Re: Bitmap Heap Scan anomaly

Tom,
Did you restart Postgres and drop file system caches?

What I am suspecting is that some sort of prefetching is happening.
I know that Postgres does not do prefetching.
I also understand very little about OS/FileSystem level prefetching.

----- Original Message ----
From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Jeff Davis <pgsql(at)j-davis(dot)com>
Cc: jaba the mobzy <makaronaforna(at)yahoo(dot)com>; pgsql-hackers(at)postgresql(dot)org
Sent: Thursday, May 3, 2007 11:42:32 PM
Subject: Re: [HACKERS] Bitmap Heap Scan anomaly

Jeff Davis <pgsql(at)j-davis(dot)com> writes:
> On Thu, 2007-05-03 at 14:33 -0700, jaba the mobzy wrote:
>> mycorr_100 took 11.4 s to run although it had to fetch 100000 row from
>> the base table.
>> mycorr_10 took 24.4 s to run although it had to fetch 10563 row from
>> the base table.

> This is because the physical distribution of data is different. The
> mycorr_10 table has tuples in which a and b are > 15.9M spread all
> throughout. mycorr_100 has them all collected together at the end of the
> physical file. Less disk seeking.

If the OP had generated the data randomly, as claimed, the rows
shouldn't be particularly more clumped in one table than the other.
But I sure agree that it sounds like a nonrandom distribution in the
mycorr_100 table. FWIW I tried to duplicate the behavior, and could
not, using tables made up like this:

create table src as
select int4(16*1024*1024*random()) as key,
int4(16*1024*1024*random()) as a,
int4(16*1024*1024*random()) as b
from generate_series(1,16*1024*1024);

create table mycorr_10 as
select key, a,
case when random() < 0.1 then a else b end as b
from src;

create table mycorr_100 as
select key, a, a as b
from src;

create index mycorr_10i on mycorr_10(a,b);

create index mycorr_100i on mycorr_100(a,b);

vacuum analyze mycorr_10;

vacuum analyze mycorr_100;

regards, tom lane

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