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Home -> Community -> Mailing Lists -> Oracle-L -> RE: type of join in sql
It uses more then presence of indexes. It also looks at the sort_area_size
and hash_area_size and hash...mutlblock... and even db_file..multi.. to
determine how expensive/inexpensive a hash join might be versus a nested
loops join. You can force a specific join type by using hints - there are
hints for each type of join. Consider how much data, how much data would be
joined from each table, whether you are running in parallel or not. Oh
yeah, the CBO will consider parallelism if you have it setup I believe which
may cause it to choose a different type of join.
-----Original Message-----
Sent: Wednesday, July 24, 2002 10:13 AM
To: Multiple recipients of list ORACLE-L
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
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Author:
INET: Paula_Stankus_at_doh.state.fl.us
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