Re: Same SQL, 104296ms of difference between 7.4.12 and 8.0.7

I wrote:
> Rafael Martinez Guerrero <r(dot)m(dot)guerrero(at)usit(dot)uio(dot)no> writes:
>> I have a sql statement that takes 108489.780 ms with 8.0.7 in a
>> RHEL4/amd64linux server with 2xAMD Opteron(tm) Processor 275 2.00GHz /
>> 8GB RAM and only 4193.588 ms with 7.4.12 in a RHEL3/386linux server with
>> 2xIntel(R) Xeon(TM) CPU 2.40GHz / 4GB RAM.

> I think you've discovered a planner regression.
> Simplified test case using the regression database:

> explain select * from tenk1 a, tenk1 b
> where (a.ten = b.ten and (a.unique1 = 100 or a.unique1 = 101))
> or (a.hundred = b.hundred and a.unique1 = 42);

I've repaired the assertion crash in 8.1/HEAD, but I don't think it's
practical to teach 8.0 to optimize queries like this nicely. The reason
7.4 can do it is that 7.4 forces the WHERE condition into CNF, ie

(a.hundred = b.hundred OR a.ten = b.ten) AND
(a.unique1 = 42 OR a.ten = b.ten) AND
(a.hundred = b.hundred OR a.unique1 = 100 OR a.unique1 = 101) AND
(a.unique1 = 42 OR a.unique1 = 100 OR a.unique1 = 101)

from which it's easy to extract the index condition for A. We decided
that forcing to CNF wasn't such a hot idea, so 8.0 and later don't do
it, but 8.0's logic for extracting index conditions from joinquals isn't
up to the problem of handling sub-ORs. Fixing that looks like a larger
change than I care to back-patch into an old release.

My recommendation is to update to 8.1.4 when it comes out.

regards, tom lane