Re: Seqscan rather than Index

Tom Lane <tgl@sss.pgh.pa.us> writes:

> Greg Stark <gsstark@mit.edu> writes:
> > Postgres is also more pessimistic about the efficiency of index scans. It's
> > willing to use a sequential scan down to well below 5% selectivity when other
> > databases use the more traditional rule of thumb of 10%.
>
> However, other databases are probably basing their analysis on a
> different execution model.  Since we have to visit both heap and index
> in all cases, we do indeed have a larger penalty for index use.

It's only in special cases that other databases do not have to look at the
heap. For simple queries like "select * from x where foo > ?" they still have
to look at the heap. I never looked into how much of a bonus Oracle gives for
the index-only case, I'm not sure it even takes it into account.

> I've looked pretty closely at the cost model for index access, believe me.
> It's not pessimistic; if anything it is undercharging for index access.

I think there's another effect here beyond the physical arithmetic. There's a
kind of teleological reasoning that goes something like "If the user created
the index chances are it's because he wanted it to be used".

I guess that argues more for more aggressive selectivity estimates than for
biased index costing though. If I'm doing "where foo > ?" then if there's an
index on foo I probably put it there for a reason and want it to be used even
if postgres doesn't really have a clue how selective the query will be.

> I think the one effect that's not being modeled is amortization of index
> fetches across successive queries.

And across multiple fetches in a single query, such as with a nested loop.

It seems like the effective_cache_size parameter should be having some
influence here.

--
greg