1. Hash index also does not support partial index column matching search. If you build an index on the data (a, b), the data cannot use the index when querying data a;
2. Hash index only supports equal value comparison query, all range search is not supported;
3. If the hash conflict is serious, the maintenance pressure on the database index is relatively large. If an index is created on a column with more duplicate values, when a piece of data is deleted, it is necessary to traverse the linked list of the corresponding hash value Every line.