Can AUTO_INCREMENT be safely used in a BEFORE TRIGGER in MySQL

Question

Instagram\'s Postgres method of implementing custom Ids for Sharding is great, but I need the implementation in MySQL.

So, I converted the method found at the bottom of

Answer 1

The following SQL Fiddle generates an output as shown below:

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 45
Server version: 5.5.35-1

Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> select `id` from `tablename`;
+-------------------+
| id                |
+-------------------+
| 11829806563853313 |
| 11829806563853314 |
| 11829806563853315 |
| 11829806563853316 |
| 11829806563853317 |
| 11829806563853318 |
| 11829806563853319 |
| 11829806563853320 |
| 11829806563853321 |
| 11829806563853322 |
| 11829806563853323 |
| 11829806563853324 |
| 11829806563853325 |
| 11829806563853326 |
| 11829806563853327 |
| 11829806563853328 |
| 11829806563853329 |
| 11829806563853330 |
| 11829806563853331 |
| 11829806563853332 |
| 11829806563853333 |
| 11829806563853334 |
| 11829806563853335 |
| 11829806563853336 |
| 11829806563853337 |
| 11829806563853338 |
| 11829806563853339 |
| 11829806563853340 |
| 11829806563853341 |
| 11829806563853342 |
| 11829806563853343 |
| 11829806563853344 |
| 11829806563853345 |
| 11829806563853346 |
| 11829806563853347 |
| 11829806563853348 |
| 11829806563853349 |
| 11829806563853350 |
| 11829806563853351 |
| 11829806563853352 |
+-------------------+
40 rows in set (0.01 sec)

Accept the answer if it really solves your need.

UPDATE

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 46
Server version: 5.5.35-1

Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> DELIMITER //

mysql> DROP FUNCTION IF EXISTS `nextval`//
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> DROP TRIGGER IF EXISTS `shard_insert`//
Query OK, 0 rows affected (0.00 sec)

mysql> DROP TABLE IF EXISTS `tablename_seq`, `tablename`;
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE `tablename_seq` (
    ->   `seq` BIGINT UNSIGNED AUTO_INCREMENT PRIMARY KEY
    -> )//
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE `tablename` (
    ->   `id` BIGINT UNSIGNED PRIMARY KEY
    -> )//
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE FUNCTION `nextval`()
    -> RETURNS BIGINT UNSIGNED
    -> DETERMINISTIC
    -> BEGIN
    ->   DECLARE `_last_insert_id` BIGINT UNSIGNED;
    ->   INSERT INTO `tablename_seq` VALUES (NULL);
    ->   SET `_last_insert_id` := LAST_INSERT_ID();
    ->   DELETE FROM `tablename_seq`
    ->   WHERE `seq` = `_last_insert_id`;
    ->   RETURN `_last_insert_id`;
    -> END//
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TRIGGER `shard_insert` BEFORE INSERT ON `tablename`
    -> FOR EACH ROW
    -> BEGIN
    ->   DECLARE `seq_id`, `now_millis` BIGINT UNSIGNED;
    ->   DECLARE `our_epoch` BIGINT UNSIGNED DEFAULT 1314220021721;
    ->   DECLARE `shard_id` INT UNSIGNED DEFAULT 1;
    ->   SET `now_millis` := `our_epoch` + UNIX_TIMESTAMP();
    ->   SET `seq_id` := `nextval`();
    ->   SET NEW.`id` := (SELECT (`now_millis` - `our_epoch`) << 23 |
    ->                            `shard_id` << 10 |
    ->                            MOD(`seq_id`, 1024)
    ->                   );
    -> END//
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO `tablename`
    -> VALUES
    -> (0), (0), (0), (0), (0),
    -> (0), (0), (0), (0), (0),
    -> (0), (0), (0), (0), (0),
    -> (0), (0), (0), (0), (0),
    -> (0), (0), (0), (0), (0),
    -> (0), (0), (0), (0), (0),
    -> (0), (0), (0), (0), (0),
    -> (0), (0), (0), (0), (0)//
Query OK, 40 rows affected (0.00 sec)
Records: 40  Duplicates: 0  Warnings: 0

mysql> DELIMITER ;

mysql> SELECT `id` FROM `tablename`;
+-------------------+
| id                |
+-------------------+
| 12581084357198849 |
| 12581084357198850 |
| 12581084357198851 |
| 12581084357198852 |
| 12581084357198853 |
| 12581084357198854 |
| 12581084357198855 |
| 12581084357198856 |
| 12581084357198857 |
| 12581084357198858 |
| 12581084357198859 |
| 12581084357198860 |
| 12581084357198861 |
| 12581084357198862 |
| 12581084357198863 |
| 12581084357198864 |
| 12581084357198865 |
| 12581084357198866 |
| 12581084357198867 |
| 12581084357198868 |
| 12581084357198869 |
| 12581084357198870 |
| 12581084357198871 |
| 12581084357198872 |
| 12581084357198873 |
| 12581084357198874 |
| 12581084357198875 |
| 12581084357198876 |
| 12581084357198877 |
| 12581084357198878 |
| 12581084357198879 |
| 12581084357198880 |
| 12581084357198881 |
| 12581084357198882 |
| 12581084357198883 |
| 12581084357198884 |
| 12581084357198885 |
| 12581084357198886 |
| 12581084357198887 |
| 12581084357198888 |
+-------------------+
40 rows in set (0.00 sec)

See db-fiddle.