The optimization process and recommendations:

1. Avoid Calling Functions With Indexed Columns (query line: 25): 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 `_table_suffix` is indexed, the index won’t be used as it’s wrapped with the function `parse_date`. 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.

The optimized query:

SELECT
        TIMESTAMP_SECONDS(visitStartTime) AS stamp,
        customDimension.value AS UserID,
        CONCAT(CAST(fullVisitorId AS STRING),
        CAST(visitId AS STRING)) AS visit_ref,
        COUNT(DISTINCT CONCAT(CAST(fullVisitorId AS STRING),
        CAST(visitId AS STRING))) OVER (PARTITION 
    BY
        customDimension.value ) AS total_visits_in_cycle,
        RANK() OVER (PARTITION 
    BY
        CONCAT(CAST(fullVisitorId AS STRING),
        CAST(visitId AS STRING)),
        TIMESTAMP_SECONDS(visitStartTime) 
    ORDER BY
        TIMESTAMP_SECONDS(visitStartTime)) AS visitrank,
        COUNT(DISTINCT transaction.transactionid) AS orders 
    FROM
        `xxx.xxx.ga_sessions_20*` AS t CROSS 
    JOIN
        UNNEST(hits) AS hits CROSS 
    JOIN
        UNNEST(t.customdimensions) AS customDimension 
    WHERE
        parse_date('%y%m%d', _table_suffix) BETWEEN DATE_sub(current_date(), INTERVAL 3 day) AND DATE_sub(current_date(), INTERVAL 1 day) 
        AND customDimension.index = 2 
    GROUP BY
        1,
        2,
        3,
        fullVisitorId,
        visitid,
        visitStartTime 
    ORDER BY
        UserID LIMIT 500