What is a ideal column length to set while creating a BTREE index?

I'm creating a BTREE index for a large table with data length of about 1.5GB. What would be a ideal Column le开发者_如何学编程ngth while creating a BTREE index?

Are you talking about the index prefix length for a column? It depends on your data. If you expect that your data will usually be unique after X characters, use X.