different queries and their efficiencies

Hi,

I'm curious as to what the general opinion is on these different
queries and their relative merits, especially in terms of speed:

-----
select count(*) from zstore_temp where product_code not in (select
product_code from zstore)

QUERY PLAN
--------------------------------------------------------------------------------
Aggregate (cost=17200040774.42..17200040774.42 rows=1 width=0)
-> Seq Scan on zstore_temp (cost=0.00..17200038808.85 rows=786226 width=0)
Filter: (NOT (subplan))
SubPlan
-> Seq Scan on zstore (cost=0.00..19747.74 rows=851574 width=21)
(5 rows)
-----

select count(foo) from (select product_code from zstore_temp except
select product_code from zstore) as foo;

QUERY PLAN
----------------------------------------------------------------------------------------------------------
Aggregate (cost=502201.07..502201.07 rows=1 width=32)
-> Subquery Scan foo (cost=487050.91..501595.06 rows=242403 width=32)
-> SetOp Except (cost=487050.91..499171.03 rows=242402 width=21)
-> Sort (cost=487050.91..493110.97 rows=2424025 width=21)
Sort Key: product_code
-> Append (cost=0.00..79125.50 rows=2424025 width=21)
-> Subquery Scan "*SELECT* 1"
(cost=0.00..50862.02 rows=1572451 width=21)
-> Seq Scan on zstore_temp
(cost=0.00..35137.51 rows=1572451 width=21)
-> Subquery Scan "*SELECT* 2"
(cost=0.00..28263.48 rows=851574 width=21)
-> Seq Scan on zstore
(cost=0.00..19747.74 rows=851574 width=21)

----

select count(zstore_temp.product_code) from zstore_temp left join
zstore on zstore_temp.product_code = zstore.product_code where
zstore.product_code is null;

QUERY PLAN
----------------------------------------------------------------------------------------
Aggregate (cost=100965813.69..100965813.69 rows=1 width=21)
-> Merge Right Join (cost=448485.57..100882122.54 rows=33476460 width=21)
Merge Cond: ("outer"."?column2?" = "inner"."?column2?")
Filter: ("outer".product_code IS NULL)
-> Sort (cost=154686.81..156815.74 rows=851574 width=21)
Sort Key: (zstore.product_code)::text
-> Seq Scan on zstore (cost=0.00..19747.74
rows=851574 width=21)
-> Sort (cost=293798.76..297729.88 rows=1572451 width=21)
Sort Key: (zstore_temp.product_code)::text
-> Seq Scan on zstore_temp (cost=0.00..35137.51
rows=1572451 width=21)

-----

It seems as if there are multiple order-of-magnitude differences
between these queries given the above costs:

17200040774
100965813
502201

Is there anything that can be done to improve the performance of the
subquery or left join strategies?

Thanks, and happy Thanksgiving to all the USians,
--
David N. Welton
- http://www.dedasys.com/davidw/

Apache, Linux, Tcl Consulting
- http://www.dedasys.com/