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https://issues.apache.org/jira/browse/HIVE-6189?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13871508#comment-13871508
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Navis commented on HIVE-6189:
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I'm pretty sure Harish is more proficient than me for this kind of works. Good
to hear that hive supports top level union!
> Support top level union all statements
> --------------------------------------
>
> Key: HIVE-6189
> URL: https://issues.apache.org/jira/browse/HIVE-6189
> Project: Hive
> Issue Type: Bug
> Reporter: Gunther Hagleitner
> Assignee: Gunther Hagleitner
> Attachments: HIVE-6189.1.patch, HIVE-6189.2.patch, HIVE-6189.3.patch
>
>
> I've always wondered why union all has to be in subqueries in hive.
> After looking at it, problems are:
> - Hive Parser:
> - Union happens at the wrong place (insert ... select ... union all select
> ...) is parsed as (insert select) union select.
> - There are many rewrite rules in the parser to force any query into the a
> from - insert -select form. No doubt for historical reasons.
> - Plan generation/semantic analysis assumes top level "TOK_QUERY" and not top
> level "TOK_UNION".
> The rewrite rules don't work when we move the "UNION ALL" recursion into the
> select statements. However, it's not hard to do that in code.
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