On Fri, May 13, 2011 at 3:20 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Tue, Apr 26, 2011 at 9:04 PM, Merlin Moncure <mmoncure@gmail.com> wrote:
>>> The very first thing to check is effective_cache_size and to set it to
>>> a reasonable value.
>
>> Actually, effective_cache_size has no impact on costing except when
>> planning a nested loop with inner index scan. So, a query against a
>> single table can never benefit from changing that setting.
>
> That's flat out wrong. It does affect the cost estimate for plain
> indexscan (and bitmap indexscan) plans.
<rereads code>
OK, I agree. I obviously misinterpreted this code the last time I read it.
I guess maybe the reason why it didn't matter for the OP is that - if
the size of the index page in pages is smaller than the pro-rated
fraction of effective_cache_size allowed to the index - then the exact
value doesn't affect the answer.
I apparently need to study this code more.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
