ClassifyConversion takes two ColumnTypes and determines "how hard" the conversion is.

ClassifyConversionFromTree is a wrapper for ClassifyConversion when we want to take into account the parsed AST for ALTER TABLE .

ValidateAlterColumnTypeChecks performs validation checks on the proposed type change.

ColumnConversionDangerous indicates that we could effect a type conversion via built-in cast methods, but that the semantics are not well-defined enough that a user would want to rely on this.

ColumnConversionGeneral indicates that we will end up rewriting the existing data into a new format.

ColumnConversionImpossible indicates that it is not possible to effect the desired type conversion.

ColumnConversionTrivial indicates that a conversion is byte-for-byte compatible with existing data and there is no possibility of introducing a constraint validation error.

ColumnConversionValidate indicates that a conversion is byte-for-byte compatible with existing data, but that we need to validate the existing data against the new rule.

ColumnConversionKind represents "how hard" a semantic type conversion will be.