Please observe the following queries. Why PostgreSQL is favouring MergeJoin
eventhough, it leading to higher execution times than NestedLoopJoin. Any
suggestions to fix this problem.
bench=# EXPLAIN ANALYZE SELECT * FROM tenk1 t1, tenk2 t2 WHERE t1.unique1 < 50
AND t1.unique2 = t2.unique2;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------
Merge Join (cost=665.09..4704.60 rows=166701 width=488) (actual
time=10.128..40.843 rows=50 loops=1)
Merge Cond: ("outer".unique2 = "inner".unique2)
-> Index Scan using tenk2_unique2 on tenk2 t2 (cost=0.00..1514.00
rows=10000 width=244) (actual time=0.031..20.520 rows=10000 loops=1)
-> Sort (cost=665.09..673.42 rows=3334 width=244) (actual
time=9.601..9.646 rows=50 loops=1)
Sort Key: t1.unique2
-> Seq Scan on tenk1 t1 (cost=0.00..470.00 rows=3334 width=244)
(actual time=0.154..9.140 rows=50 loops=1)
Filter: (unique1 < 50)
Total runtime: 41.101 ms
(8 rows)
bench=# SET enable_mergejoin = off;
SET
bench=# EXPLAIN ANALYZE SELECT * FROM tenk1 t1, tenk2 t2 WHERE t1.unique1 < 50
AND t1.unique2 = t2.unique2;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------
Hash Join (cost=588.34..11841.35 rows=166701 width=488) (actual
time=9.028..70.453 rows=50 loops=1)
Hash Cond: ("outer".unique2 = "inner".unique2)
-> Seq Scan on tenk2 t2 (cost=0.00..445.00 rows=10000 width=244) (actual
time=0.007..11.846 rows=10000 loops=1)
-> Hash (cost=470.00..470.00 rows=3334 width=244) (actual
time=8.378..8.378 rows=0 loops=1)
-> Seq Scan on tenk1 t1 (cost=0.00..470.00 rows=3334 width=244)
(actual time=0.135..8.093 rows=50 loops=1)
Filter: (unique1 < 50)
Total runtime: 70.659 ms
(7 rows)
bench=# SET enable_hashjoin = off;
SET
bench=# EXPLAIN ANALYZE SELECT * FROM tenk1 t1, tenk2 t2 WHERE t1.unique1 < 50
AND t1.unique2 = t2.unique2;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------
Nested Loop (cost=0.00..633218.15 rows=166701 width=488) (actual
time=0.178..9.389 rows=50 loops=1)
-> Seq Scan on tenk1 t1 (cost=0.00..470.00 rows=3334 width=244) (actual
time=0.135..8.349 rows=50 loops=1)
Filter: (unique1 < 50)
-> Index Scan using tenk2_unique2 on tenk2 t2 (cost=0.00..189.16 rows=50
width=244) (actual time=0.009..0.011 rows=1 loops=50)
Index Cond: ("outer".unique2 = t2.unique2)
Total runtime: 9.552 ms
(6 rows)
--
Regards.
Anjan Kumar A.
MTech2, Comp Sci.,
www.cse.iitb.ac.in/~anjankumar
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