I am wondering why it uses the O(n^2) nested loop when there is a O(N)
methoud using btree indexes for a merg join. I am using 7.2.1 would
upgrading fix my problime or is it somthing else?
Given the schema:
drop table Entry_Pairs;
create table Entry_Pairs (
left_entry int REFERENCES Entry ON DELETE RESTRICT,
right_entry int REFERENCES Entry ON DELETE RESTRICT,
relation int NOT NULL ,
subtract bool NOT NULL ,
comment int NULL REFERENCES Comment ON DELETE SET NULL,
UNIQUE (left_entry, right_entry, relation)
);
CREATE INDEX entry_pairs_left_index ON entry_pairs (left_entry);
CREATE INDEX entry_pairs_right_index ON entry_pairs (right_entry);
--
You get this"
dblex=> explain select A.left_entry from entry_pairs A, entry_pairs B
where A.right_entry != B.left_entry;
NOTICE: QUERY PLAN:
Nested Loop (cost=100000000.00..102876671.17 rows=97545252 width=12)
-> Seq Scan on entry_pairs a (cost=0.00..167.77 rows=9877 width=8)
-> Seq Scan on entry_pairs b (cost=0.00..167.77 rows=9877 width=4)
EXPLAIN
That is dum. If you just walk both B-Tree indexes there is a O(n)
search. I tryed to turn off netsed loops but it still did it. (the
reason the cost is 100000000.00 is a artifact from turing off loops)
-Jonathan