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Don Burleson Blog 







  Disabling sort merge and hash joins tips

Oracle Database Tips by Donald BurlesonApril 8, 2015

Question:  How do I disable sort merge and hash joins?

Answer:  Vadim Tropashko's  is the author of "SQL Design Patterns: The Expert Guide to SQL Programming", the definitive reference for SQL design patterns, a critical knowledge area for any Oracle developer.

Tropashko notes that the hash joins may not be the fastest table join method and he removes them  by unsetting the hash_join_enabled parameter and reviews the resulting nested loops table join method:

alter session set "_hash_join_enabled" = false;
alter session set "_optimizer_sortmerge_join_enabled" = false;

The particular query that we are studying in this section has 5 query blocks: 2 pairs of selects from the tables A and B, and one set operation. After this query is transformed, it would consist of only 3 query blocks which we might witness from the QBLOCK_NAME column of the PLAN_TABLE.

Execution plan for comparing the contents of two tables  with sort merge disabled

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