Please help me understanding MySQL InnoDB Transactions

Question

I'm trying to understand the locking mechanisms for InnoDB.

Let's say i have the following PHP script:

mysql_query('BEGIN;');
$a=mysql_query('SELECT id, status FROM testtable WHERE id=123 LIMIT 1 FOR UPDATE;'); 
$res=mysql_fetch_assoc($a);
$b=somefunction($res['id']);
$c=someotherfunction($b);
mysql_query('UPDATE testtable SET status=1 WHERE id=123;');
mysql_query('COMMIT;');

inside somefunction() and someotherfunction() we'll do a lot of other queries from the same table, but also from other tables. It even selects other fields from testtable where id=123.

Do i get it right that within this session i can do whatever i want with id=123, but other sessions will wait for the lock to be released?

Because i use 'for update' in the first query, that's the only row in testtable which will be locked. Any other tables or rows selected/updated/inserted in the same session are left untouched (unlocked)?

What will happen if someotherfunction() selects and updates a different field on id=123? Is this possible? Does the lock stay?

Answer 1

I'm not sure about the locking part, but transactions are made to allow "all or nothing" on a group of queries.

For example, if you were to run the following two queries for a payment of some sort:

UPDATE customer SET credit=credit - 1 WHERE id=1;
INSERT INTO subscriptions VALUES (...);

If the INSERT query fails, the customer will have their credit decremented, but not get the subscription, for example.

By wrapping it in a transaction, if any part of the query fails, it goes back to being like it was before.

BEGIN;
UPDATE customer SET credit=credit - 1 WHERE id=1;
INSERT INTO subscriptions VALUES (...);
COMMIT;

Either both queries will succeed, or no change will be made on failure.

EDIT: I don't believe InnoDB will do any locking for you automatically, look up SELECT ... LOCK IN SHARE MODE and SELECT ... FOR UPDATE