What are the reasons for the failure of a MySQL composite index?

There are several possible reasons for the failure of a MySQL composite index to work effectively.

1. The order of index fields is not appropriate: The order of fields in a composite index is crucial. If the query conditions are not in the same order as the composite index, the index will not be used effectively.
2. If a function is applied to the indexed column in the search condition, the index will be invalidated.
3. Range query: If the query includes a range search, such as using BETWEEN, >, < and other operators, the index will become ineffective.
4. If the data type in the query condition does not match the data type of the indexed column, the index will become invalid.
5. If there are NULL values in the indexed column, the index will become invalid if the query contains NULL values in the indexed column.
6. When the amount of data in the table is too small, using an index for querying may not be advantageous compared to a full table scan, leading to the index being ineffective.
7. Index statistics expiration: If the statistics of an index expire, MySQL may incorrectly choose to use the index for queries, leading to index inefficiency.
8. Forced index usage: If an index hint is used in a query but the index is not suitable for the current query, then the index will become ineffective.

It is worth noting that the above are just some possible reasons that may cause index failure. The specific reasons will need to be analyzed based on the specific query statements and table structure.