Re: Index scan and redundant sorting
- From: Tanel Põder <tanel.poder.003@xxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 26 Feb 2004 05:16:20 +0200
MessageThat's also the reason why index rebuild requires sorting, controversary
to a myth that it doesn't...
Tanel.
----- Original Message -----
From: Bobak, Mark
To: oracle-l@xxxxxxxxxxxxx
Sent: Wednesday, February 25, 2004 9:37 PM
Subject: RE: Index scan and redundant sorting
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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