Current RexNode-to-Expression implementation relies on BlockBuilder's incorrect “optimizations” to inline unsafe operations. As illustrated in
CALCITE-3173, when this cooperation is broken in some special cases, it will cause exceptions like NPE, such as CALCITE-3142, CALCITE-3143, CALCITE-3150.
Though we can fix these problems under current implementation framework with some efforts like the PR in
CALCITE-3142, the logic will become more and more complex. To pursue a thorough and elegant solution, we implement a new one. Moreover, it also ensures the correctness for non-optimized code.
- Visitor Pattern: Each RexNode will be visited only once in a bottom-up way, rather than recursively visiting a RexNode many times with different NullAs settings.
- Conditional Semantic: It can naturally guarantee the correctness even without BlockBuilder’s “optimizings”. Each line of code generated for a RexNode is null safe.
- Interface Compatibility: The implementation only updates RexToLixTranslator and RexImpTable. Interfaces such as CallImplementor keep unchanged.
For each RexNode, the visitor will generally generate two declaration statements, one for value and one for nullable. The code snippet is like:
The visitor’s result will be the variable pair (isNullVariable, valueVariable).
(1) ReImplement different RexCall implementations (e.g., CastImplementor, BinaryImplementor and etc.) as seperated files and remove them into the newly created package org.apache.calcite.adapter.enumerable.rex, and organize them in RexCallImpTable.
(2) move some util functions into EnumUtils.
Take a simple test case as example, in which the "commission" column is nullable.
The codegen progress and non-optimized code are demonstrated in the figure below.
- When visiting RexInputRef (commission), the visitor generates three lines of code, the result is a pair of ParameterExpression (input_isNull, input_value).
- Then the visitor visits RexLiteral (10) and generates two lines of code. The result is (literal_isNull, literal_value).
- After that, when visiting RexCall(Add), (input_isNull, input_value) and (literal_isNull, literal_value) can be used to implement the logic. The visitor also generates two lines of code and return the variable pair.
In the end, the result Expression is constructed based on (binary_call_isNull, binary_call_value)