Mysql – Can having a LIMIT on DELETE degrade performance dramatically

You could delete data in batches.

In SQL Server, the syntax is delete top X rows from a table. You then do it in a loop, with a transaction for each batch (if you have more than one statement, of course), so to keep transactions short and maintain locks only for short periods.

In MySQL syntax: DELETE FROM userTable LIMIT 1000

There are restrictions on that (can't use LIMIT in deletes with joins, for instance) but in this case you might be able to do it that way.

There is an additional danger to using LIMIT with DELETE when it comes to replication; the rows deleted are sometimes not deleted in the same order on the slave as it was deleted on the master.

Any application that runs queries against a SQL Server (or any database engine) instance will decrease the performance because it's extra load the server has to deal with. I assume the warning is just to cover themselves, legally.

What it comes down to is the scripts you run, and how much activity the application generates on its own. There may be options to disable the latter, which is usually a good idea when working against a production instance.

I'm not familiar with the particular product you mentioned (Azure?), so for example from Management Studio, I always disable IntelliSense by default on a production management box, because this runs queries behind the scenes to do its work. Activity Monitor is also a big consumer of resources, and sometimes people leave that tab open and forget about it.

So again, their warning is a standard thing; I would probably just ignore it.