Question: I want to understand the
no_merge hint and understand when to use the no_merge
hint. Is it true that the no_merge hint has
nothing to do with a merge join?
Answer: You are correct, the no_merge
hint has nothing to do with a merge join (or a sort merge
join).
Rather, the no_merge is commonly used in
in-line views (a case where a sub-select exists in the FROM
clause of a SQL statement:
select
/*+ no_merge(MY_VIEW)
*/
stuff
from
(select
col1,
col2,
col3,
from view1) MY_VIEW,
. . . . In
the case above, the no_merge hint tells the SQL optimizer
not to do complex view merging
and to execute the
in-line view before executing the outer query components.
This will make the optimization phase of the SQL
statement run faster because there are less permutations of
the view.
In other words, the no_merge hint prevents
Oracle from re-writing the inner query (the in-line view)
and the outer query into a merged view, which may be less
efficient that addressing the inner query directly.
In a nutshell, the no_merge hint instructs the optimizer not
to combine the outer query and any inline view queries into
a single query.
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