【MySQL源码】PT_query_expression_body

class PT_query_expression_body : public Parse_tree_node {
 public:
  virtual bool is_union() const = 0;

  /**
    True if this query expression can absorb an extraneous order by/limit
    clause. The `ORDER BY`/`LIMIT` syntax is mostly consistestent, i.e. a
    trailing clause may not refer to the tables in the `<query primary>`, with
    one glaring exception:

        (...( SELECT ... )...) ORDER BY ...

    If the nested query expression doesn't contain `ORDER BY`, the statement
    is interpreted as if the `ORDER BY` was absorbed by the innermost query
    expression, i.e.:

        (...( SELECT ... ORDER BY ... )...)

    There is no rewriting of the parse tree nor AST happening here, the
    transformation is done by the contextualizer (see
    PT_query_expression::contextualize_order_and_limit), which interprets the
    parse tree, and builds the AST according to this interpretation. This
    interpretation is governed by the following rule: An `ORDER BY` can be
    absorbed if none the nested query expressions contains an `ORDER BY` *or*
    `LIMIT`. The rule is complex, so here are some examples for illustration:

    In these cases the `ORDER BY` *is* absorbed:

        ( SELECT * FROM t1 ) ORDER BY t1.a;
        (( SELECT * FROM t1 )) ORDER BY t1.a;

    In these cases the ORDER BY is *not* absorbed:

        ( SELECT * FROM t1 ORDER BY 1 ) ORDER BY t1.a;
        (( SELECT * FROM t1 ) ORDER BY 1 ) ORDER BY t1.a;
        ( SELECT * FROM t1 LIMIT 1 ) ORDER BY t1.a;
        (( SELECT * FROM t1 ) LIMIT 1 ) ORDER BY t1.a;

    The same happens with `LIMIT`, obviously, but the optimizer is freeer to
    choose when to apply the limit, and there are name no resolution issues
    involved.

    @param order  True if the outer query block has the ORDER BY clause.
    @param limit  True if the outer query block has the LIMIT clause.
  */
  virtual bool can_absorb_order_and_limit(bool order, bool limit) const = 0;
  virtual bool has_into_clause() const = 0;
  virtual bool has_trailing_into_clause() const = 0;

  virtual bool is_table_value_constructor() const = 0;
  virtual PT_insert_values_list *get_row_value_list() const = 0;
};