Nick Torenvliet wrote:
Hey all...
I am having a problem with sub-queries that I cannot trouble shoot.
I run query a:
select symbol from names where market like 'NYMEX' and name like 'natural
gas {%';
and get 168 names that I manually insert into query b:
select * from endOfDayData where endOfDayData.market like 'NYMEX' and
endOfDayData.symbol IN
(... big list clipped ...);
Running query b gives me a result set as follows:
| 2010-01-15 | NYMEX | NGZ22 | 8.9620 | 8.9680 | 8.9620 | 8.9680
| 0 |
+------------+--------+--------+---------+---------+---------+---------+--------+
86765 rows in set (4.46 sec)
I then because I want to generalize query b I continue by creating query c
as follows:
mysql> select * from endOfDayData where endOfDayData.market like 'NYMEX'
and endOfDayData.symbol IN (select names.symbol from names where
names.market like 'NYMEX' and names.name like 'natural gas {%');
Query c seems to have good syntax as neither the command line mysql
interface nor the gui spit it back but it literally takes forever to run;
I've waited at least twenty minutes and not got anything back. I'm running
Ubuntu 9.10 on an intel core i7 with 4GB RAM and 12GB swap... the process
monitor doesn't even flinch so I'm not thinking hardware here... why is the
sub-query running so slow?
Thanks for you help!!
Subqueries are also not indexed. Even if this is an independent
subquery, the optimizer will still need to scan the results of each
subquery for every line of the outer query.
Try rewriting this as a joinl
SELECT eod.*
from endOfDayData eod
INNER JOIN names n
on n.symbol = eod.symbol
and n.market like 'NYMEX'
and names.name like 'natural gas {%';
Try that and see what a difference it makes.
--
Shawn Green, MySQL Senior Support Engineer
Sun Microsystems, Inc.
Office: Blountville, TN
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