[Solved] Accessing outer query column in sub query

How to optimize this SQL query?

In case you have your own slow SQL query, you can optimize it automatically here.

For the query above, the following recommendations will be helpful as part of the SQL tuning process.
You'll find 3 sections below:

  1. Description of the steps you can take to speed up the query.
  2. The optimal indexes for this query, which you can copy and create in your database.
  3. An automatically re-written query you can copy and execute in your database.
The optimization process and recommendations:
  1. Avoid Subqueries (query line: 8): 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.
  2. Avoid Subqueries (query line: 14): 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.
  3. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
Optimal indexes for this query:
CREATE INDEX users_table_idx_user_name ON users_table (user_name);
The optimized query:
SELECT
        (SELECT
            Listagg(full_name,
            ',') WITHIN 
        GROUP (ORDER BY
            full_name) AS CC_Recipients 
        FROM
            (SELECT
                users_table.first_name || ' ' || users_table.last_name AS full_name 
            FROM
                users_table 
            WHERE
                users_table.user_name IN (
                    SELECT
                        DISTINCT Trim(Regexp_substr(cc_recipients,
                        '[^,]+',
                        1,
                        dual.LEVEL)) 
                    FROM
                        dual CONNECT 
                    BY
                        Regexp_substr(cc_recipients,
                        '[^,]+',
                        1,
                        dual.LEVEL) IS NOT NULL
                )
            ) t) AS cc_rec 
    FROM
        email_queue 
    WHERE
        cc_recipients IS NOT NULL

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* original question posted on StackOverflow here.