Thread: postgresql is slow with larger table even it is in RAM

postgresql is slow with larger table even it is in RAM

From
"sathiya psql"
Date:
     I have a table with 32 lakh record in it. Table size is nearly 700 MB, and my machine had a 1 GB + 256 MB RAM, i had created the table space in RAM, and then created this table in this RAM.

    So now everything is in RAM, if i do a count(*) on this table it returns 327600 in 3 seconds, why it is taking 3 seconds ????? because am sure that no Disk I/O is happening. ( using vmstat i had confirmed, no disk I/O is happening, swap is also not used )

Any Idea on this ???

I searched a lot in newsgroups ... can't find relevant things.... ( because everywhere they are speaking about disk access speed, here i don't want to worry about disk access )

If required i will give more information on this.

Re: postgresql is slow with larger table even it is in RAM

From
Tomasz Ostrowski
Date:
On 2008-03-25 09:51, sathiya psql wrote:

> Table size is nearly 700 MB,
> and my machine had a 1 GB + 256 MB RAM, i had created the table space in
> RAM, and then created this table in this RAM.
>
>     So now everything is in RAM, if i do a count(*) on this table it returns
> 327600 in 3 seconds, why it is taking 3 seconds ?????

PostgreSQL needs to scan the whole table. And 700MB is a lot of data
even when in RAM. Check this:

$ dd if=/dev/zero bs=1M count=700 | cat > /dev/null
734003200 bytes (734 MB) copied, 1.38732 s, 529 MB/s

This command will just copy 700MB of zeros around in RAM, and it takes
over 1s to do it on my Pentium D 3,4GHz. PostgreSQL would need to do
much more with this data.

Maybe you just got bitten by count(*) gotcha:
http://sql-info.de/en/postgresql/postgres-gotchas.html#1_7
Maybe you'll get by with an estimate.

Regards
Tometzky
--
...although Eating Honey was a very good thing to do, there was a
moment just before you began to eat it which was better than when you
were...
                                                      Winnie the Pooh


Re: postgresql is slow with larger table even it is in RAM

From
Alvaro Herrera
Date:
Please stop reposting your questions to multiple groups.  Since all your
questions are about performance, please stick to the pgsql-performance
list.  Posting to pgsql-sql is not really appropriate, and in
pgsql-admin you're totally off-topic.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

Re: postgresql is slow with larger table even it is in RAM

From
"sathiya psql"
Date:
ok

On Tue, Mar 25, 2008 at 5:33 PM, Alvaro Herrera <alvherre@commandprompt.com> wrote:
Please stop reposting your questions to multiple groups.  Since all your
questions are about performance, please stick to the pgsql-performance
list.  Posting to pgsql-sql is not really appropriate, and in
pgsql-admin you're totally off-topic.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support