Bill,
I too would love to hear from better experts than me.
I also have implemented queries using db links.
In my rudimentary testing, I found that you need to create the view on the
remote machine to get any decent kind of performance, and then access that
view thru the db link. My guess as a result of my testing eas that Oracle
does a very poor job here. It looked to me like the data was brought across
the link, and then the where clause (meaning any table joins) was applied.
This will not work at all, performance wise, if the tables in question are
large.
On the other hand, if the view exists on the remote machine, the where
clause gets properly executed by the optimizer on that machine.
As for joining tables across database links - again - it did not perform
well for me. We ended up writing a cross-db audit/update process to get the
data we wanted to query into our local database to support our queries.
Anybody else have any better luck? I would love to hear more experienced
opinions
Hope this helps
Tom Mercadante
Oracle Certified Professional
-Original Message-
Sent: Wednesday, September 12, 2001 2:15 PM
To: Multiple recipients of list ORACLE-L
Hello,
Oracle 8.1.6 on Solaris 2.7.
Can any one provide me with information or a link regarding
performance of views across database links?
I am thinking of the case where instance A has a view defined
as SELECT FLD1,FLD2,... FROM TABLE_T1@DBLINK.
Consider the case when a view such as this is referenced within
a sql statement that joins this view with other local tables and
also contains some WHERE conditions which reference a field in
TABLE_T1.
Are the entire contents of TABLE_T1 transferred across the network,
then the joins and where conditions are processed within the local
instance? Or is the WHERE condition applied in the remote instance,
and only rows which satisfy the condition are returned across the network?
I was under the impression that the former was true. Did the behaviour
change in some previous version?
Thanks to any responders.
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Author: Mercadante, Thomas F
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