SQL Implicit Type Cast
This package contains rules for implicit type coercion, it works during
the process of SQL validation. The transformation entrance are all kinds of
When user do validation for a
SqlNode, and the type coercion is turned on(default), the validator will check the operands/return types of all kinds of operators, if the validation passes through, the validator will just cache the data type (say RelDataType) for the
SqlNodeit has validated;
- If the validation fails, the validator will ask the
TypeCoercionabout if there can make implicit type coercion, if the coercion rules passed, the
TypeCoercioncomponent will replace the
SqlNodewith a casted one, (the node may be an operand of an operator or a column field of a selected row).
TypeCoercionwill then update the inferred type for the casted node and the containing operator/row column type.
- If the coercion rule fails again, the validator will just throw the exception as is before.
For some cases, although the validation passes, we still need the type coercion, e.g. for expression 1 > '1', Calcite will just return false without type coercion, we do type coercion eagerly here: the result expression would be transformed to "1 > cast('1' as int)" and the result would be true.
Conversion SQL Contexts
The supported conversion contexts are: Conversion Expressions
Strategies for Finding Common Type:
- If the operator has expected data types, just take them as the desired one. i.e. the UDF.
- If there is no expected data type but data type families are registered, try to coerce operand to the family's default data type, i.e. the STRING family will have a VARCHAR type.
- If neither expected data type nor families are specified, try to find the tightest common type of the node types, i.e. INTEGER and DOUBLE will return DOUBLE, the numeric precision does not lose for this case.
- If no tightest common type is found, try to find a wider type, i.e. STRING and INT will return int, we allow some precision loss when widening DECIMAL to fractional, or promote to STRING.
Type Conversion Matrix
ClassDescriptionBase class for all the type coercion rules.Default strategies to coerce differing types that participate in operations into compatible ones.Factory for
TypeCoercionobjects.Default implementation of Calcite implicit type cast.Factory class for type coercion instantiation of different sql dialects.