First up. A large amount of my work has to do with ETL processes. There are a lot of things that can go wrong there, but one of the main issues is wrong estimations on size. When we read data from a source system to transfer it to the datawarehouse environment we have to match the datatypes. A varchar(10) in the source will have to be a varchar(10) in the target. Easy enough. But now the source gets an update and with that update the source datatype goes from varchar(10) to varchar(12). When the supplier informs us, we change the datatype accordingly and everything is fine. When for some reason the update is missed, issues will arise. Because off course it’s the primary key that got enlarged and duplicates will start to form.
The other way around happens as well. Some tools check out the source, see a varchar column and, when no-one notices, will create a nvarchar(2000) column. Joy will arise when this column contains one or two characters when the optimizer expects at least a thousand characters.
I’m in almost complete agreement with this notion, with the exception that I think
sql_variant is an abomination and its existence in a database is ipso facto proof that the designer came up with (or was forced into) a bad solution.