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From "Shengsheng Huang (JIRA)" <>
Subject [jira] [Created] (HIVE-3577) Subquery Unnesting
Date Mon, 15 Oct 2012 06:53:03 GMT
Shengsheng Huang created HIVE-3577:

             Summary: Subquery Unnesting
                 Key: HIVE-3577
             Project: Hive
          Issue Type: Sub-task
    Affects Versions: 0.10.0
            Reporter: Shengsheng Huang

Hive requires subquery to be used only in the FROM clause. But real-world queries can be much
more complex and subqueries could appear in WHERE, having and SELECT clauses. Many of such
subqueries can be transformed/unnested to various join operations. This is an umbrella id
to include all subquery unnesting work, specifically:

1. subquery in WHERE conditions, including subquery used as operand of IN, EXISTS, ISNULL
ALL/ANY/SOME, and etc.

2. correlated subquery (a correlated subquery is a subquery that refers to a column of a table
not in its FROM clause)
e.g. select * from t1 where exists ( select * from t2 where t1.b = t2.y );

3. scalar subquery (a subquery that returns exactly one column value from one row) 
e.g. select a,b,c,d,e,(select z from t2 where t2.y = t1.b and z != 99 ) from t1;

4. top-level subquery
(select * from t1) union all (select * from t2) union (all select * from t3 order by 1);


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