| Oracle FAQ | Your Portal to the Oracle Knowledge Grid | |
 |
 | |||
| Oracle FAQ | Your Portal to the Oracle Knowledge Grid | |
|
| |||
Home -> Community -> Mailing Lists -> Oracle-L -> RE: type of join in sql
The optimizer will use the availability of indexes in deciding which type of
join to use.
no indexes = merge join
indexes = nested loops, although the optimizer may dynamically choose to
perform a hash join.
Your tweaking of the join conditions is what causes the different joins to be used.
HTH p.s. See the "Database Performance Guide and Reference" section of the docs. for more info.
-----Original Message-----
Sent: Tuesday, July 23, 2002 9:59 PM
To: Multiple recipients of list ORACLE-L
can some body shed some light on how does the optimizer decides to choose
the kind of join i.e. nested loop, sort merge or hash join . In one of
queries if i tweak the join condition it changes the type of join and start
using index , otherwise it doesn't .
I thought it depends on statistics .. if optimize finds that are more
qualifying records in inner table then it will prefer to go for sort merge
and will do full scan of inner table , but if it thinks there are less
records in inner table it will user nested loop . am I correct ?
TIA ,
bp
--
Please see the official ORACLE-L FAQ: http://www.orafaq.com
--
Author: Richard Huntley
INET: rhuntley_at_mindleaders.com
Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051 San Diego, California -- Public Internet access / Mailing Lists --------------------------------------------------------------------To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). Received on Wed Jul 24 2002 - 09:13:25 CDT
 |
 |