My information on this may be a bit outdated. I had read it somewhere that statistics are not to be generated for SYS tables. Oracle will handle the dictionary information access, irrespective of the optimizer mode being used. The setting of the optimizer_mode should affect the application queries being generated for other schemas. So if you are using CBO, setting the Optimizer_mode to first_rows looks more applicable than doing an alter session every time. I am not able to comment on the poor performance when accessing the dict.
rgds
amar
-----Original Message-----
From: Boivin, Patrice J [mailto:[EMAIL PROTECTED]]
Sent: Friday, December 14, 2001 11:00 PM
To: Multiple recipients of list ORACLE-L
Subject: FIRST_ROWS and the data dictionary
FYI,
I just logged a call with Oracle re. Designer 6.0 vs. all_cons_columns.
A developer noticed that whenever he queries that data dictionary table as
part of generating database from server models, the process slows down to a
crawl.
If the target doesn't have any constraints, no problem.
I am starting to wonder if it isn't because FIRST_ROWS was specified on this
database, most of the developers are developing forms (6i) so I set the
init.ora parameter accordingly.
The Oracle documentation however implies that the CBO only switches to rule
when there are no stats if optimizer_mode had been set to CHOOSE, if it's
either ALL_ROWS or FIRST_ROWS it tries to use the CBO against the data
dictionary anyway. How the CBO can run when there are no stats probably
means it defaults to full table scans (?).
OPTIMIZER_MODE = { all_rows | first_rows | rule | choose }
The OPTIMIZER_MODE parameter specifies the approach and mode of the
optimizer for your session.
See Also: Oracle8i Concepts <../../server.817/a76965/toc.htm> and Oracle8i
Performance Guide and Reference <../../server.817/a76992/toc.htm> for
information on how to choose a goal for the cost-based approach based on the
characteristics of your application
* all_rows specifies the cost-based approach and optimizes for best
throughput.
* first_rows specifies the cost-based approach and optimizes for best
response time.
* rule specifies the rule-based approach. (The rule-based optimizer
does not use function-based indexes.)
* choose causes the optimizer to choose an optimization approach based
on the presence of statistics in the data dictionary.
Is this correct? If so, then I suppose I should set the init.ora parameter
back to optimizer_mode=choose, and tell each developer to put an alter
session statement on their clients to alter their sessions automatically to
first_rows. The developer using designer will then be able to run it in a
timely manner.
Please tell me if my suspicion is correct.
If correct, it begs the question: why are first_rows and all_rows available
for the init.ora file???
Regards,
Patrice Boivin
Systems Analyst (Oracle Certified DBA)
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