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.