There are two structures that describe selects:

The latter item represents UNION operation (the absence of UNION is a union with only one SELECT and this structure is present in any case). In the future, this structure will be used for EXCEPT and INTERSECT as well.

For example:

(SELECT ...) UNION (SELECT ... (SELECT...)...(SELECT...UNION...SELECT))
   1           2      3           4             5        6       7

will be represented as:

------------------------------------------------------------------------
                                                                 level 1
SELECT_LEX_UNIT(2)
|
+---------------+
|               |
SELECT_LEX(1)   SELECT_LEX(3)
                |
--------------- | ------------------------------------------------------
                |                                                level 2
                +-------------------+
                |                   |
                SELECT_LEX_UNIT(4)  SELECT_LEX_UNIT(6)
                |                   |
                |                   +--------------+
                |                   |              |
                SELECT_LEX(4)       SELECT_LEX(5)  SELECT_LEX(7)

------------------------------------------------------------------------

Note: Single subquery 4 has its own SELECT_LEX_UNIT.

The uppermost SELECT_LEX_UNIT (#2 in example) is stored in LEX. The first and uppermost SELECT_LEX (#1 in example) is stored in LEX, too. These two structures always exist.

At the time of creating or performing any JOIN::* operation, LEX::current_select points to an appropriate SELECT_LEX.

Only during parsing of global ORDER BY and LIMIT clauses (for the whole UNION), LEX::current_select points to SELECT_LEX_UNIT of this unit, in order to store this parameter in this SELECT_LEX_UNIT. SELECT_LEX and SELECT_LEX_UNIT are inherited from st_select_lex_node.