Question

Which one is more reliable and has better performance? Setting MySQL unique key and using INSERT IGNORE or first checking if data exists on database and act according to the result?

If the answer is the second one, is there any way to make a single SQL query instead of two?

UPDATE: I ask because my colleagues in the company I work believe that deal with such issues should be done in application part which is more reliable according to them.

Answer 1

You application won’t catch duplicates.

Two concurrent calls can insert the same data, because each process doesn’t see the other while your application checks for uniqueness. Each process thinks it’s OK to INSERT.

You can force some kind of serialisation but then you have a bottleneck and performance limit. And you will have other clients writing to the database, even if it is just a release script-

That is why there are such things as unique indexes and constraints generally. Foreign keys, triggers, check constraints, NULL/NIOT NULL, datatype constraints are all there to enforce data integrity

There is also the arrogance of some code monkey thinking they can do better.

See programmers.se: Constraints in a relational databases – Why not remove them completely? and this Enforcing Database Constraints In Application Code (SO)