You can degrade the performance of your database by overindexing. You should expect to see gains when you index and remove the newly created indexes if you don't.

The space involved in creating index also involves maintenance. In addition, indexes on tables that are updated or deleted frequently should be rebuilt when they become fragmented. This takes some time and attention.

There are two types of indexes that are commonly used. The bitmap index is used when a column has very few distinct values (5% of the overall rows) and is part of table that is not frequently updated.

A B-Tree index is used for columns in which no one value has more than 20% of the columns, but there are many different values in the column.

It's a long answer, but datatype of NUMBER vs VARCHAR shouldn't matter.

Good luck.