Sql-server – Whats SQL Server NVARCHAR(max) equivalent in MySQL

You're seeing the cost overhead of using MAX types.

While NVARCHAR(MAX) is identical to NVARCHAR(n) in TSQL and can be stored in-row, it is handled separately by the storage engine because it can be pushed off-row. When off-row it is a LOB_DATA allocation unit, rather than ROW_OVERFLOW_DATA allocation unit and we can assume from your observations that this carries an overhead.

You can see the two types are internally stored differently with a little DBCC PAGE spelunking. Mark Rasmussen posted example page dumps that show the differences in What is the Size of the LOB Pointer for (MAX) Types Like Varchar, Varbinary, Etc?

We can probably assume it's the GROUP BY on the MAX column that causes the performance difference in your case. I've not tested other operations on a MAX type but it might be interesting to do so and see if similar results are seen.

Well, since you have data currently stored in SQL Server, and it's already in an NVARCHAR column, then either it's an NVARCHAR <= 4000 (in which case you can't lose any data, and should just change all instances of NVARCHAR(8000) to NVARCHAR(4000)), or it's an NVARCHAR(MAX) column, in which case you change all instances of NVARCHAR(8000) to NVARCHAR(MAX). Or just leave out those CASTs - do you really need them?

As an aside, you should probably change as NTEXT to as NVARCHAR(MAX) as well.