Undocumented behavior:
setting optimizer_index_cost_adj = 1 is simply begging the optimizer to use
any index, just kidding :}
I think it's simply b/c the total cost which is = cost of sequential read *
number of reported rows in the index (adjusted by optimizer_index_cost_adj)
is the lowest calculated one.
Regards,
Waleed
-----Original Message-----
Sent: Thursday, May 09, 2002 5:06 PM
To: Multiple recipients of list ORACLE-L
choice
Hi Johnathan,
here is the skinny:
db_file_multiblock_read_count = 64
number of rows=15m
blocks=251071
empty_blocks=0
db_block_size=16384
total plan cost=4924
tablescan cost = 4924
Jack
- Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
wrote:
>
> There seems to be a perfectly good theoretical
> reason
> for this. But it would be interesting to know:
> Number of blocks below HWM
> Setting for db_file_multiblock_read_count
> Maximum usable value for db_file_mbrc
> The cost given by Explain Plan for the tablescan
>
> before I confuse the issue further by expounding a
> hypothesis that may be totally misleading.
>
> Jonathan Lewis
> http://www.jlcomp.demon.co.uk
>
> Author of:
> Practical Oracle 8i: Building Efficient Databases
>
> Next Seminar - Australia - July/August
> http://www.jlcomp.demon.co.uk/seminar.html
>
> Host to The Co-Operative Oracle Users' FAQ
> http://www.jlcomp.demon.co.uk/faq/ind_faq.html
>
>
>
> -----Original Message-----
> To: Multiple recipients of list ORACLE-L
> <ORACLE-L_at_fatcity.com>
> Date: 09 May 2002 14:30
>
>
> |Good morning listers,
> |
> |Some weeks ago I noticed a query plan that was
> using a
> |bizarre index choice and opened a TAR. Here is that
> |query:
> |
> |select DSS_EMPR.V_CLAIM_EMPR_HX.AMT_COPAY from
> |DSS_EMPR.V_CLAIM_EMPR_HX where
> |DSS_EMPR.V_CLAIM_EMPR_HX.BATCH_NUM between
> |to_date('2000-01-01','yyyy-mm-dd') and
> |to_date('2000-01-31','yyyy-mm-dd');
> |
> |Here is the query plan:
> |
> | SELECT STATEMENT (all_rows) Cost
> |(4924,636953,12739060)
> |
> | 1 0 1 2
> | TABLE ACCESS (analyzed) DSS_EMPR
> T_CLAIM_EMPR_HX
> |(by index rowid) Cost (4924,636953,12739060)
> |
> | 2 1 1
> | BITMAP CONVERSION (to rowids)
> |
> | 3 2 1
> | BITMAP INDEX DSS_EMPR X_CLAIM_EMPR_HX_N01
> |(full scan)
> |
> |
> |The index x_claim_empr_hx_n01 is on the phmcy_gid
> |column of the t_claim_empr_hx table.
> |
> |Now, here is the crux of the matter: phmcy_gid is
> |referred to nowhere in the query, not in select nor
> in
> |where nor even in order by.
> |
> |Worked through the tar with Oracle and they advised
> |that Oracle can and will cost ALL indexes during a
> |plan parse and eval, so it became a matter of
> |discovering why the index was being incorrectly
> |costed.
> |
> |Remembered that our optimizer_index_cost_adj was
> set
> |to 1 (don't ask). When I upped this value to 2 or
> more
> |and reran the query, it returned the appropriate
> FTS
> |plan.
> |
> |hth,
> |
> |Jack Silvey
> |
>
>
> --
> Please see the official ORACLE-L FAQ:
> http://www.orafaq.com
> --
> Author: Jonathan Lewis
> INET: jonathan_at_jlcomp.demon.co.uk
>
> Fat City Network Services -- (858) 538-5051 FAX:
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Do You Yahoo!?
Yahoo! Shopping - Mother's Day is May 12th!
http://shopping.yahoo.com
--
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--
Author: Jack Silvey
INET: jack_silvey_at_yahoo.com
Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051
San Diego, California -- Public Internet access / Mailing Lists
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Author: Khedr, Waleed
INET: Waleed.Khedr_at_FMR.COM
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Received on Thu May 09 2002 - 17:53:20 CDT
