Proper way of handling and preventing deadlocks - php

Question

I've understood that deadlocks occur when an sql query tries to lock an already locked row and I'm currently experiencing deadlocks. here's my sql query below:
INSERT INTO transactions (product_id, category, C, amount, date)
SELECT 'SomeProduct', 'SomeCategory', v.username, 10, '2016-3-31' FROM balance v
WHERE v.username = 'SomeUsername' AND v.balance + 10 >= 0
balance is a virtual table that sums transactions to get user's balance.
This error usually is noticed when having a reasonable amount of users which makes it hard to test, any tips on how to avoid deadlocks or any possible solution because I'm inserting rows into the transaction table in a very numerous way and looking to solve it!
I've also tried tried to catch the exception, but I couldn't create a loop that would redo the query until it is finished.

Answer 1

General answer
Deadlocks can only occur when you have two or more resources, two or more processes, and the processes lock the resources in different order.
Say, process 1 wants to lock resource A, then B, then C. Process 2 wants to lock B, then A, then C.
This may lead to a dead lock if 1 gets A, then 2 gets B, then 1 waits for B and 2 waits for A - indefinitely.
The solution is, thankfully quite simple: anytime if a process needs to lock two or more resources, it must do so in a "sorted" fashion. In this example,
if process 2 also gets A, then B, then C, a deadlock can never happen.
Specific answer
I your case, you seem to be locking different table rows within one transaction in more or less random order. Try to find out how to release locks with mysql and make sure you are only holding as many as you actually need. If you need to hold more than one at a time, try to order your requests in some way.
Hard to tell without knowing more about your code... the first Google hit for "mysql deadlock" shows some promising stuff though: https://www.percona.com/blog/2014/10/28/how-to-deal-with-mysql-deadlocks

Answer 2

I have create a sample table with 2 field. id has primary key
MariaDB [who]> DESC mynum;
+-------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| num | float | YES | | NULL | |
+-------+------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
I have filled with 100000 records
MariaDB [who]> SELECT * FROM mynum LIMIT 10;
+----+----------+
| id | num |
+----+----------+
| 1 | 0.47083 |
| 2 | 0.670773 |
| 3 | 0.941373 |
| 4 | 0.69455 |
| 5 | 0.648627 |
| 6 | 0.159488 |
| 7 | 0.851557 |
| 8 | 0.779321 |
| 9 | 0.341933 |
| 10 | 0.371704 |
+----+----------+
10 rows in set (0.00 sec)
MariaDB [who]> SELECT count(*) FROM mynum;
+----------+
| count(*) |
+----------+
| 100000 |
+----------+
1 row in set (0.02 sec)
Now i select row and calculate +10 to the id. You see that he must read ALL rows
MariaDB [who]> EXPLAIN SELECT * FROM mynum WHERE id +10 > 20;
+------+-------------+-------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+------+---------------+------+---------+------+--------+-------------+
| 1 | SIMPLE | mynum | ALL | NULL | NULL | NULL | NULL | 100464 | Using where |
+------+-------------+-------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)
and now i compare the id with a constant. You can see the only reads the row they use and use a index
MariaDB [who]> EXPLAIN SELECT * FROM mynum WHERE id < 10;
+------+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | mynum | range | PRIMARY | PRIMARY | 4 | NULL | 9 | Using where |
+------+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)