code: func BuildNotNullExpr(ctx sessionctx.Context, expr Expression) Expression {
  	isNull := NewFunctionInternal(ctx, ast.IsNull, types.NewFieldType(mysql.TypeTiny), expr)
  	notNull := NewFunctionInternal(ctx, ast.UnaryNot, types.NewFieldType(mysql.TypeTiny), isNull)
  	return notNull
  }
docs: // BuildNotNullExpr wraps up `not(isnull())` for given expression.