Julian Hyde created CALCITE-5208:
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Summary: Reduce size of generated parser by optimizing lookahead
Key: CALCITE-5208
URL: https://issues.apache.org/jira/browse/CALCITE-5208
Project: Calcite
Issue Type: Bug
Reporter: Julian Hyde
We could reduce the size of generated parser by optimizing lookahead.
The main problem is identifiers and non-reserved keywords. Every point in the
grammar where the parser needs to choose between an identifier and something
else the parser needs to do lookahead, so it generates a {{switch}} expression.
But that {{switch}} contains not just the identifier tokens ({{IDENTIFIER}},
{{UNICODE_QUOTED_IDENTIFIER}} and 5 others) but about 300 non-reserved keywords.
Switches like this, which are typically 300 to 460 lines long, occur 53 times
in the current parser. That's 24,000 lines of code, and the entire parser is
38,000 lines of code. And there are several parsers (core, babel, server, test).
{noformat}
$ find . -name SqlParserImpl.java
./core/build/javacc/javaCCMain/org/apache/calcite/sql/parser/impl/SqlParserImpl.java
$ wc
core/build/javacc/javaCCMain/org/apache/calcite/sql/parser/impl/SqlParserImpl.java
38242 86680 989569
core/build/javacc/javaCCMain/org/apache/calcite/sql/parser/impl/SqlParserImpl.java
$ grep 'case ZONE:'
core/build/javacc/javaCCMain/org/apache/calcite/sql/parser/impl/SqlParserImpl.java
|wc
53 106 865
$ grep 'case [A-Z]*:'
core/build/javacc/javaCCMain/org/apache/calcite/sql/parser/impl/SqlParserImpl.java
|wc
13507 27014 255827
{noformat}
We can optimize by putting common patterns into shared methods. For example,
this pattern occurs several times, and could be a method {{AliasOpt()}}:
{code}
( [ <AS> ] alias = SimpleIdentifier() | { alias = null; } )
{code}
It might be possible to remove a choice. Rather than choosing whether or not to
fire a rule, we could fire the rule, call the method, and have the method
return null.
I don't know how many of the 53 switches we can remove. If we can remove just
10 that would be a significant number of lines of code saved.
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