The optimization process and recommendations:

1. Avoid Calling Functions With Indexed Columns (query line: 59): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `Field1_1` is indexed, the index won’t be used as it’s wrapped with the function `UPPER`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
2. Avoid LIKE Searches With Leading Wildcard (query line: 59): The database will not use an index when using like searches with a leading wildcard (e.g. '%abc%'). Although it's not always a satisfactory solution, please consider using prefix-match LIKE patterns (e.g. 'TERM%').
3. Avoid Subqueries (query line: 9): 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.
4. 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:

ALTER TABLE `CMS_Correspondence_Primary` ADD INDEX `cms_primary_idx_field12_1_field0_field1_1` (`Field12_1`,`Field0`,`Field1_1`);
ALTER TABLE `Items` ADD INDEX `items_idx_itemid_status` (`ItemId`,`Status`);
ALTER TABLE `Links` ADD INDEX `links_idx_parenttype_parentid` (`ParentType`,`ParentId`);

The optimized query:

SELECT
        SHOWN0,
        SHOWN1,
        LVL_1,
        LVL_2,
        LVL_3,
        LVL_4 
    FROM
        (SELECT
            CMS_Correspondence_Primary.Field0 SHOWN0,
            CMS_Correspondence_Primary.Field1_1 SHOWN1,
            (SELECT
                L.LEVEL 
            FROM
                Links L 
            WHERE
                L.ParentType = 1 
                AND L.ParentId = 'UG205' START WITH L.ItemId = Items.ItemId CONNECT 
            BY
                L.ItemId = PRIOR L.ParentId 
                AND PRIOR L.ParentType = 2) LVL_1,
            (SELECT
                L.LEVEL 
            FROM
                Links L 
            WHERE
                L.ParentType = 1 
                AND L.ParentId = 'UG206' START WITH L.ItemId = Items.ItemId CONNECT 
            BY
                L.ItemId = PRIOR L.ParentId 
                AND PRIOR L.ParentType = 2) LVL_2,
            (SELECT
                L.LEVEL 
            FROM
                Links L 
            WHERE
                L.ParentType = 1 
                AND L.ParentId = 'UG230' START WITH L.ItemId = Items.ItemId CONNECT 
            BY
                L.ItemId = PRIOR L.ParentId 
                AND PRIOR L.ParentType = 2) LVL_3,
            (SELECT
                L.LEVEL 
            FROM
                Links L 
            WHERE
                L.ParentType = 1 
                AND L.ParentId = 'UG281' START WITH L.ItemId = Items.ItemId CONNECT 
            BY
                L.ItemId = PRIOR L.ParentId 
                AND PRIOR L.ParentType = 2) LVL_4 
        FROM
            Items,
            CMS_Correspondence_Primary 
        WHERE
            (
                (
                    (
                        UPPER(CMS_Correspondence_Primary.Field1_1) LIKE UPPER('%abc%') 
                        AND CMS_Correspondence_Primary.Field1_1 IS NOT NULL
                    )
                ) 
                AND (
                    (
                        CMS_Correspondence_Primary.Field12_1 = 210
                    ) 
                    AND (
                        1 = 1
                    ) 
                    AND (
                        1 = 1
                    ) 
                    AND (
                        1 = 1
                    ) 
                    AND (
                        1 = 1
                    )
                )
            ) 
            AND (
                Items.Status <> 1
            ) 
            AND (
                Items.ItemId = CMS_Correspondence_Primary.Field0
            )
        ) INNER_QUERY 
    WHERE
        LVL_1 IS NULL 
        AND LVL_2 IS NULL 
        AND LVL_3 IS NULL 
        AND LVL_4 IS NULL