Question

I’ve been using mysql_pconnect to establish all DB connections on my PHP site, with the theory that it’s more efficient (debatable, I know).

I went to use mysql_insert_id to get the ID from a recent INSERT and it occurred to me that given the multi-threaded nature of web requests, I can’t guarantee that another PHP script using the same pconnection has made a DB INSERT before my call to mysql_insert_id.

This is kind of a huge deal as I see no other way to guarantee atomicity of the INSERT and ID retrieval, as the ID is not returned by the INSERT query (or I don’t know how to get it).

So basically I can never use mysql_pconnect if I want to have thread-safe INSERTS and ID retrieval?

Answer 1

mysql_insert_id() returns the last id within context of the current connection session. Thus avoiding race-condition problems. There’s a bunch of notes/comments regarding this on the php.net manual for mysql_insert_id()

More Info:

• PHP/MySQL insert row then get 'id'
• How bad is using SELECT MAX(id) in MYSQL instead of mysql_insert_id() in PHP?
• get id of last inserted record without using mysql_insert_id()

This should be easy to test:

<?php
// connection already established
mysql_query("INSERT INTO table VALUES('foo', 'bar')");
sleep(15);
echo mysql_insert_id();

Then see if you can fool it by slamming it with requests during the sleep period.. and check the insert_id results.