RE: Database link performance

[Michael Rosenblum]

Title: Message

Tom,

 

I tried both ways:

- creating the view on db_B and query it from db_A

- creating view of db_A using link to db_B

 

No difference.

 

Michael Rosenblum,

Dulcian Inc.

-----Original Message-----
From: Mercadante, Thomas F [mailto:[EMAIL PROTECTED]]
Sent: Thursday, June 06, 2002 1:07 PM
To: '[EMAIL PROTECTED]'
Cc: '[EMAIL PROTECTED]'
Subject: RE: Database link performance

Mike,

 

you created the view on the db_B machine and called it from db_A, right? 

 

 

"To Tom Mercadante: creating of a view, that does filtering, did not help. Absolutely the same results!"

 

Tom Mercadante
Oracle Certified Professional

-----Original Message-----
From: Michael Rosenblum [mailto:[EMAIL PROTECTED]]
Sent: Thursday, June 06, 2002 12:03 PM
To: Multiple recipients of list ORACLE-L
Subject: Re: Database link performance

Hi, all!

 

Thanks for your help and suggestions.  Here is some feedback on what I have found on  the performance problem of the querying data between two databases using DB links.

 

To John Kanagaraj: filter by ROWNUM does not make any difference – I have absolutely the same speed, when querying table with 5000 rows and filtering ‘where ROWNUM <5000’.

 

To Tom Mercadante: creating of a view, that does filtering, did not help. Absolutely the same results!

 

Just remind you the case:

declare

   v_record B%rowtype;

 

   cursor cB

   is

   select *

   from [EMAIL PROTECTED]

   where rownum < 5000;

begin

   for c in cB

   loop

     v_record :=c;

   end loop;

end;

 

My next step has been to analyze session-level statistics.

 

When using link we have physically two sessions – from a client to db_A (opened explicitly) and from db_A to db_B (opened by Oracle automatically). After execution of a script:
- in the session to db_A we have 10000 SQL*Net roundtrips to/from db_link

- in the session to db_B we have 10000 SQL*Net roundtrips to/from client.

 

But when I am running the same script directly (from client to db_B) – there is only ONE SQL*Net roundtrip to/from the client.

 

I asked Paul Dorsey to check it out with Tom Kyte at yesterday’s NYOUG meeting.

 

Tom told us that Oracle 8.* does single record fetches when using database links. In general, database links have been tuned to support database replication. It means, that ‘create table new_A as select … from table_A@db_B’ runs quickly but querying from new_A is quite slow.

 

The remaining question: has anybody tried to use database links in Oracle9i? Does the same single fetch limitation apply?

 

Thank you,

Michael Rosenblum

Dulcian Inc.