I know it's been a very long time since I posted anything, but I felt the itch today. A question came up earlier this week at work: How much is JOIN performance affected when using columns that are not the exact same data type?

This is important for me because entity-attribute-value tables require a lot of self-joins. Let me start by saying that we mitigate one of the common drawbacks to EAVs - mashing diverse data types into a single column - by separating numeric, date, and character data into different tables. However, we mashed a lot of integer data into a DECIMAL(13,4) column right alongside our financial data. I recently noticed that most of the data in this EAV table has no fractional part, and to determine whether it would be worth moving it all into another table - as well as determine what column type to use - I spent an afternoon running …