Cost Estimation for queries

Posted in 2005

PARAMESHWAR PATHRI — Tue, 15 Mar 2005 04:52:49 — source: IIUG Forums & Mailing Lists

hi, Does the optimizer take into consideration the number of buffers(BUFFERS) available and RA_PAGES into account to estimated the no of IO's required to complete the query. bye.

↪ replying to PARAMESHWAR PATHRI

Michael Mueller — Tue, 15 Mar 2005 09:04:01 — source: IIUG Forums & Mailing Lists

No. It assumes that it has to read everything from disk. I think it would be extremely difficult to quickly find out how many pages needed by a query are cached in the buffer pool. Michael PARAMESHWAR.... wrote: > hi, > Does the optimizer take into consideration the number of buffers(BUFFERS) available and RA_PAGES into account to estimated the no of IO's required to complete the query. > > > bye. > > > >

↪ replying to Michael Mueller

Samar Desai — Tue, 15 Mar 2005 21:56:22 — source: IIUG Forums & Mailing Lists

Again optimizer considering BUFFERS/RA_PAGES while considering estimated no of I/Os is not a good idea since buffers are part of volatile memory where at a point of time we cannot be sure how many pages reside in memory satisfying the query for which optimizer is evaluating the best possible access plan. Regards Samar --- "Michael Mue...." <m.mueller99@kay-mueller.de> wrote: > No. It assumes that it has to read everything from > disk. I think it > would be extremely difficult to quickly find out how > many pages needed > by a query are cached in the buffer pool. > > Michael > > PARAMESHWAR.... wrote: > > hi, > > Does the optimizer take into consideration the > number of buffers(BUFFERS) available and RA_PAGES > into account to estimated the no of IO's required to > complete the query. > > > > > > bye. > > > > > > > > > > > __________________________________ Do you Yahoo!? Yahoo! Small Business - Try our new resources site! http://smallbusiness.yahoo.com/resources/