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Home -> Community -> Mailing Lists -> Oracle-L -> RE: Index scan and redundant sorting
Here is an article hosted on the cooperative FAQ on Jonathan's site that
demonstrates that Oracle sorts on an index rebuild and that an index rebuild
will sometimes perform a full table scan rather than read the index:
When I rebuild an index, I see Oracle doing a sort. Why should this be necessary, why doesn't it simply read the existing index ?<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
http://www.jlcomp.demon.co.uk/faq/ind_rebuild.html <http://www.jlcomp.demon.co.uk/faq/ind_rebuild.html>
-----Original Message-----
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org]On
Behalf Of Tanel Põder
Sent: Wednesday, February 25, 2004 10:16 PM
To: oracle-l_at_freelists.org
Subject: Re: Index scan and redundant sorting
That's also the reason why index rebuild requires sorting, controversary to a myth that it doesn't...
Tanel.
Dan,
Only an INDEX FULL SCAN (walks the tree, does single block reads) provides
sorted output.
An INDEX FAST FULL SCAN (does not walk tree structure, does multi-block
reads, discards branch blocks) does NOT provide sorted output.
-Mark
Mark J. Bobak
Oracle DBA
ProQuest Company
Ann Arbor, MI
"Imagination was given to man to compensate him for what he is not, and a
sense of humor was provided to console him for what he is." --Horace
Walpole
-----Original Message-----
From: Daniel Fink [mailto:Daniel.Fink_at_Sun.COM]
Sent: Wednesday, February 25, 2004 2:34 PM
To: oracle-l_at_freelists.org
Subject: Index scan and redundant sorting
A query (with an order by) is able to satisfy it's column list by scanning an index. This scan will return the rows in sorted order, but the query still executes a sort (confirmed by 10046 trace). Should not the result set from the fast full scan be correctly ordered? This would make the sort redundant, but very expensive in terms of response time.
Table:
random_data
Name Null? Type ------------------- -------- ------------------- REC_NO NOT NULL NUMBER INSERT_TEXT VARCHAR2(200) INSERT_DATE DATE LARGE_RANDOM_NUM NUMBER SMALL_RANDOM_NUM NOT NULL NUMBER <--- COLUMN OF INTEREST ROWID_BLOCKNUM NUMBER ROWID_ROWNUM NUMBER
select column_name
from user_ind_columns
where index_name = 'IX_RD_SMALL_RN'
COLUMN_NAME
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=7477 Card=1000000 Bytes=2000000)
1 0 SORT (ORDER BY) (Cost=7477 Card=1000000 Bytes=2000000) <------ Is this sort needed?
2 1 INDEX (FAST FULL SCAN) OF 'IX_RD_SMALL_RN' (NON-UNIQUE) (Cost=722 Card=1000000 Bytes=2000000)
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