Re: Question about caching on full table scans

Hi Laurenz,

>
> In your approach 1 to 3, what do you mean with "load into main memory"?

I forgot to say: I use Java and connect with JDBC.

in approach 1 I do an initial loading of the entire relation, by executing 1 SQL query to load all edges in main memory, where I create my main memory structure
as an adjacency list.

> Do you
> a) make sure that the data you talk about are in the PostgreSQL buffer
> cache
> or

> b) retrieve the data from PostgreSQL and store it somewhere in your
> application?

In approach 1 I do that, as described before.

But after each experiment I restart a new java process.

>
> To clear PostgreSQL's cache, restart the server.
> That should be a fast operation.
> Since version 8.3, PostgreSQL is smart enough not to evict the
> whole cache for a large sequential scan.

>
> To flush the filesystem cache (from Linux 2.6.16 on), use
> sync; echo 3 > /proc/sys/vm/drop_caches

I started to do that , and
yes, this solves my problem!!

I assume that deleting file system cache implies that also postgres cache is deleted, isn't it ?

so i will invoke after each experiment this command

thanks a lot!!

Markus