RE: Index scan and redundant sorting
- From: "Bobak, Mark" <Mark.Bobak@xxxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Wed, 25 Feb 2004 14:37:33 -0500
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@xxxxxxx]
Sent: Wednesday, February 25, 2004 2:34 PM
To: oracle-l@xxxxxxxxxxxxx
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
-----------------
SMALL_RANDOM_NUM
set autotrace traceonly explain
select small_random_num
from random_data
order by small_random_num;
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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