Question for the query engine experts

I'm wondering if it's considered normal for a complex query with multiple CTE to work fast when broken
up into multiple queries that output to work files.

An example:

with t1 as (select ... from table1 ...)
,t2 as (select ... from table2 ...)
, t3 as(select ... from t1 .... UNION select ... from t2 ...)
, t4 as (select .... from table3 ... left outer join table4 ... left outer join table4 )
, t5 as (select ... from t4 UNION ALL ... select ... from t4 UNION ALL ... select ... from t4)
select ... from ( t5 inner join t3 ...) left outer join table5 .... left outer join table6 ... left
outer join table7

works much faster when I break the query into three parts with two physical temporary files:

with t1 as (select ... from table1 ...)
,t2 as (select ... from table2 ...)
select ... from t1 .... UNION select ... from t2 ...
---> output to tmptable3

with t4 as (select .... from table3 ... left outer join table4 ... left outer join table4 )
select ... from t4 UNION ALL ... select ... from t4 UNION ALL ... select ... from t4
---> output to tmptable5


select ... from ( tmptable5 inner join tmptable3 ...) left outer join table5 .... left outer join
table6 ... left outer join table7


It would seem to me that the query engine should be faster at creating virtual tables than writing to
physical tables.
In fact, the CPU processing time is close for the two methods:
1 query - 209 seconds
3 querys w/tmp files - 225 seconds.

But clock time was quite a bit different:
1 query - 90min
3 querys w/tmp files - 20min.

All the queries were run via Query Manager as batch jobs on a v5r2 box.

Thoughts or comments from anyone as to why physical temporary tables were quicker performing than
embedded CTE's?

If this always the case? Or does the CTE version possibly need indexes that the physical temporary
table one doesn't?

Thanks!
Charles Wilt








Charles Wilt

Software Engineer

CINTAS Corporation - IT 92B

513.701.1307

wiltc@xxxxxxxxxx <mailto:wiltc@xxxxxxxxxx>



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