See and clear Postgres caches/buffers?

Question

Sometimes I run a Postgres query it takes 30 seconds. Then, I immediately run the same query and it takes 2 seconds. It appears that Postgres has some sort of caching. Can I somehow see what that cache is holding? Can I force all caches to be cleared for tuning purposes?

Note: I'm basically looking for a postgres version of the following SQL Server command:

DBCC FREEPROCCACHE
DBCC DROPCLEANBUFFERS

But I would also like to know how to see what is actually contained in that buffer.

Thanks for any help.

Answer 1

I havn't seen any commands to flush the caches in PostgreSQL. What you see is likely just normal index and data caches being read from disk and held in memory. by both postgresql and the caches in the OS. To get rid of all that, the only way I know of:

What you should do is:

1. Shutdown the database server (pg_ctl, sudo service postgresql stop, etc.)
2. echo 3 > /proc/sys/vm/drop_caches This will clear out the OS file/block caches - very important though I don't know how to do that on other OSs.
3. Start the database server

Answer 2

Yes, postgresql certainly has caching. The size is controlled by the setting *shared_buffers*. Other than that, there is as the previous answer mentions, the OS file cache which is also used.

If you want to look at what's in the cache, there is a contrib module called *pg_buffercache* available (in contrib/ in the source tree, in the contrib RPM, or wherever is appropriate for how you installed it). How to use it is listed in the standard PostgreSQL documentation.

There are no ways to clear out the buffer cache, other than to restart the server. You can drop the OS cache with the command mentioned in the other answer - provided your OS is Linux.

Answer 3

You can see what's in the PostgreSQL buffer cache using the pg_buffercache module. I've done a presentation called "Inside the PostgreSQL Buffer Cache" that explains what you're seeing, and I show some more complicated queries to help interpret that information that go along with that.

It's also possible to look at the operating system cache too on some systems, see pg_osmem.py for one somewhat rough example.

There's no way to clear the caches easily. On Linux you can stop the database server and use the drop_caches facility to clear the OS cache; be sure to heed the warning there to run sync first.