This behavior likely depends on how the data is loaded into the DBMS. If the records you are fetching are distributed widely among the 3M records on disk, then
On 1/12/07 4:31 PM, "Mark Dobbrow" <mdobbrow@xxxxxxxxx> wrote:
Hello -
I have a fairly large table (3 million records), and am fetching 10,000 non-contigous records doing a simple select on an indexed column ie
select grades from large_table where teacher_id = X
This is a test database, so the number of records is always 10,000 and i have 300 different teacher ids.
The problem is, sometimes fetching un-cached records takes 0.5 secs and sometimes (more often) is takes more like 10.0 seconds
(fetching the same records for a given teacher_id a second time takes about 0.25 secs)
Has anyone seen similar behavior or know what the solution might be?
any help much appreciated,
Mark
ps. My shared_buffers is set at 5000 (kernal max), and work_mem=8192
