MySQL: unexpected SELECT result after INSERT into table with autoincrement column

Question

I'm seeing a weird behavior when I INSERT some data into a table and then run a SELECT query on the same table. This table has an auto-increment primary key (uid), and this problem occurs when I try to then select results where 'uid IS NULL'.

I've golfed this down to the following SQL commands:

DROP TABLE IF EXISTS test_users;
CREATE TABLE test_users (uid INTEGER PRIMARY KEY AUTO_INCREMENT, name varchar(20) NOT NULL);
INSERT INTO test_users(name) values('foo');
SELECT uid FROM test_users WHERE uid IS NULL;
SELECT uid FROM test_users WHERE uid IS NULL; -- no output from this query

I'd expect SELECT uid FROM test_users WHERE uid IS NULL to never return anything, but it does, sometimes. Here's what I've found:

• Version of MySQL/MariaDB seems to matter. The machine having this problem is running MySQL 5.1.73 (CentOS 6.5, both 32-bit and 64-bit). My other machine running 5.5.37-MariaDB (Fedora 19, 64-bit). Both running default configs, aside from being configured to use MyISAM tables.
• Only the first SELECT query after the INSERT is affected.
• If I specify a value for uid rather than let it auto-increment, then it's fine.
• If I disconnect and reconnect between the INSERT and SELECT, then I get the expected no results. This is easiest to see in something like Perl where I manage the connection object. I have a test script demonstrating this at https://gist.github.com/avuserow/1c20cc03c007eda43c82

Answer 1

This behavior is by design.

It's evidently equivalent to SELECT * FROM t1 WHERE id = LAST_INSERT_ID(); which would also work only from the connection where you just did the insert, exactly as you described.

It's apparently a workaround that you can use in some environments that make it difficult to fetch the last inserted (by your connection) row's auto-increment value in a more conventional way.

To be precise, it's actually the auto_increment value assigned to the first row inserted by your connection's last insert statement. That's the same thing when you only inserted one row, but it's not the same thing when you insert multiple rows with a single insert statement.

http://dev.mysql.com/doc/connector-odbc/en/connector-odbc-usagenotes-functionality-last-insert-id.html