[jira] [Created] (CALCITE-3224) New RexNode-to-Expression CodeGen Implementation

["Feng Zhu (JIRA)" <ji...@apache.org>]

Feng Zhu created CALCITE-3224:
---------------------------------

             Summary: New RexNode-to-Expression CodeGen Implementation
                 Key: CALCITE-3224
                 URL: https://issues.apache.org/jira/browse/CALCITE-3224
             Project: Calcite
          Issue Type: Bug
          Components: core
    Affects Versions: 1.20.0
            Reporter: Feng Zhu
            Assignee: Feng Zhu
         Attachments: codegen.png

h3. *Background*

    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.
h3. *Major Features*
 * *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 “optimizing” operation. 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.

h3. *Implementation*

    For each RexNode, the visitor will generally generate two declaration statements, one for value and one for nullable. The code snippet is like:

 
{code:java}
{valueVariable} = {valueExpression}

{isNullVariable} = {isNullExpression}
{code}
 

The visitor’s result will be the variable pair (*_isNullVariable_*, *_valueVariable_*).
h3. *Example Demonstration*

Take a simple test case as example, in which the "commission" column is nullable.
{code:java}
@Test public void testNPE() {
  CalciteAssert.hr()
    .query("select \"commission\" + 10 as s\n"
      + "from \"hr\".\"emps\"")
    .returns("S=1010\nS=510\nS=null\nS=260\n");
}
{code}
The codegen progress and non-optimized code are demonstrated in the figure below.

!codegen.png!
 # 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*_)



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