Hint That Disables Subquery Pull-up

Function

To optimize query logic, the optimizer usually pulls up subqueries for execution. However, sometimes the pulled up subqueries do not run much faster than others, and may even be slower due to enlarged search scope. In this case, you can specify the no merge hint to disable pull-up. This hint is not recommended in most cases.

Syntax

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no merge [(subquery_name)]

Description

subquery_name indicates the name of a subquery. It can also be a view or CTE name. The specified subquery will not be unnested during logic optimization. If subquery_name is not specified, the current query will not be unnested.

Example

Create tables t1, t2, and t3.

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create table t1(a1 int,b1 int,c1 int,d1 int);
create table t2(a2 int,b2 int,c2 int,d2 int);
create table t3(a3 int,b3 int,c3 int,d3 int);

The original statement is as follows:

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explain select * from t3, (select a1,b2,c1,d2 from t1,t2 where t1.a1=t2.a2) s1 where t3.b3=s1.b2;

In this query, you can use the following methods to disable the pull-up of subquery s1:

• Method 1:

  1	explain select /*+ no merge(s1) */ * from t3, (select a1,b2,c1,d2 from t1,t2 where t1.a1=t2.a2) s1 where t3.b3=s1.b2;

• Method 2:

  1	explain select * from t3, (select /*+ no merge */ a1,b2,c1,d2 from t1,t2 where t1.a1=t2.a2) s1 where t3.b3=s1.b2;

Outcome: