Re: WAL question

Hello.

So, what happens if I set checkpoint_segments and checkpoint_timeout very high,
and, shared_buffers to a very low value?
Will PostgreSQL force a checkpoint when it has not enough memory in shared_buffers for some new "un-checkpointed" data ?

On Fri, 12 Jan 2007 10:49:04 -0300
Alvaro Herrera <alvherre(at)commandprompt(dot)com> wrote:

> Evgeny Gridasov wrote:
> > Hello.
> >
> > Lets's imagine a sequence in time:
> > (1) Some transactions commit data (simple table inserts/updates).
> > (2) I get data from a table using select.
> > (3) A CHECKPOINT occurs, data from pg_xlog is copied to actual table data files.
> >
> > Questions:
> >
> > Where does PostgreSQL read data during (2) process?
>
> Shared buffers, which means that data that's not in memory is read from
> disk, and data that is in memory is read from there. A checkpoint
> evicts stuff from shared buffers to disk.
>
> > Would it scan both pg_xlog(for committed data that dind't pass
> > CHECKPOINT) and table data files?
>
> pg_xlog is not read except in crash recovery.
>
> > What happens if an index could be used?
>
> Same thing.
>
> --
> Alvaro Herrera http://www.CommandPrompt.com/
> The PostgreSQL Company - Command Prompt, Inc.
>