On Thu, Nov 11, 2010 at 3:43 AM, <tv@xxxxxxxx> 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 -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
