One on one teen chats free Updating multiple rows in oracle

For the fifth sub query in the where clause, I guess the whole of the restof the query will be the outer query ?

October 03, 2001 - am UTC 1) yes more or less -- the ANSWER will be the same (16,886 rows processed).

Both of not in and not exists can be very efficient when there are no nulls (and not in WITH THE CBO is pretty good -- using an "anti join" -- see the design/tuning for performance guide for details on that). ) " do you mean not exists should be substituted for not in when there are nulls ??????? A not in when the subquery cannot contain NULLS can be alot like an IN (processed as an ANTI-JOIN instead of a JOIN). COM analyze table small compute statistics 2 for table 3 for all indexes 4 for all indexed columns; Table analyzed. Also take care to note that this query processed 0 rows - no data found. COM Select * from big b0 2 where object_id IS NOT NULL 3 and object_id NOT IN ( select object_id 4 from small ) 5 / 16886 rows selected. That is, it returns rows that fail to match (NOT IN) the subquery on the right side.

updating multiple rows in oracle-15

But the table T1 is relatively small and executing ( select null from t2 where y = x.x ) is very very fast (nice index on t2(y)).Then the exists will be faster as the time to full scan T1 and do the index probe into T2 could be less then the time to simply full scan T2 to build the subquery we need to distinct on.If your goal is the FIRST row -- exists might totally blow away IN. to sum it up finally , can you give us examples of when the use of 'not in' is appropriate and when 'not exists' is appropriate. COM select * from big 2 where NOT EXISTS (select null 3 from small 4 where small.object_id = big.object_id) 5 / 16886 rows selected. COM select * from big 2 where object_id NOT IN ( select object_id 3 from small ) 4 / no rows selected Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=7 Card=845 Bytes=81965) 1 0 FILTER 2 1 TABLE ACCESS (FULL) OF 'BIG' (Cost=7 Card=845 Bytes=81965) 3 1 TABLE ACCESS (FULL) OF 'SMALL' (Cost=1 Card=1 Bytes=3) Statistics ---------------------------------------------------------- 0 recursive calls 202743 db block gets 84706 consistent gets 0 physical reads 0 redo size 862 bytes sent via SQL*Net to client 319 bytes received via SQL*Net from client 1 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 0 rows processed Wow -- what happened here? The hash anti join is similar: [email protected] Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=145 Card=16885 Bytes=1857350) 1 0 HASH JOIN (ANTI) (Cost=145 Card=16885 Bytes=1857350) 2 1 TABLE ACCESS (FULL) OF 'BIG' (Cost=7 Card=16894 Bytes=1638718) 3 1 VIEW OF 'VW_NSO_1' (Cost=1 Card=9 Bytes=117) 4 3 INDEX (FULL SCAN) OF 'SMALL_IDX' (UNIQUE) (Cost=1 Card=9 Bytes=36) Statistics ---------------------------------------------------------- 0 recursive calls 15 db block gets 237 consistent gets 31 physical reads 0 redo size 2350779 bytes sent via SQL*Net to client 125305 bytes received via SQL*Net from client 1127 SQL*Net roundtrips to/from client 3 sorts (memory) 0 sorts (disk) 16886 rows processed 2b) see 1b) 3b) see 1b) where the NOT IN returns 0 rows, but NOT EXISTS returns 16k -- they are NOT the same when NULLs are involved.If you are a batch process (and hence getting to the LAST row is vital) .... The point is: be aware of their differences, try them both when tuning, understand conceptually what they do and you'll be able to use them to maximum effect. let me try to sum up what you are saying -- in and exists can be substitutes for each other, and we have to use one in the place of other depending upon the rowns returned by the outer and inner query, where as 'not in' and 'not exists' are not substitutes for each other , and they are not related in any way. October 01, 2001 - pm UTC you got the first part right (in/exists) the second part is too strong. NOT IN is different then NOT exists but NOT EXISTS and NOT IN are the same when the subquery you use in the NOT IN does not contain NULLS. And how is not exists better over not in when there are nulls. What do you mean by saying "and many people substitute a not exists for it (not realzing the ANSWER just changed!!! As it stands now -- lets see what happens with NOT IN and NOT EXISTS: [email protected] Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=7 Card=845 Bytes=81965) 1 0 FILTER 2 1 TABLE ACCESS (FULL) OF 'BIG' (Cost=7 Card=845 Bytes=81965) 3 1 INDEX (UNIQUE SCAN) OF 'SMALL_IDX' (UNIQUE) Statistics ---------------------------------------------------------- 305 recursive calls 15 db block gets 18278 consistent gets 0 physical reads 0 redo size 1961046 bytes sent via SQL*Net to client 125305 bytes received via SQL*Net from client 1127 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 16886 rows processed So, for each row in BIG it did an INDEX PROBE into SMALL to see if that row existed or not. 18k consistent reads (high) but not as high as: [email protected] Well, for each row in BIG -- we FULL SCANNED small -- yes, for each row, we full scanned small -- we full scanned small some 16,000 times (imagine if small weren't so small). Optimizing Anti-Joins and Semi-Joins An anti-join returns rows from the left side of the predicate for which there is no corresponding row on the right side of the predicate.So, when is where exists appropriate and in appropriate?

Lets say the result of the subquery ( select y from T2 ) is "huge" and takes a long time.

All of the necessary code is there -- it is very easy to do these sorts of tests. The "select * from big where object_id in ( select object_id from small )" will sort BIG once and SMALL once and join them (sort merge join) in all likelyhood.

The "select * from big where exists ( select null from small where small.object_id = big.object_id )" will tend to FULL SCAN big once and FOR EVERY ROW in big -- FULL SCAN small.

SELECT * FROM dept WHERE EXISTS (SELECT * FROM emp WHERE dept.ename = emp.ename AND emp.bonus 5000); What difference does it make when there is an index on the bonus column.

A semi join is already being executed in the above query, right ?

Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=269 Card=16885 Bytes=1857350) 1 0 MERGE JOIN (ANTI) (Cost=269 Card=16885 Bytes=1857350) 2 1 SORT (JOIN) (Cost=267 Card=16894 Bytes=1638718) 3 2 TABLE ACCESS (FULL) OF 'BIG' (Cost=7 Card=16894 Bytes=1638718) 4 1 SORT (UNIQUE) (Cost=3 Card=9 Bytes=117) 5 4 VIEW OF 'VW_NSO_1' (Cost=1 Card=9 Bytes=117) 6 5 INDEX (FULL SCAN) OF 'SMALL_IDX' (UNIQUE) (Cost=1 Card=9 Bytes=36) Statistics ---------------------------------------------------------- 0 recursive calls 19 db block gets 237 consistent gets 236 physical reads 0 redo size 1863304 bytes sent via SQL*Net to client 125305 bytes received via SQL*Net from client 1127 SQL*Net roundtrips to/from client 4 sorts (memory) 1 sorts (disk) 16886 rows processed 1.