Advantages and disadvantages of High_Non_Group

Due to compression algorithms used, disk space requirements can be reduced without sacrificing performance.

If the column has a high number of unique values, this is the fastest index, with few exceptions described below.

This index is not recommended for GROUP BY queries.

Index not possible if uniqueness enforced.

Cannot use this index if data in your columns is FLOAT, REAL, DOUBLE, BIT, BINARY, VARBINARY, CHAR > 255 bytes, or VARCHAR > 255 bytes.