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https://issues.apache.org/jira/browse/CALCITE-3414?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16952170#comment-16952170
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Julian Hyde commented on CALCITE-3414:
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I haven't looked at the PR, but let me state the strategy. Stuff in linq4j
should follow the rules of the Java language, whereas stuff in core follows the
rules of the SQL language (or our relational algebra), and the stuff layered on
top by the java runtime libraries (e.g. java.sql.Timestamp and
java.math.BigDecimal).
I think that {{Expressions.convert}} and {{Types.castIfNecessary}} follow that
policy. {{RexToLixTranslator.convert}} has a mixture - some stuff regarding
boxing/unboxing, and primitive to primitive (e.g. float to double), that should
be in linq4j, and some stuff such as converting java.sql.Timestamp to long and
BigDecimal to String that would remain in core.
> 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
> Priority: Major
> Labels: pull-request-available
> Attachments: RexToLixTranslator.png, TypeConversion.txt, Types.png
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> 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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