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From "Zheng Shao (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-416) Get rid of backtrack in Hive.g
Date Fri, 17 Apr 2009 05:40:15 GMT

    [ https://issues.apache.org/jira/browse/HIVE-416?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12700013#action_12700013
] 

Zheng Shao commented on HIVE-416:
---------------------------------

@Raghu: I just had a second thought on that approach. The new production you add is left-recursive
and it's not permitted in LL(k), but it's possible to use precedence rules to fix that.
However given all the change including the flattening, it seems to me that's too much work
with very little benefit - who care about the optional brackets, for usage it's exactly the
same.

For Venky's case, it's a separate problem. Venky's case is more like supporting "a" and "((((a))))".
We should be able to support it easily once we allow omitting the sub query alias. 

> Get rid of backtrack in Hive.g
> ------------------------------
>
>                 Key: HIVE-416
>                 URL: https://issues.apache.org/jira/browse/HIVE-416
>             Project: Hadoop Hive
>          Issue Type: Improvement
>          Components: Query Processor
>    Affects Versions: 0.4.0
>            Reporter: Zheng Shao
>            Assignee: Zheng Shao
>             Fix For: 0.4.0
>
>         Attachments: HIVE-416.1.1.patch, HIVE-416.1.patch
>
>
> Hive.g now still uses "backtrack=true". "backtrack" not only slows down the parsing in
case of error, it can also produce wrong syntax error messages (usually based on the last
try of the backtracking).
> We should follow http://www.antlr.org/wiki/display/ANTLR3/How+to+remove+global+backtracking+from+your+grammar
to remove the need of doing backtrack.

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