[jira] [Created] (FLINK-40978) Cast from VARIANT through FromVariantConverter

["Ramin Gharib (Jira)" <ji...@apache.org>]

Ramin Gharib created FLINK-40978:
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             Summary: Cast from VARIANT through FromVariantConverter
                 Key: FLINK-40978
                 URL: https://issues.apache.org/jira/browse/FLINK-40978
             Project: Flink
          Issue Type: Improvement
          Components: Table SQL / Planner, Table SQL / Runtime
    Affects Versions: 2.4.0
            Reporter: Ramin Gharib
            Assignee: Ramin Gharib


Every \{{CAST(v AS T)}} from \{{VARIANT}} generates its own code today. Six planner cast rules each emit the walk for their target: primitives, ARRAY, ROW, MAP and character strings, plus a shared base class. For \{{CAST(v AS ARRAY<INT>)}} the generated code is:

{code:java}
int arraySize$2 = VariantCastUtils.arraySize(variant$1, "ARRAY<INT>");
java.lang.Integer[] objArray$3 = new java.lang.Integer[arraySize$2];
for (int i$4 = 0; i$4 < arraySize$2; i$4++) {
    Variant element$5 = variant$1.getElement(i$4);
    if (!element$5.isNull()) {
        result$6 = ((int) VariantCastUtils.toIntegral(element$5, -2147483648L, 2147483647L, "INTEGER"));
        objArray$3[i$4] = result$6;
    }
}
result$0 = new GenericArrayData(objArray$3);
{code}

The write side went the other way in FLINK-40826. \{{CAST(x AS VARIANT)}} calls a \{{ToVariantConverter}} in flink-table-common, built once per type. The read side has no such class. A format that reads a \{{VARIANT}} into SQL types cannot reuse the cast, and the null and shape rules are spread over several rules.

h3. Proposal

Add \{{FromVariantConverter}} to flink-table-common as the twin of \{{ToVariantConverter}}. It is built once per target type and returns internal data. The generated code shrinks to one call:

{code:java}
result$1 = ((ArrayData)(fromVariantConverter$2.convert(variant$0)));
{code}

* Merge the six read-side rules into one \{{FromVariantCastRule}}. Printing keeps its own path through \{{VariantCastUtils#toPrintString}}, because a printed value must never fail.
* Move \{{VariantCastUtils}} from flink-table-runtime to flink-table-common. The converter calls it, and common cannot depend on runtime. The class is \{{@Internal}}.
* Add \{{getSessionZoneId()}} to \{{CodeGeneratorCastRule.Context}}. The converter needs the zone when it is created. \{{CastRule.Context}} already exposes it.

h3. Behavior

No user-visible change. Every result, error message and null rule stays the same. Existing tests pass unchanged, apart from the rule names in \{{CastRuleProviderTest}}.

Internal differences:
* A \{{ROW}} result is a \{{GenericRowData}} instead of a \{{BinaryRowData}}, because \{{BinaryRowWriter}} is not in common. A \{{VARIANT}} below a \{{ROW}} is still copied out of its parent.
* An \{{ARRAY}} of a \{{NOT NULL}} primitive holds boxed values instead of a primitive array.
* A primitive target boxes its value through the converter. The write side accepted the same cost.
* A \{{DISTINCT}} target now casts like its source type. It used to fail during code generation. SQL cannot create such a type.



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