Most datatypes can be used in indexes, but there are restrictions
when using LongText data types.
Indexes in the store database are B+tree structures with a fixed size key. This means that text keys may easily generate large indexes, and a maximum length text column cannot be indexed.
Keys in DBMS may be truncated by specifying the length of a text column to be used in the index key. Such a key column must always be the last one in the key, to ensure that the index provides a true ordering. If a text key column does not supply a truncation length, the full length of the table column is used.
Apart from Binary and LongBinary columns,
all types can be used in an index key including LongText8 and LongText16,
although these must supply a truncation length (as they have no maximum length)
and must therefore be the last key columns in the key.