When MySQL Indexes Fail: Common Scenarios

Here are some common scenarios that can cause MySQL indexes to become ineffective:

1. The query criteria not adhering to the principle of the most left prefix: When the columns in the query criteria are not the leftmost prefix of the index, MySQL is unable to use the index to speed up the query, resulting in the index becoming ineffective.
2. When using function operations in query conditions, such as using a function that includes an indexed column or converting the type of an indexed column, MySQL cannot utilize the index to speed up the query, resulting in the index being ineffective.
3. Perform a fuzzy search using the LIKE operator: When performing a fuzzy search using the LIKE operator, if the fuzzy matching pattern begins with a wildcard character (such as ‘%abc’) or two consecutive wildcard characters (such as ‘%abc%’), MySQL cannot use an index to speed up the query, resulting in the index being ineffective.
4. When using the OR operator in a query and at least one of the conditions cannot be accelerated using an index, the entire query will not be able to utilize the index, leading to index inefficiency.
5. Uneven data distribution: When the data distribution of the indexed column is uneven, with some values appearing very frequently and others appearing very rarely, MySQL may choose not to use the index and instead perform a full table scan.
6. When using JOIN operations to connect multiple tables, if the columns in the join conditions are not indexed, MySQL is unable to use indexes to speed up the JOIN operation, resulting in index inefficiency.
7. When the amount of data is very small in a table, such as only a few dozen or a few hundred rows, MySQL may choose not to use an index and instead perform a full table scan.
8. Frequently updated tables: When the data in a table is constantly being updated, if the frequency of these updates is too high, MySQL may opt not to use an index and instead perform a full table scan.

Please note that the above are just some common scenarios, the actual situation may vary depending on specific database design and query statements. To avoid index inefficiency, you can improve query performance by optimizing query statements, designing indexes appropriately, and avoiding excessive use of function operations.