MySQL - 触发器调用两次导致死锁

Question

I have a table with millions of rows and I have to use count it divided by groups.

CREATE TABLE `customers` (
    `id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
    `group_id` INT(10) UNSIGNED NULL DEFAULT NULL
)

so calls I made very often are

SELECT COUNT(*) FROM customers WHERE group_id=XXX

But unfortunately MySQL is really slow (>10 sec for one call) when counting in tables with dozens millions of rows.

So I decided to create a new table to keep counters only:

CREATE TABLE `customer_stats` (
    `group_id` INT(11) NOT NULL,
    `value` INT(11) NOT NULL,
)

where I can keep current counters and make sure it's up to date using triggers.

So I have a triggers for insert/update/delete, here's example of insert one:

CREATE TRIGGER `customers_insert` AFTER INSERT ON `customers` FOR EACH ROW 
BEGIN
    UPDATE customer_stats
    SET
      `value` = `value` + 1
    WHERE
      customer_stats.group_id = NEW.group_id;
END

and it works fine for most cases, but on high load (dozens of calls per seconds) I got deadlocks.

2016-09-21T20:14:30.639907Z 2057 [Note] InnoDB: Transactions deadlock detected, dumping detailed information.
2016-09-21T20:14:30.639926Z 2057 [Note] InnoDB:
*** (1) TRANSACTION:

TRANSACTION 10390, ACTIVE 0 sec starting index read
mysql tables in use 2, locked 2
LOCK WAIT 10 lock struct(s), heap size 1136, 5 row lock(s), undo log entries 1
MySQL thread id 2059, OS thread handle 140376644818688, query id 85330 test_test-php-fpm_1.test_default 172.19.0.12 root updating
UPDATE customer_stats
SET
  `value` = `value` + 1
WHERE
  customer_stats.group_id = NEW.group_id;
2016-09-21T20:14:30.639968Z 2057 [Note] InnoDB: *** (1) WAITING FOR THIS LOCK TO BE GRANTED:

RECORD LOCKS space id 85 page no 3 n bits 72 index customer_stats_key_group_id_unique of table `test`.`customer_stats` trx id 10390 lock_mode X locks rec but not gap waiting
Record lock, heap no 4 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 21; hex 637573746f6d657264657461696c735f636f756e74; asc customerdetails_count;;
 1: len 4; hex 80000002; asc     ;;
 2: len 6; hex 000000002890; asc     ( ;;
 3: len 7; hex 34000002341224; asc 4   4 $;;
 4: len 4; hex 80000666; asc    f;;

2016-09-21T20:14:30.640302Z 2057 [Note] InnoDB: *** (2) TRANSACTION:

TRANSACTION 10391, ACTIVE 0 sec starting index read
mysql tables in use 2, locked 2
10 lock struct(s), heap size 1136, 5 row lock(s), undo log entries 1
MySQL thread id 2057, OS thread handle 140376513820416, query id 85333 test_test-php-fpm_1.test_default 172.19.0.12 root updating
UPDATE customer_stats
SET
  `value` = `value` + 1
WHERE
  customer_stats.group_id = NEW.group_id;
2016-09-21T20:14:30.640334Z 2057 [Note] InnoDB: *** (2) HOLDS THE LOCK(S):

2016-09-21T20:14:30.640850Z 2057 [Note] InnoDB: *** WE ROLL BACK TRANSACTION (2)

It exists only on high load and I wonder if there is some easy way to change the trigger to make sure, that they don't try to execute that UPDATE customer_stats in the same time, as this is causing deadlock. So two customer record must be created in the same time to raise deadlock.

Tables and system of triggers I have is a little bit more complicated, but I tried to simplify it as much I can to explain you what is my problem.

Answer 1

You need a composite INDEX(key, group_id) , in either order.

Let's simplify the trigger: Step 1: VALUES is simpler than a SELECT :

BEGIN
    DECLARE originalGroupId INT;
    SET originalGroupId = NEW.group_id;
    INSERT IGNORE INTO table_stats(`key`, value, group_id)
        VALUES ('customers_count', 0, originalGroupId);   -- line changed
    UPDATE table_stats
    SET
      table_stats.`value` = table_stats.`value` + 1
    WHERE
      table_stats.`key` = "customers_count"
      AND table_stats.group_id = originalGroupId;
END

Step 2: Use IODKU. At this point, that needs to be UNIQUE(key, group_id) , in either order.

BEGIN
    DECLARE originalGroupId INT;
    SET originalGroupId = NEW.group_id;
    INSERT INTO table_stats(`key`, value, group_id)
        VALUES ('customers_count', 1, originalGroupId)  -- note 1 not 0
        ON DUPLICATE KEY UPDATE
            `value` = `value` + 1;
END

Steps 1 and 2 make it run faster, thereby decreasing the frequency of deadlock.

Step 3: Deal with deadlocks! They are not totally preventable. So, plan on repeating the entire transaction whenever a deadlock occurs.

Answer 2

ok, I think I found out what was the problem.

I tried to simplify the problem to show you here, but it looked like after I made it simpler - the problem was not existing anymore.

My original trigger was:

BEGIN
    DECLARE originalGroupId INT;
    SET originalGroupId = NEW.group_id;
    INSERT IGNORE INTO table_stats(`key`, value, group_id)
    SELECT 'customers_count', 0, originalGroupId;
    UPDATE table_stats
    SET
      table_stats.`value` = table_stats.`value` + 1
    WHERE
      table_stats.`key` = "customers_count"
      AND table_stats.group_id = originalGroupId;
END

and I looks like deadlock is caused by INSERT IGNORE or variable as when I deleted it - it started to work without any issues. Thanks!