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Home -> Community -> Mailing Lists -> Oracle-L -> Re: Why does this query need a table access by rowid?
You are granting too much intelligence to the CBO.
The index is on a virtual column.
The virtual column is just like any other column that holds a value.
The optimizer does not "know" anything about what that value represents, it merely knows that if it executes a function that is stored against the column definition then it may, or may not, find that value in the column.
Regards
Jonathan Lewis
http://www.jlcomp.demon.co.uk/faq/ind_faq.html The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/seminar.html Optimising Oracle Seminar - schedule updated July 20th
Hi,
I've got a table, DOCUMENTS, with lots of columns (and lots of rows). DOC_ID is the PK, and DOC_HOLD_STATUS is another column in the table, which is NOT NULL and will always have 'Y' or 'N'.
Now, I've got a query where, given the DOC_ID, I want to determine if a particular document is on hold. Note that DOCUMENTS contains on the order of 170M rows, of which approximately 200k rows are flagged as DOC_HOLD_STATUS=3D'Y', or, docs that are on hold.
So, given the large disparity in the number of docs on hold vs. not on
hold, I created a function-based index defined as:
Create DOC_ON_HOLD on
DOCUMENTS(DECODE(DOC_HOLD_STATUS,'Y',DOC_ID,NULL));
Now, that gives me a convenient index of just the docs that are on hold.
(And with only around 200k DOC_IDs, it's *much* smaller than if I just
had an index on DOC_HOLD_STATUS, which makes it a lot more cache
friendly.)
So, finally, I get to the problem.
When I execute this query:
SELECT /*+ index(doc doc_on_hold) */ DOC_ID=20
FROM DOCUMENTS doc=20
WHERE DECODE(DOC_HOLD_STATUS,'Y',DOC_ID,NULL) =3D :B1;
I get this execution plan:
Execution Plan
0 SELECT STATEMENT Optimizer=3DCHOOSE (Cost=3D1 Card=3D1430947 Bytes=3D10016629)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'DOCUMENTS' (Cost=3D1 Card=3D1430947 Bytes=3D10016629)
2 1 INDEX (RANGE SCAN) OF 'DOC_ON_HOLD' (NON-UNIQUE) (Cost=3D3 Card=3D1430947)
So, the index is recognized, and that's great. My question is, why is
the TABLE ACCESS BY ROWID required?
Since only the DOC_ID is in the select list, why the table access?
Thanks,
-Mark
PS Granted, this is an efficient query, and the elimination of the table access amounts to one less consistent get. However, this is a VERY heavily hit query, and reducing 4 or 5 consistent gets to 3 or 4 could potentially be a big savings on a query that gets executed as much as this one does.
Mark J. Bobak
Oracle DBA
ProQuest Company
Ann Arbor, MI
"Post Hoc Ergo Propter Hoc"
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