On Thu, Nov 11, 2010 at 3:43 AM, <tv@fuzzy.cz> wrote:
>> Okay, I want to know how the planner computes the cost of constructing
>> bitmap. And when the planner computes the cost of 'Bitmap Index Scan', if
>> it considers the influence of memory cache? As when I do not clear the
>> memory cache, I find the 'Bitmap Index Scan' is real fast than 'Seq
>> Scan'.
>
> There are two things here - loading the data from a disk into a cache
> (filesystem cache at the OS level / shared buffers at the PG level), and
> then the execution itself.
>
> PostgreSQL estimates the first part using an effective_cache_size hint,
> and uses that to estimate the probability that the data are already in the
> filesystem cache.
No, it does not do that.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company