[jira] [Created] (CALCITE-3173) RexNode Code Generation Problem

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

Feng Zhu created CALCITE-3173:
---------------------------------

             Summary: RexNode Code Generation Problem
                 Key: CALCITE-3173
                 URL: https://issues.apache.org/jira/browse/CALCITE-3173
             Project: Calcite
          Issue Type: Improvement
          Components: core
    Affects Versions: next
            Reporter: Feng Zhu
         Attachments: pic.png

*Abstract:* Both RexImpTable and BlockBuilder have codegen issues, but it is interesting that they can work together well for most cases.


    We can illustrate the problem with a simple test case in JdbcTest, in which the "commission" column is nullable.
{code:java}
@Test public void testNullSafeCheck() {
    CalciteAssert.hr()
      .query("select \"commission\" + 10 as s from \"hr\".\"emps\"")
      .returns("S=1010\n"
               + "S=510\n"
               + "S=null\n"
               + "S=260\n");
}
{code}
    This test case can pass as the BlockBuilder is in default optimization mode. However, when we set it into un-optimization mode in _EnumerableCalc_, this test will fail due to NPE. The following picture demonstrates their differences.

!pic.png!

*1.RexImpTable generates unsafe code*
    Before translating the RexCall (_*Add*_), RexImpTable firstly translate its operands with (nullAs=*IS_NULL*) [1] to detect whether it is null (i.e., {color:#FF0000}_inp4_unboxed_{color}). Then RexImpTable sets this operand's nullable in RexToLixTranslator as false [2]. After that, the operand will be translated again with *NOT_POSSIBLE* [3] to get the value (i.e., {color:#FF0000}_inp4_0_unboxed_{color}). In the end, the RexCall is implemented by NotNullImplementor.However, it is not safe to conduct operations in the second translation phase. 
*2.BlockBuiler optimization's semantic issue buries NPE*
    BlockBuilder.optimize() changes the code semantic in this case. For conditional-like clauses (if...else, ?:, etc), InlineVariableVisitor will wrongly make variables inlined.

    In general, they can work together for most cases. However, when some special branch is triggered by query, the problem will be exposed. For example, the NewExpression in CALCITE-3143 breaks the optimization phase.

*How to fix?*
    I have digged into this problem a couple of days and tried many approaches to fix it. But in this progress, I found the limitation in current implementation. 
    The whole recursive framework essentially conducts a sequential codegen beheavior, and may visit a RexNode again and again with different NullAs setting. Due to the limitation, it is difficult to meet null-safe codegen semantics. We can also find that there are many branches for special cases in current implementation. Even we can handle issues every time, the logic will becomes more and more complex  and unfriendly for maintenance.   

Therefore, I propose to re-consider this part, including several initial points:
(1) {color:#FF0000}_Visitor Pattern_{color} (RexVisitor<Result>). Theoretically, RexNode can be translated into Expression by visiting the node only once. We can implement RexVisitor rather than current recursive translation.
(2)The {color:#FF0000}Result{color} consists of three items (code: BuilderStatement, isNull: ParameterExpression, value: Expression).So it is easy to decide how  to implement a RexNode according to its children.

Please correct me if I make something wrong. Look forward to suggestions!

 

[1][https://github.com/apache/calcite/blob/1748f0503e7b626a8d0165f1698adb8b61bbc31e/core/src/main/java/org/apache/calcite/adapter/enumerable/RexImpTable.java#L1062]

[2][https://github.com/apache/calcite/blob/1748f0503e7b626a8d0165f1698adb8b61bbc31e/core/src/main/java/org/apache/calcite/adapter/enumerable/RexImpTable.java#L1064]

[3][https://github.com/apache/calcite/blob/1748f0503e7b626a8d0165f1698adb8b61bbc31e/core/src/main/java/org/apache/calcite/adapter/enumerable/RexImpTable.java#L1113]

 



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