Data types for index fields that do not support index creation include TEXT and BLOB fields, which cannot directly create an index.
The length of the index field data is too long, exceeding the database limit.
Some databases do not allow the creation of indexes on columns that contain NULL values.
The index has lost its optimization due to excessively repetitive values in the indexed field.
The length of the indexed field does not meet the database requirements, for example, when using prefix indexing in MySQL, the field length must be less than 767 bytes.
The index name is duplicated, there is already an index with the same name in the database.
The index fields contain special characters, such as spaces, commas, and so on.
The index field names are not standardized, such as using database keywords.
The index field types do not match, such as when creating a composite index, the field types must be consistent.
The index fields do not match the table fields, and the number of index fields does not match the number of table fields.
The database does not have enough space to store index data.