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https://issues.apache.org/jira/browse/CALCITE-2847?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16809449#comment-16809449
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Hongze Zhang commented on CALCITE-2847:
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Thanks for the review, [~julianhyde] and [~krisden]! But there is a tiny
issue[1] founded in the PR (thanks, [~zhenw]!), and I'll summarize the problem
by 2 phases:
# The parser is generated with correct lookahead code for `<CROSS> <JOIN>`
under global settings `LOOKAHEAD=2`;
# The parser is generated with incorrect lookahead code for `<CROSS> <JOIN>`
under global settings `LOOKAHEAD=1` and local lookahead hint `LOOKAHEAD(2)
Natural() JoinType()`.
After a little debug and investigation I see that JavaCC generates minimum
lookahead code[2] incorrectly for the expansion `<CROSS> <JOIN>` when we use
the global lookahead 1 and lookahead hint 2 for expansion `Natural()
JoinType()` (phase 2). Where we didn't meet the issue under the phase 1 because
JavaCC assigns lookahead 2 everywhere including the expansion `<CROSS> <JOIN>`.
For solving the "incorrect minimum lookahead code" problem, I've opened a PR[3]
for JavaCC project. And currently I am about to push this forward (maybe merge
after next 24 hours) by treating the LOOKAHEAD(3) as a necessary workaround,
since we already have a "+1" in previous disscussion. Also, any further review
will be much appreciated too.
[1] [https://github.com/apache/calcite/pull/1041#discussion_r264518804]
[2]
[https://github.com/zhztheplayer/javacc-calcite-generated-sources/blob/with-7.0.4/SqlParserImpl.java#L24929-L24933]
[3] [https://github.com/javacc/javacc/pull/87]
> Optimize global LOOKAHEAD for SQL parsers
> -----------------------------------------
>
> Key: CALCITE-2847
> URL: https://issues.apache.org/jira/browse/CALCITE-2847
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: Hongze Zhang
> Assignee: Hongze Zhang
> Priority: Major
> Labels: pull-request-available
> Fix For: 1.20.0
>
> Time Spent: 1.5h
> Remaining Estimate: 0h
>
> Currently global LOOKAHEAD is set to 2 for the built-in SQL parsers[1], we'd
> like to optimize LOOKAHEAD to 1 to enable performance enhancement if possible.
> [1]https://github.com/apache/calcite/blob/883666929478aabe07ee5b9e572c43a6f1a703e2/core/pom.xml#L304
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