TL; DR
SELECT … FOR UPDATE has a (not so) surprising side effect on non-existent rows: it could cause a (serious) performance penalty and even prevent you from inserting new rows at all.
Locking rows for update
A development team of ours was working on an application that needed to ensure an update on a single row item isn’t modified by another transaction. Naturally they started making use of SELECT … FOR UPDATE to lock the row before updating it. This worked excellent to keep anyone else from updating this row. However they started to get some lock wait timeouts on new inserts of totally unrelated items during a load test and they asked me to look into this.
SELECT … FOR UPDATE is described as following in the
MySQL documentation:
A SELECT ... FOR UPDATE …