Question

I’m in a situation where an entire column in a table (used for user tokens) needs to be wiped, i.e., all user tokens are reset simultaneously. There are two ways of going about it: reset each user’s token individually with a separate UPDATE query; or make one big query that affects all rows.

The advantage of one big query is that it will obviously be much faster, but I’m worried about the implications of a large UPDATE query when the database is big. Will requests that occur during the query be affected?

Answer 1

Afraid it’s not that simple. Even if you enable dirty reads, running one big update has a lot of drawbacks:

• long running transaction that updates one column will effectively block other insert, update and delete transactions.
• long running transaction causes enourmous load on disk because server is having to write to a log file everything that is taking place so that you can roll back that huge transaction.
• if a transaction fails, you would have to rerun it entirely, it is not restartable.

So if simultaneous requirement can be interpreted “in one batch that may take a while to run”, I would opt for batching it. A good research write up on performance of DELETEs in MySql is here: http://mysql.rjweb.org/doc.php/deletebig, and I think most of the findings are applicable to UPDATE.

The trick will be finding the optimal “batch size”.

Added benefits of batching is that you can make this process resilient to failures and restart-friendly.