I have a Drupal 6 website which is hosted on a shared hosting service and which is used to communicate with a desktop software application. Over the years I extended the user profile with over 70 different profile fields. With over 45.000 users this resulted in a profile_values table containing over 1.200.000 rows and I was getting complaints about slow queries and I’ve read it might lead to performance issues. Switching host or upgrading Drupal 6 to Drupal 7 is not an option!

Since the profile data is only used when the software communicates with the website, which is done once per application start I did not really have a need for such a large profile table so I created a custom module which stores the profile data per user in a single row as a serialized array. This also has the benefit that I don’t need to update my website every time a new profile value is added in the software.

But my MySQL slow query log is filling up again. The query that’s logged is SELECT * FROM {my_table}. As I know this table is quite large I was careful not do call such a query myself and therefore I cannot find this query in my module or any of the modules that use my module. I always select a single column and pass a specific parameter to check.

As far as I can see there are only two functions that might have an influence on this. I implement hook_user() for insert and update operations and I call drupal_write_record() when saving the user data. But running through the code I fail to see where such a query might be called.

From where might this query originate from?