The optimization process and recommendations:

1. Avoid Correlated Subqueries (query line: 12): A correlated subquery is a subquery that contains a reference (column: client_no) to a table that also appears in the outer query. Usually correlated queries can be rewritten with a join clause, which is the best practice. The database optimizer handles joins much better than correlated subqueries. Therefore, rephrasing the query with a join will allow the optimizer to use the most efficient execution plan for the query.
2. Avoid Selecting Unnecessary Columns (query line: 6): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
3. Avoid Subqueries (query line: 5): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, it's recommended to join a newly created temporary table that holds the data, which also includes the relevant search index.

The optimized query:

SELECT
        some_field,
        some_other_field 
    FROM
        (SELECT
            * 
        FROM
            some_view a 
        WHERE
            a.some_criteria 
            AND a.client_no || ':' || a.engagement_no || ':' || a.registered_date = (
                SELECT
                    b.client_no || ':' || b.engagement_no || ':' || MAX(b.registered_date) 
                FROM
                    some_view b 
                JOIN
                    some_engagement_view e 
                        ON e.client_no = b.client_no 
                        AND e.engagement_no = b.engagement_no 
                JOIN
                    some_client_view c 
                        ON c.client_no = b.client_no 
                WHERE
                    some_other_criteria 
                    AND b.client_no = a.client_no 
                    AND b.engagement_no = a.engagement_no 
                GROUP BY
                    b.client_no,
                    b.engagement_no
            )
        )