Concatenation and Union all

Question

Hi,
From Oracle 9i performance guide:
"Concatenation is useful for statements with different conditions combined with an OR clause"
"If a query contains a WHERE clause with multiple conditions combined with OR operators, then the optimizer transforms it into an equivalent compound query that uses the UNION ALL set operator, if this makes the query execute more efficiently"
Can anyone tell me when concatenation is better than union and vice versa?

Answer 1

skde wrote:
Hi,
From Oracle 9i performance guide:
"Concatenation is useful for statements with different conditions combined with an OR clause"
"If a query contains a WHERE clause with multiple conditions combined with OR operators, then the optimizer transforms it into an equivalent compound query that uses the UNION ALL set operator, if this makes the query execute more efficiently"
Can anyone tell me when concatenation is better than union and vice versa?Concatenation is not exactly the same as UNION ALL (the ALL is important, by the way), as it has to eliminate repetitions of data. You may rewrite an 'OR' query as a UNION ALL, concatenation is simply Oracle's strategy for doing this. There's an example on my blog which might help you understand the issues: http://jonathanlewis.wordpress.com/2007/02/26/subquery-with-or/
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk
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