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https://issues.apache.org/jira/browse/BEAM-7724?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16892960#comment-16892960
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Rui Wang commented on BEAM-7724:
--------------------------------
A little more background about code generation in BeamSQL:
The code generation starts from RexNode, it follows RexNode -> Expression
(Calcite's ling4j library) -> Expression.toString() gives human written style
code -> [Janino|https://janino-compiler.github.io/janino/] complier to compile
the code and run it at runtime.
What is RexNode? It's a Class representation of SQL expression. For example,
SELECT "abc". "abc" will be converted to a child class of RexNode (which is
called RexLiteral).
What is Expression (Calcite's ling4j library)? You can start thinking from a
small piece of code: int a = 10; This assignment contains three pieces: an int
with variable name "a", an assignment and a constant. Therefore, you can
actually use an object oriented way to represent "int a = 10". You can start
from a class called "ExpressionVariable". You can then do
ExpressionVariable.of(type.INT, "a", New
ExpressionAssignment(ExpressionConstant(10)). ExpressionVariable will include
enough information to tell you 10 is assigned to an int with name "a". If you
think in this way, you will see that given an Expression, we can generate code
reversely. For example, if you call ExpressionVariable.toString(), it will very
likely do something like this:
{code:java}
ExpressionVariable.toString() {
return "int a" + ExpressionAssignment.toString();
}
ExpressionAssignment.toString() {
return " = " + ExpressionConstant.toString();
}
ExpressionConstant.toString() {
return "10";
}
{code}
We reuse many of Calcite code to generate Expression from RexNode. Calcite has
already many functions implementation and it knows how to convert those to
Expression. After have an Expression, you can call string to get code.
So the resolution I was looking for, was to find the place where RexLiteral
(which is null in our case) is converted to Expression. So in that conversion,
I expect add something similar to ExpressionCast whose toString gives "(toType)
ExpressionConstant.toString()"
> Codegen should cast(null) to a type to match exact function signature
> ---------------------------------------------------------------------
>
> Key: BEAM-7724
> URL: https://issues.apache.org/jira/browse/BEAM-7724
> Project: Beam
> Issue Type: Improvement
> Components: dsl-sql
> Reporter: Rui Wang
> Assignee: sridhar Reddy
> Priority: Major
>
> If there are two function signatures for the same function name, when input
> parameter is null, Janino will throw exception due to vagueness:
> A(String)
> A(Integer)
> Janino does not know how to match A(null).
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