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) */
*
from
t1
where
not exists
(
select
/*+ qb_name(nested_query) */
null
from
t1 t1_inner
where
t1_inner.id = t1.id
);
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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