Subqueries with distinct and the ONCE-UNIQUE aggregate

Evaluate Grouped ONCE-UNIQUE AGGREGATE.
Evaluate Ungrouped ONCE-UNIQUE AGGREGATE.

When the subquery includes distinct, the ONCE-UNIQUE aggregate indicates that duplicates are being eliminated:

select pub_name from publishers
where pub_id =
(select distinct titles.pub_id from titles
    where publishers.pub_id = titles.pub_id
    and price > $1000)
QUERY PLAN FOR STATEMENT 1 (at line 1).

    STEP 1
        The type of query is SELECT.

        FROM TABLE
            publishers
        Nested iteration.
        Table Scan.
        Forward scan.
        Positioning at start of table.

        Run subquery 1 (at nesting level 1).
        Using I/O Size 2 Kbytes for data pages.
        With LRU Buffer Replacement Strategy for data pages.

NESTING LEVEL 1 SUBQUERIES FOR STATEMENT 1.

  QUERY PLAN FOR SUBQUERY 1 (at nesting level 1 and at line 3).

    Correlated Subquery.
    Subquery under an EXPRESSION predicate.


    STEP 1
        The type of query is SELECT.
        Evaluate Ungrouped ONCE-UNIQUE AGGREGATE.

        FROM TABLE
            titles
        Nested iteration.
        Index : pub_id_ix
        Forward scan.
        Positioning by key.
        Keys are:
            pub_id  ASC
        Using I/O Size 16 Kbytes for index leaf pages.
        With LRU Buffer Replacement Strategy for index leaf pages.
        Using I/O Size 2 Kbytes for data pages.
        With LRU Buffer Replacement Strategy for data pages.

  END OF QUERY PLAN FOR SUBQUERY 1.