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I have a fairly complicated query. I have written two different query to accomplish my goal. First one is using WITH, other one is using global temporary tables.

First One:

WITH A
(
 KNO 
 ..
 )
 , B
 (
 KNO
 ...
 )
 , C
 (
 KNO
 ...
 )
 SELECT * from 
 A INNER JOIN B 
 on A.KNO = B.KNO
 INNER JOIN C
 on B.KNO = C.KNO

Second One:

Truncate Table tempA;
Truncate Table tempB;
Truncate Table tempC;

INSERT INTO tempA SELECT -- Same select which constructs WITH A
INSERT INTO tempB SELECT -- Same select which constructs WITH B
INSERT INTO tempC SELECT -- Same select which constructs WITH C

 SELECT * from 
 tempA A INNER JOIN tempB B 
 on A.KNO = B.KNO
 INNER JOIN tempC C
 on B.KNO = C.KNO

They are on par. What I mean is I observe same query time with both of them. But When I add KNO index to tempA,tempB,tempC temporary tables. Second one skyrockets.
What I mean is : It gets faster, a lot. I query for 1 year of values. using WITH: about 58 minutes. using Temp Table with INDEX: about 30 minutes.

Query times for 4 months of values are given below.

Using WITH

DENEME@DENEME   Finished    15:37:02    15:40:38    03:36 mins  Select  500 WITH A

Total:216000 msecs

Using Temporary Tables + Index

DENEME@DENEME   Finished    15:41:54    15:41:55    1 sec   Select  500 WITH D
DENEME@DENEME   Finished    15:41:38    15:41:38    665 msecs   Insert  34660   INSERT INTO C
DENEME@DENEME   Finished    15:41:21    15:41:35    14 secs Insert  34660   INSERT INTO B 
DENEME@DENEME   Finished    15:41:17    15:41:17    109 msecs   Insert  1804    INSERT INTO  A
DENEME@DENEME   Finished    15:41:12    15:41:12    33 msecs    Unknown 0   TRUNCATE TABLE C
DENEME@DENEME   Finished    15:41:12    15:41:12    39 msecs    Unknown 0   TRUNCATE TABLE B
DENEME@DENEME   Finished    15:41:11    15:41:11    203 msecs   Unknown 0   TRUNCATE TABLE A

Total: 16049 msecs

My question is :

Can I give hint to oracle to add index when it is constructing WITH temporary tables?

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"Second one skyrockets" you mean it gets slower or it gets faster? –  Jack Douglas Nov 24 '11 at 12:53
    
It gets faster, a lot. I query for 1 year of values. using WITH: about 58 minutes. using Temp Table with INDEX: about 30 minutes. I also added this information to question. –  Atilla Ozgur Nov 24 '11 at 13:21
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2 Answers

up vote 3 down vote accepted

No, I don't think defining an index for a common table expression is possible.

You can experiment with the materialize hint, but I don't think you'll get anything faster than the solution with (indexed) temporary tables.

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GTTs still generate some undo (though minimal in the case of inserts), which might not be the case with a materialized CTE (though at least in some cases/versions internally Oracle creates GTTs for materialized CTEs!) –  Jack Douglas Nov 24 '11 at 12:58
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Can I give hint to oracle to add index when it is constructing WITH temporary tables?

  • A CTE or 'with' clause does not necessarily mean Oracle 'constructs temporary tables' - this is SQL, you are generally telling the database what you want, rather than how to go about getting it
  • there is a hint to ask the CBO to 'construct a temporary table - but hints are a last resort - if the CBO is choosing the wrong plan, the first thing to do is find out why (eg: bad stats)
  • you are joining the whole of tempA, tempB and tempC so the positive impact of an index will be minimal at best (in most cases the CBO will choose a hash join so you will not even save a sort) - ie don't try and make it faster magically by adding indexes
  • are you comparing apples with apples? the 'query time' for the case with temporary tables needs to include the three insert statements as well as the select
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From execution plan: oracle seems to construct view or temporary tables. 1 Every row in the table SYS.SYS_TEMP_0FD9FC87B_B1CA4F26 is read. 2 A view definition was processed, either from a stored view XXXX. or as defined by steps 1. –  Atilla Ozgur Nov 24 '11 at 12:53
    
yes, it might - but with a different execution plan it might not. Materializing is sometimes chosen if the CTE is used multiple times - does your real query do that? –  Jack Douglas Nov 24 '11 at 13:01
    
Yes, Information in A and B is queried in C for aggregation purposes. Also hint /*+ materialize */ give a small (%1-%5) boost to query time. –  Atilla Ozgur Nov 24 '11 at 13:20
    
Ok, you need to post either your real queries or a test simplified test case that demonstrates the same issue for us to be able to help further –  Jack Douglas Nov 24 '11 at 13:49
    
have you checked the explain plan has sensible estimated cardinalities for each step? –  Jack Douglas Nov 24 '11 at 14:18
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