Which update is faster using join or sequential? - mysql

Question

This question is in sequence of my previous Question required update same table on deletion a row.
I could write two solutions using Stored Procedure instead of trigger or nested-query .
Both use a helper function my_signal(msg).
A Stored Procedure to delete employee from Employee Table.
Fist Solution: use UPDATE rows in table, without join operation:
CREATE PROCEDURE delete_employee(IN dssn varchar(64))
BEGIN
DECLARE empDesignation varchar(128);
DECLARE empSsn varchar(64);
DECLARE empMssn varchar(64);
SELECT SSN, designation, MSSN INTO empSsn, empDesignation, empMssn
FROM Employee
WHERE SSN = dssn;
IF (empSsn IS NOT NULL) THEN
CASE
WHEN empDesignation = 'OWNER' THEN
CALL my_signal('Error: OWNER can not deleted!');
WHEN empDesignation = 'WORKER' THEN
DELETE FROM Employee WHERE SSN = empSsn;
WHEN empDesignation = 'BOSS' THEN
BEGIN
UPDATE Employee
SET MSSN = empMssn
WHERE MSSN = empSsn;
DELETE FROM Employee WHERE SSN = empSsn;
END;
END CASE;
ELSE
CALL my_signal('Error: Not a valid row!');
END IF;
END//
Second solution: as I was suggested in my previous question using INNER JOIN
CREATE PROCEDURE delete_employee(IN dssn varchar(64))
BEGIN
DECLARE empDesignation varchar(128);
DECLARE empSsn varchar(64);
DECLARE empMssn varchar(64);
SELECT SSN, designation, MSSN INTO empSsn, empDesignation, empMssn
FROM Employee
WHERE SSN = dssn;
IF (empSsn IS NOT NULL) THEN
IF (empDesignation = 'OWNER') THEN
CALL my_signal('Error: OWNER can not deleted!');
END IF;
UPDATE `Employee` A INNER JOIN `Employee` B ON A.SSN= B.MSSN
SET B.MSSN = A.MSSN WHERE A.SSN = empSsn;
DELETE FROM `Employee` WHERE SSN = empSsn;
ELSE
CALL my_signal('Error: Not a valid row!');
END IF;
END//
I read here that using join is efficient for Efficient SELECT. But my problem includes only one table and I feel my solution(first) is much efficient than second because join will consume memory comparatively.
Please suggest me which is better and efficient, if Employee table is sufficiently large. Which is better for me? Reason
EDIT: I checked for small table consist of 7 rows only, and both solution take same time.
mysql> CALL delete_employee(4);
Query OK, 1 row affected (0.09 sec)
I know SQL function behaves non-deterministically because table heuristics. Which choice is better? Either if you have some idea How query can be further optimised.

Answer 1

After a while of thinking I am almost sure it doesn't make a difference, first solution may be even slightly slower, but in a not measurable dimension.
First intention would be, that the first solution is faster because you first fetch data by id and update only if nessesary.
But MySQL internally does nothing else in the UPDATE .. JOIN statement, just internally and as a result of this probably more efficiently as well.
Your first solution doesn't catch a default case - what happens if I neither get WORKER or BOSS?
Also your execution time (0.09s) is extremely high, which can't be explained with what I know about your database so far.
Did you set any index?
EDIT:
After looking at the table structure you've posted here
I have some improvement offers for the structure itself.
1. Use type int when you are storing integer values. The database can handle integer way more efficient
2. Why generate SSN by yourself? Using auto_increment on the PRIMARY KEY is much simpler to handle and saves you a lot of work when you add new employees
ALTER TABLE `Employee`
CHANGE `SSN` `SSN` int(11) NOT NULL AUTO_INCREMENT ,
CHANGE `MSSN` `MSSN` int(11) DEFAULT NULL,
ADD KEY `KEY_Employee_MSSN` ( `MSSN` );
3. Do you use the name for lookups? If so, it needs to be unique as well
ALTER TABLE `Employee`
ADD UNIQUE KEY `UNI_KEY_Employee` ( `name` );
4. Do you have a fixed range of designations? enum forces the input to be one of the defined values
ALTER TABLE `Employee`
CHANGE `designation` `designation` ENUM( 'BOSS', 'WORKER' ) NOT NULL DEFAULT 'WORKER',
ADD KEY `KEY_Employee_designation` ( `designation` );
Final structure
mysql> EXPLAIN `Employee`;
+-------------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-----------------------+------+-----+---------+----------------+
| SSN | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(64) | YES | UNI | NULL | |
| designation | enum('BOSS','WORKER') | NO | MUL | WORKER | |
| MSSN | int(11) | YES | MUL | NULL | |
+-------------+-----------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)