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https://issues.apache.org/jira/browse/HIVE-416?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12699898#action_12699898
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Zheng Shao commented on HIVE-416:
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> 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...
We do support "(a+b)" and "a+b".
The problem is that there is no easy way of supporting both "((((a))), b)" and
"((((a)))), b". No matter what is k, it's not possible to determine whether the
first "(" is the optional bracket for the expression list, or just part of the
first expression.
I will need to go over the antlr book to know more about Semantic/Syntactic
predicate to know whether that is possible.
> Identifier DOT Identifier.
Treating it as a lexical rule won't allow both T.a.b and a.b. I am making a
first Identifier a TOK_TABLE_OR_COL. I will let SemanticAnalyzer to decide
whether it is a table name or column name. Not sure that should go into the
same transaction or not since it's a much bigger change.
> 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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