Table aliases are ambiguous
---------------------------
Key: HIVE-1395
URL: https://issues.apache.org/jira/browse/HIVE-1395
Project: Hadoop Hive
Issue Type: Bug
Reporter: Adam Kramer
Consider this query:
SELECT a.num FROM (
SELECT a.num AS num, b.num AS num2
FROM foo a LEFT OUTER JOIN bar b ON a.num=b.num
) a
WHERE a.num2 IS NULL;
...in this case, the table alias 'a' is ambiguous. It could be the outer table
(i.e., the subquery result), or it could be the inner table (foo).
In the above case, Hive silently parses the outer reference to a as the inner
reference. The result, then, is akin to:
SELECT foo.num FROM foo WHERE bar.num IS NULL. This is bad.
The bigger problem, however, is that Hive even lets people use the same table
alias at multiple points in the query. We should simply throw an exception
during the parse stage if there is any ambiguity in which table is which, just
like we do if the column names are ambiguous.
Or, if for some reason we need people to be able to use 'a' to refer to
multiple tables or subqueries, it would be excellent if the exact parsing
structure were made clear and added to the wiki. In that case, I will file a
separate bug JIRA to complain about how it should be different. :)
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