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https://issues.apache.org/jira/browse/HIVE-416?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12699840#action_12699840
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Namit Jain commented on HIVE-416:
---------------------------------
I had some questions.
1. Wont it be better to factor out the common part in a seperate rule instead
of providing the look-ahead.
For eg:
instead of:
alterStatement
options {k=5;}
@init { msgs.push("alter statement"); }
@after { msgs.pop(); }
: alterStatementRename
| alterStatementAddCol
| alterStatementDropPartitions
| alterStatementAddPartitions
| alterStatementProperties
| alterStatementSerdeProperties
;
wont it be better to factor out < ALTER TABLE identifier> in a common and then
have the remaining rules ?
2. On the same lines, I could not understand the reason for brackets around
SELECT TRANSFORM and
no brackets around MAP/REDUCE.
Instead of this:
selectClause
@init { msgs.push("select clause"); }
@after { msgs.pop(); }
:
KW_SELECT (KW_ALL | dist=KW_DISTINCT)?
selectList -> {$dist == null}? ^(TOK_SELECT selectList)
-> ^(TOK_SELECTDI selectList)
|
trfmClause ->^(TOK_SELECT ^(TOK_SELEXPR trfmClause) )
;
if we factor out:
KW_SELECT for the first part and the transform clause, brackets should become
optional.
Am I missing something here ?
> 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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