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What is the exact meaning of foldable column in this context?

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Returns true when an expression is a candidate for static evaluation before the query is executed. The following conditions are used to determine suitability for constant folding:

• A Coalesce is foldable if all of its children are foldable
• A BinaryExpression is foldable if its both left and right child are foldable
• A Not, IsNull, or IsNotNull is foldable if its child is foldable
• A Literal is foldable
• A Cast or UnaryMinus is foldable if its child is foldable