thanks. Actually, what you mentioned – was my very first recommendation for the customer. Thing is, in this case – there is a number of different tables in place of tb2 joined and in each case – different prefix is used. As query is really executed only occasionally, having it executed in less than a second (instead of few minutes) is enough and it does not cause additional disk/memory waste.

But, I agree- schema should be planned to avoid that sort of join in advance.

I understand where your coming from and whilst I don’t recommend wrapping crap design with hacks, you could actually eliminate the concat from the select query by adding an additional column to the table and use a trigger which updates this additional column by performing the concat at point of data insert/update. Then by changing the select query to use our new column we have eliminated the concat on where clause and have got all the speed benefits of a properly designed schema.

I wanted to add one more thing about it – generally this choice of making Number a binary string is counter intuitive to me.

The number is a string and for numbers it does not matter if it is case sensitive and case insensitive so I’d see CONCAT to simply handle numbers by converting them to the type of other argument.

Though I do not know may be this more intuitive solution would some ugly side effects.

This would be the case if we would live in the perfect world. True in the database with good design you would not join on CONCAT but there are a lot of databases which are written with not so good design, and these are the ones which we’re called to fix quite commonly.

Often to do it right you would need to redo quite a lot, however the customer may not be ready for that in many cases so we end up squeezing as much as we can from the current schema which brings us to deal with strange queries which bring various weird issues.