I was hoping someone could shed some light on the
following problem:
The subselect under 6.4 ignores indexes built on
tables. For example, consider the following two
tables:
\d sales;
Table = sales
supplysource varchar() not null 16
supply varchar() not null 16
supplyunit varchar() not null 2
quantity float8 not null 8
target varchar() not null 16
costcntr varchar() not null 8
saletype varchar() not null 16
saledate datetime not null 8
Indices: k_sales k_sales_saledate k_sales_supply k_sales_target
\d locations;
Table = locations
location varchar() not null 16
costcntr varchar() not null 5
supplypath varchar() not null 64
formpath varchar() not null 64
engineerpath varchar() not null 64
Indices: k_locations k_locations_costcntr k_locations_location
with the following query:
explain select supply from sales where target in
(select location from locations);
NOTICE: QUERY PLAN:
Seq Scan on sales (cost=5738.60 size=116806 width=12) SubPlan -> Seq Scan on locations (cost=7.49 size=136
width=12)
EXPLAIN
This results in a full table scan on the outer table
where there are 150K rows. As a result, it is the
equivalent of a full table scan on 150K (sales) *
100 (locations) rows = 15,000,000 row table scan.
Oracle 8.0.5.00 for Linux Early Adaptor Program
correctly uses the index on the same two tables
resulting on a sequential access on the subselected
table (locations) and an index look-up on the
select table (sales), according to EXPLAIN PLAN.
Due to the failure of PostgreSQL 6.4 to correctly
use indexes with subselects, it makes subselects
practically unusable.
(NOTE: This occurs regardless of whether or not it
is executed immediately following a VACUUM)
Are there any patches available to fix this problem?
Will this fixed in the 6.4 release?
Thanks for any information,
Marcus Mascari
(mascarim@yahoo.com)
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