Is inserting a new database entry faster than checking if the entry exists first?

Question

I was once told that it is faster to just run an insert and let the insert fail than to check if a database entry exists and then inserting if it is missing.

I was also told that that most databases are heavily optimized for reading reading rather than writing, so wouldn't a quick check be faster than a slow insert?

Is this a question of the expected number of collisions? (IE it's faster to insert only if there is a low chance of the entry already existing.) Does it depend on the database type I am running? And for that matter, is it bad practice to have a method that is going to be constantly adding insert errors to my error log?

Thanks.

Answer 1

If the insert is going to fail because of an index violation, it will be at most marginally slower than a check that the record exists. (Both require checking whether the index contains the value.) If the insert is going to succeed, then issuing two queries is significantly slower than issuing one.