Yes, explain plan will become increasingly unreliable as the cbo takes more
and more factors and current conditions current into account. Some of the
factors that can change the outcome of a parse from session to session are:
for Oracle 8
- different session parameters (db_file_multiblock_read_count,
hash_multiblock_io_count, sort_area_size, hash_area_size)
for Oracle 9i additionally
- you can let Oracle dynamically set the sort_area_size and other memory
parameters so you have a moving target now
- bind variable peeking - the first parse determines the plan for all
following sql depending on its bind value
- system statistics in effect at the time of parse. If the system
statistics get changed, existing plans do not get invalidated, but if you
do an explain the cbo will use the current values
- dynamic sampling where the optimizer tries to improve on its estimates by
sampling predicate values at the time of parsing.
from comments I heard, it will get "worse" (as far as explain differing
from reality is concerned) with Oracle 10. The optimizer will try and learn
from past executions of a sql and modify the plan if appropriate.
At 08:35 AM 3/5/2003 -0800, you wrote:
>Just had a fellow tell me that explain plan in completely unreliable in 9.2
>and getting accurate results requires direct SGA access on executing SQL (he
>is working in a RAC environment). They are running Precise, a good product,
>but this sounds like something a sales person told him. I can only recall
>that occasionally the plan executed is not the plan you see in explain plan.
>Anyone know the truth of this issue?
Wolfgang Breitling
Centrex Consulting Corporation
http://www.centrexcc.com
********************
This email communication is intended as a private communication for the sole use of the primary addressee and those individuals listed for copies in the original message. The information contained in this email is private and confidential and if you are not an intended recipient you are hereby notified that copying, forwarding or other dissemination or distribution of this communication by any means is prohibited. If you are not specifically authorized to receive this email and if you believe that you received it in error please notify the original sender immediately. We honour similar requests relating to the privacy of email communications.
Cette communication par courrier électronique est une communication privée à l'usage exclusif du destinataire principal ainsi que des personnes dont les noms figurent en copie. Les renseignements contenus dans ce courriel sont confidentiels et si vous n'êtes pas le destinataire prévu, vous êtes avisé, par les présentes que toute reproduction, tout transfert ou toute autre forme de diffusion de cette communication par quelque moyen que ce soit est interdit. Si vous n'êtes pas spécifiquement autorisé à recevoir ce courriel ou si vous croyez l'avoir reçu par erreur, veuillez en aviser l'expéditeur original immédiatement. Nous respectons les demandes similaires qui touchent la confidentialité des communications par courrier électronique.
