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Home -> Community -> Mailing Lists -> Oracle-L -> 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_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 listby 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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