[jira] [Created] (CALCITE-3414) Unify Expression'type cast and conversion as a robust one

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

Feng Zhu created CALCITE-3414:
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             Summary: Unify Expression'type cast and conversion as a robust one
                 Key: CALCITE-3414
                 URL: https://issues.apache.org/jira/browse/CALCITE-3414
             Project: Calcite
          Issue Type: Bug
          Components: core
    Affects Versions: 1.21.0
            Reporter: Feng Zhu
            Assignee: Feng Zhu
         Attachments: RexToLixTranslator.png, TypeConversion.txt, Types.png

 Current now, there are two functions in calcite that can be used to cast/convert Expression to a specific Type.
 *_Types.castIfNecessary_* and _*RexToLixTranslator.convert*_.

We make a deep investigation on their implementations and demonstrate them as below.

{color:#ff0000}                                                                     !RexToLixTranslator.png!{color}

{color:#ff0000}                                                                      *RexToLixTranslator.convert*{color}

  !Types.png!

                                           {color:#ff0000}                           *Types.castIfNecessary*{color}

It can be seen that: 
 (1) They have a lot of overlaps; 
 (2) *_RexToLixTranslator.cast_* can cover more cases with tools like _SqlFunctions_ and etc.
 (3) Both of them have limitations and may generate incorrect code, which is listed in attachment(TypeConversion.txt).

Multiple choices usually bring confusion to developers and resulting to the misuse of them. 
 For example, CALCITE-3245 exposes that Types.castIfNecessary cannot cast the Expression to BigDecimal.class.


 Fixing the issue in *_Types.castIfNecessary_* directly seems to be not a good idea. 
 On one hand, it is not convenient to call _SqlFunctions_ in linq4j. One the other hand, it will brings duplicate with _*RexToLixTranslator.cast*_. However, due to some unique logic in _*Types.castIfNecessary*_, we cannot replace it as _*RexToLixTranslator.cast*_ neither.

Therefore, it is a good idea to integrate implementations into RexToLixTranslator.cast.



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