[CALCITE-6228] ELEMENT function infers incorrect return type - ASF JIRA

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Details

Type: Bug
Status: Closed
Priority: Minor
Resolution: Fixed
Affects Version/s: 1.36.0
Fix Version/s: 1.37.0
Component/s: core
Labels:
- pull-request-available

Description

The ELEMENT function is defined in the documentation as follows: https://calcite.apache.org/docs/reference.html#collection-functions

Returns the sole element of an array or multiset; null if the collection is empty; throws if it has more than one element.

However, the type inference returns just the type of the element of the collection, without changing its nullability.

The type inference is implemented as follows in SqlStdOperatorTable:

public static final SqlFunction ELEMENT =
      SqlBasicFunction.create("ELEMENT",
          ReturnTypes.MULTISET_ELEMENT_NULLABLE,
          OperandTypes.COLLECTION);

However, reading the definition of MULTISET_ELEMENT_NULLABLE in ReturnTypes.java:

 public static final SqlReturnTypeInference MULTISET_ELEMENT_NULLABLE =
      MULTISET.andThen(SqlTypeTransforms.TO_COLLECTION_ELEMENT_TYPE);

we notice that it is not forced to be nullable. Probably the correct implementation would be

  public static final SqlReturnTypeInference MULTISET_ELEMENT_NULLABLE =
      MULTISET.andThen(SqlTypeTransforms.TO_COLLECTION_ELEMENT_TYPE)
          .andThen(SqlTypeTransforms.FORCE_NULLABLE);