Feng Zhu created CALCITE-3224:
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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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