Shengsheng Huang created HIVE-3577:
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Summary: Subquery Unnesting
Key: HIVE-3577
URL: https://issues.apache.org/jira/browse/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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