On Wed, 2007-07-04 at 10:20 +0200, Csaba Nagy wrote:
> On Wed, 2007-07-04 at 00:53, Christopher Browne wrote:
> > One at a time took about 15 minutes
> > 100 at a time took 3:15
> > 1000 at a time took longer than 100 at a time (curious, that!)
> > all in one shot took 43 seconds.
> 
> Check the plans for the 100 vs. 1000 cases: I'm pretty sure 100 goes for
> bitmap index scan and 1000 goes for sequential scan... and 10 * 100
> bitmap index scans are probably somewhat faster than 1 sequential scan
> on your table/box. I guess 1000 is close to the limit between the
> performance turnover between the index scan and sequential scan on your
> table/box/setup, but the sequential scan is slightly underestimated by
> the planner.
> 
> BTW, the bitmap index scan case should theoretically be the fastest, so
> aiming for the highest chunk size where the planner still chooses bitmap
> index scan (or downright forcing it to do so if possible) would give the
> best performance.
> 

Why would a bitmap index scan be faster than a sequential scan when
deleting the entire table?

Regards,
	Jeff Davis