What are the types of MySQL indexes?

Analysis & Answer

From the perspective of functional logic, it can be divided into:

• Ordinary index INDEX (ordinary index) ALTER TABLE  table_name ADD INDEX index_name (  column )
• Unique index UNIQUE (unique index) ALTER TABLE  table_name ADD UNIQUE ( column)
• Primary key index PRIMARY KEY (primary key index) ALTER TABLE  table_name ADD PRIMARY KEY (  column )
• Composite Index Composite Index ALTER TABLE  table_name ADD INDEX index_name (  column1,  column2,  column3 )
• Full text index FULLTEXT (full text index) ALTER TABLE  table_name ADD FULLTEXT (  column )

From the perspective of data structure, it can be divided into:

• B+ tree index
• hash index
• full text index
• Spatial data index (R-Tree index) ( not required )

From the perspective of physical storage, it can be divided into:

• clustered index
• nonclustered index

Reflect & Expand

Tell me about the database index type? (Oracle)

logically:

• Single column single row index
• Concatenated multi-row index
• Unique unique index
• NonUnique non-unique index
• Function-based function index
• Domain domain index

Physically:

• Partitioned partition index
• NonPartitioned non-partitioned index
• B-tree：
• Normal normal type B tree
• Rever Key reverse B-tree 
• Bitmap bitmap index

Index structure:

B-tree：

• Suitable for a large number of additions, deletions, and changes (OLTP);
• Cannot use queries containing the OR operator;
• Suitable for columns with high cardinality (many unique values)
• Typical tree structure;
• Each node is a data block;
• Most of them are physically one layer, two layers or three layers, and logically three layers;
• Leaf block data is sorted, increasing from left to right;
• In the branch block and the root block are the ranges of the index;

Bitmap:

• fit and decision support system;
• The cost of doing UPDATE is very high;
• Very suitable for OR operator queries; 
• Bitmap indexes can only be built when the cardinality is relatively small;

The B*tree index is usually more suitable when accessing a small amount of data. For example, you access no more than 5% of the data in the table. Of course, this is only a relative ratio and applies to general situations. Bitmap is often used in data warehouses for low-cardinality columns, such as gender and other fields with many repeated values. The smaller the cardinality, the better.

Meow Interview Assistant: One-stop solution to interview questions, you can search the WeChat applet [Meow Interview Assistant]  or follow [Meow Brush Questions] -> Interview Assistant  free questions. If you have good interview knowledge or skills, look forward to your sharing!