Re: Some ideas about Vacuum

Hi,

Gokulakannan Somasundaram wrote:
> But i am just thinking of creating the DSM
> by reading through the WAL Logs, instead of asking the Inserts, updates
> and deletes to do the DSM creation.

What's the advantage of that? What's wrong with collecting the
information for DSM at transaction processing time? The overhead is
certainly smaller than the overhead for doing it later on.

> I think what Gregory is coming at is, "if we schedule the Vacuum
> after 20% of table changes, then we essentially say we need 120% of the
> disk space and hence our select operations might end up doing more I/Os."

Well, full sequential scans end up doing more I/O, but not index scans
typical for OLTP. So if autovacuum is the only thing doing full
sequential scans, you'd better reduce the number of full scans, instead
of saving only some percentage per scan, no?

Of course, depending on how much of your table fits in ram, you also
need to consider the space savings in RAM... However, I'm assuming a
reasonably low ratio of RAM size vs table size.

Regards

Markus