.         Why does the second and the later queries take the whole on second
if the dataset is the same . Shouldn't PG realise that the query is the same
so i give the user the same resultset ?

.         How do I know if one query blocks the other ?

.         Is there a way to log the long running queries in 7.4 ? If not is
it available in any newer version ?

 

thanks for your help ! 

 

Üdvözlettel/kind regards,

Faludi, Gábor

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