| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com> |
| Cc: | Jeff Davis <pgsql(at)j-davis(dot)com>, pgsql-hackers(at)postgresql(dot)org, npboley(at)gmail(dot)com |
| Subject: | Re: new correlation metric |
| Date: | 2008-10-27 20:12:53 |
| Message-ID: | 23891.1225138373@sss.pgh.pa.us |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-hackers |
Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com> writes:
> Tom Lane wrote:
>> By definition, a bitmap scan's cost isn't affected by index order
>> correlation.
> No? I think I understand that for index scans the correlation
> influenced how many data pages are estimated to get sucked in.
No, it's not about that, it's about whether we're likely to have to read
any of those pages more than once. The bitmap processing implicitly
sorts all the tuples-to-be-read into physical TID order, so the heap
access is always linear no matter how badly correlated the index is.
regards, tom lane
| From | Date | Subject | |
|---|---|---|---|
| Next Message | A. Kretschmer | 2008-10-27 20:36:06 | Re: Website request -- developer docs along with release docs |
| Previous Message | Tom Lane | 2008-10-27 20:08:48 | Re: CREATE TABLE AS WITH NO DATA |