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Don Burleson Blog 







  Rewriting Oracle SQL MINUS operator with a NOT IN subquery tips

Oracle Database Tips by Donald BurlesonApril 4, 2015

Question:  How do I rewrite the Oracle SQL MINUS operator with a NOT IN subquery?

Answer:  Oracle query rewrite might come into play where a "set operation" (the minus operator) might be converted into a join by the CBO:

It confirms our intuition on the number of table scans and sorts. It also reveals that the time scanning a single table (about 100 msec) is small compared to the time spent when sorting each table (about 450 msec). Therefore we could try to aim lower than 2.4 sec of total execution time.

This is not the only possible execution plan for the symmetric difference query. Oracle has quite sophisticated query rewrite capabilities, and transforming set operation into join is one of them.

In our example, both minus operators could be rewritten as anti-joins. As of version 10.2, however, this transformation is not enabled by default, and used as part of view merging/ query unnesting framework only. It can be enabled with

alter session set "_convert_set_to_join"= true;

The other alternative is to rewrite the SQL query manually [replacing the minus operator with a NOT IN subquery] evidences about 30% improvement in execution time . . .

select *
from A
where (col1,col2,?) not in
   (select col1,col2,? from B)
   union all
select * from B
where (col1,col2,?) not in
   (select col1,col2,? from A);

The new execution statistics are very different and faster, using hash joins:

Execution plan for comparing the contents of two tables with NOT IN subquery

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