Why is a "select * from table1 where field in (select field from table2
where condition )"
is so dramatically bad compared to:
"select * from table1, table2 where table1.field = table2.field and
condition"
I can't understand why the first query isn't optimized better than the
second one. The 'in' query forces a full table scan (it shouldn't) and
the second one uses the indexes. Does anyone know why?
I know I am no SQL guru, but my gut tells me that the 'in' operator
should be far more efficient than a join.
Here are the actual queries:
cdinfo=# explain select trackid from zsong where muzenbr in (select
muzenbr from ztitles where title = 'Mulan') ;
NOTICE: QUERY PLAN:
Seq Scan on zsong (cost=100000000.00..219321449380756.66 rows=2193213
width=4)
SubPlan
-> Materialize (cost=100000022.50..100000022.50 rows=10 width=4)
-> Seq Scan on ztitles (cost=100000000.00..100000022.50
rows=10 width=4)
cdinfo=# explain select trackid from zsong, ztitles where
ztitles.muzenbr = zsong.muzenbr and title = 'Mulan' ;
NOTICE: QUERY PLAN:
Merge Join (cost=0.00..183664.10 rows=219321 width=12)
-> Index Scan using zsong_muznbr on zsong (cost=0.00..156187.31
rows=2193213 width=8)
-> Index Scan using ztitles_pkey on ztitles (cost=0.00..61.50
rows=10 width=4)
cdinfo=# \d zsong
Table "zsong"
Attribute | Type | Modifier
-----------+-------------------+-------------------------------------------
muzenbr | integer |
disc | integer |
trk | integer |
song | character varying |
trackid | integer | not null default
nextval('trackid'::text)
artistid | integer |
acd | character varying |
Indices: zsong_muznbr,
zsong_pkey
cdinfo=# \d ztitles
Table "ztitles"
Attribute | Type | Modifier
------------+-------------------+----------
muzenbr | integer | not null
artistid | integer |
cat2 | character varying |
cat3 | character varying |
cat4 | character varying |
performer | character varying |
performer2 | character varying |
title | character varying |
artist1 | character varying |
engineer | character varying |
producer | character varying |
labelname | character varying |
catalog | character varying |
distribut | character varying |
released | character varying |
origrel | character varying |
nbrdiscs | character varying |
spar | character varying |
minutes | character varying |
seconds | character varying |
monostereo | character varying |
studiolive | character varying |
available | character(1) |
previews | character varying |
pnotes | character varying |
acd | character varying |
Index: ztitles_pkey
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