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Don Burleson Blog 







qb_name hint tips

Oracle Database Tips by Donald BurlesonMay 22, 2016

Question: What does the 10g qb_name hint do?  Can you describe when to use the qb_name hint?

Answer:  The cost-based optimizer tries to eliminate multiple select statements in a complex query by merging in-line views and subqueries into a single SQL statement.  In Oracle parlance, each in-line view and subquery is a separate and distinct "query block".

Solely for debugging purposes, Oracle has created the query block name (qb_name) hint to allow you to clearly see the individual query blocks in a complex SQL query that has subqueries (or in-line views).

For example, Oracle un-nests some correlated subqueries and replaces it with a single anti-join operation.  Ti see this, you can add a qb_name hint and then reference this nased query block in a no_unnest hint:

select /*+ no_unnest(@nested_query) */
not exists
select /*+ qb_name(nested_query) */
t1 t1_inner
where =

Oracle experts use the qb_name hint to plant "markers".  These markers have two purposes:

1 - The markers make reading complex execution plans (subqueries) easier to read.

2 - The qb_name hint allows hints to be applied to subqueries.

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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