KnownNullable¶

Overview¶

KnownNullable is a tagging expression that wraps a child expression to explicitly mark it as nullable, overriding the child's nullability determination. This expression acts as a transparent wrapper that forces the nullable property to true while preserving all other behavior of the wrapped expression.

Syntax¶

// Internal Catalyst expression - not directly exposed in SQL
KnownNullable(childExpression)

Arguments¶

Argument	Type	Description
child	Expression	The child expression to be wrapped and marked as nullable

Return Type¶

Returns the same data type as the child expression.

Supported Data Types¶

Supports all data types since it acts as a transparent wrapper around any child expression.

Algorithm¶

Wraps the child expression without modifying its evaluation logic
Forces the nullable property to return true regardless of child's nullability
Delegates all evaluation and code generation to the child expression
Acts as a metadata annotation for the Catalyst optimizer's nullability analysis
Preserves the exact return value and behavior of the wrapped expression

Partitioning Behavior¶

This expression preserves partitioning behavior:

Does not affect partitioning as it's a transparent wrapper
No shuffle operations are introduced
Partitioning characteristics are inherited from the child expression

Edge Cases¶

Null handling: Inherits null handling behavior from child expression
The expression itself never produces nulls beyond what the child produces
Used internally by Catalyst optimizer for nullability inference corrections
Does not change the actual runtime behavior, only compile-time nullability metadata

Code Generation¶

Supports full code generation (Tungsten) by delegating code generation directly to the child expression without any wrapper overhead.

Examples¶

-- Not directly accessible in SQL as this is an internal Catalyst expression
-- Used internally during query optimization phases

// Internal usage in Catalyst optimization
// This expression is typically created by optimizer rules
val childExpr = col("someColumn")
val nullableWrapper = KnownNullable(childExpr)
// Forces nullability analysis to treat the expression as nullable