Question: I see
that I can use either the EXISTS or the IN clause for a
subquery. Which
is better, the EXISTS or the IN clause? Does using
EXISTS resul in a different execution plan? When
should I use the IN clause versus the EXISTS clause?
Answer: Most Oracle IN clause
queries involve a series of literal values, and when a table
is present a standard join is better. In most cases the
Oracle cost-based optimizer will create an identical
execution plan for IN vs EXISTS, so there is no difference
in query performance.
The Exists keyword evaluates true or false, but the IN
keyword will compare all values in the corresponding subuery
column. If you are using the IN operator, the SQL
engine will scan all records fetched from the inner query.
On the other hand, if we are using EXISTS, the SQL engine
will stop the scanning process as soon as it found a match.
The EXISTS subquery is used when we want to display all
rows where we have a matching column in both tables.
In most cases, this type of subquery can be re-written with
a standard join to improve performance.
select
book_key
from
book
where
exists (select book_key from sales);
The EXISTS clause is much faster than IN when the
subquery results is very large. Conversely, the IN clause is
faster than EXISTS when the subquery results is very small.
Also, the IN clause can't compare anything with NULL
values, but the EXISTS clause can compare everything with
NULLs.
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