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Reused the Expr node to hold DISTINCT which strongly resembles the existing OP info. Define DISTINCT_EXPR which strongly resembles the existing OPER_EXPR opType, but with handling for NULLs required by SQL99. We have explicit support for single-element DISTINCT comparisons all the way through to the executor. But, multi-element DISTINCTs are handled by expanding into a comparison tree in gram.y as is done for other row comparisons. Per discussions, it might be desirable to move this into one or more purpose-built nodes to be handled in the backend. Define the optional ROW keyword and token per SQL99. This allows single-element row constructs, which were formerly disallowed due to shift/reduce conflicts with parenthesized a_expr clauses. Define the SQL99 TREAT() function. Currently, use as a synonym for CAST().