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From "Ashish Thusoo (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-416) Get rid of backtrack in Hive.g
Date Thu, 16 Apr 2009 21:26:15 GMT

    [ https://issues.apache.org/jira/browse/HIVE-416?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12699873#action_12699873

Ashish Thusoo commented on HIVE-416:

I think for Identifier DOT Identifier we should probably treat it as a lexical rule rather
than a grammar rule. It will also make it much simpler to support optional aliasing with complex

Right now

select T.a.b FROM T


select  a.b FROM T

is very hard to handle in the SemanticAnalyzer as the grammar treats a as a table alias instead
of a complex column name.

About the comment on optional brackets - clearly these are optional in expressions. So how
do we support those expression e.g (a+b) and a+b are both valid sql expressoins if we cannot
support this without backtracking...

> 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.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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