The optimization process and recommendations:

1. Avoid Calling Functions With Indexed Columns (query line: 10): 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 `Co_Acc` is indexed, the index won’t be used as it’s wrapped with the function `Substr`. 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 Calling Functions With Indexed Columns (query line: 11): 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 `Co_Acc` is indexed, the index won’t be used as it’s wrapped with the function `Substr`. 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.
3. Avoid Calling Functions With Indexed Columns (query line: 12): 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 `Co_Acc` is indexed, the index won’t be used as it’s wrapped with the function `Substr`. 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.
4. Avoid Calling Functions With Indexed Columns (query line: 13): 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 `Co_Acc` is indexed, the index won’t be used as it’s wrapped with the function `Substr`. 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.
5. 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 operation_history_idx_state_id_curr_day ON operation_history (state_id,curr_day);

The optimized query:

SELECT
        t.co_filial AS fil_code,
        t.emp_birth AS emp_code,
        to_char(t.curr_day,
        'YYYY-MM-DD') AS operation_date,
        TRUNC(t.Sum_Pay / 100) AS summa 
    FROM
        operation_history t 
    WHERE
        Substr(t.Co_Acc, 8) LIKE '12294%' 
        AND Substr(t.Co_Acc, -3) > 599 
        AND Substr(t.Co_Acc, -3) != 683 
        AND Substr(t.Co_Acc, -3) < 696 
        AND t.state_id = 41 
        AND t.curr_day >= to_date('12.08.2019', 'DD.MM.YYYY') 
        AND t.curr_day < to_date('13.08.2019', 'DD.MM.YYYY')