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Normal VACUUM locks indexes?

From: Aldor <an(at)mediaroot(dot)de>
To: pgsql-admin-owner(at)postgresql(dot)org
Subject: Normal VACUUM locks indexes?
Date: 2005-08-18 13:08:14
Message-ID: 4304883E.5010704@mediaroot.de (view raw or flat)
Thread:
Lists: pgsql-admin
Hi,

when vacuuming a big table with some indexes I experienced a locking of 
VACUUM.

As I understood the theory the normal VACUUM should never lock something 
or influence queries made to the table.

When running VACUUM it was vacuuming first the indexes, after that it 
was vacuuming the table data. While it was vacuuming the indexes I tried 
to make some queries into the table - I didn't get any lock error but 
saw that that the process which was responsible for my query was set to 
the state WAIT. After that I made a second query, again... WAIT. After 
VACUUM finished the indexes and started processing the table data the 
WAIT processes were processed and the query result appeared. Any new 
query was also not set to WAIT, it was processed immediately.

The idea of normal VACUUM is to run it while the system needs to process 
user queries in the same time.

I saw this behevior when setting the vacuum-options in postgresql.conf 
to no delay so that I can run VACUUM with nearly full power.

I'm not sure if I would experience the same problem when I would set up 
a delay and setup the cost rules in postgresql.conf but I think when 
normal VACUUM starts vacuuming the indexes of the table it locks queries 
using them. On bigger tables this can mean that your queries are 
sleeping for 10-20 Minutes until the indexes have been vacuumed and this 
is the reason why I think that this behavior is not compatible with the 
"theory" of normal VACUUM;-)

1) Question: Is this a normal behavior of Postgres to lock the indexes 
while vacuuming them?

2a) Question: If yes: Is there anything which can be done to prevent 
Postgres doing this or maybe tell VACUUM to skip the indexes and only 
vacuum the table data?

2b) Question: If no: Which parameters are important to find out why this 
happens only in my case or what kind of other information is needed to 
find out why this happen?

Aldor

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