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Concatenation and Union all

skdeFeb 28 2010 — edited Feb 28 2010
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?
This post has been answered by Jonathan Lewis on Feb 28 2010
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Locked on Mar 28 2010
Added on Feb 28 2010
#9i, #dba, #general-database-discussions, #performance
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