Re-ordering of OR conditions

IF I run the following with the a < 2900 condition first, the more  
expensive EXISTS only gets executed when needed, but if I change the  
order of the OR's, the EXISTS is always executed. It would be good if  
the optimizer could re-order the OR conditions based on estimated  
cost (granted, this wouldn't work very well if you've got functions  
in the OR, but it'd still be useful):

select * from a where a < 2900 or exists (select * from b where b.a =  
a.a);

Here's a full example. Note the loops count for the Subplan between  
both cases:

decibel=# create table a as select * from generate_series(1,3000) a;
SELECT
decibel=# create table b as select a,b from a, generate_series(1,100)  
b where a > 10;
SELECT
decibel=# create index b__a on b(a);
CREATE INDEX
decibel=# explain analyze select * from a where a < 2900 or exists  
(select * from b where b.a = a.a);                                                       QUERY PLAN
------------------------------------------------------------------------ 
------------------------------------------------
Seq Scan on a  (cost=0.00..8080.41 rows=1997 width=4) (actual  
time=0.014..1.784 rows=3000 loops=1)   Filter: ((a < 2900) OR (subplan))   SubPlan     ->  Index Scan using b__a on b
(cost=0.00..4006.44rows=1495  
 
width=8) (actual time=0.009..0.009 rows=1 loops=101)           Index Cond: (a = $0)
Total runtime: 2.151 ms
(6 rows)

decibel=# explain analyze select * from a where exists (select * from  
b where b.a = a.a) or a < 2000;                                                       QUERY PLAN
------------------------------------------------------------------------ 
-------------------------------------------------
Seq Scan on a  (cost=0.00..8080.41 rows=1997 width=4) (actual  
time=0.067..37.011 rows=3000 loops=1)   Filter: ((subplan) OR (a < 2000))   SubPlan     ->  Index Scan using b__a on b
(cost=0.00..4006.44rows=1495  
 
width=8) (actual time=0.011..0.011 rows=1 loops=3000)           Index Cond: (a = $0)
Total runtime: 37.497 ms
(6 rows)

decibel=#

(This is on HEAD as of a few minutes ago)
--
Jim Nasby                               jim.nasby@enterprisedb.com
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)