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https://issues.apache.org/jira/browse/HIVE-6189?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13870516#comment-13870516
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Lefty Leverenz commented on HIVE-6189:
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Apparently the nonsupport of top-level UNION ALL never got documented, so for
starters that needs to be fixed in the wiki for Hive 0.12.0 and earlier. Then
we'll need documentation for this ticket, which could go in a release note or
directly into the wiki: [Union
Syntax|https://cwiki.apache.org/confluence/display/Hive/LanguageManual+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
>
>
> 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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