Informix database index and query optimization principle

Informix database index and query optimization principle Overview: Index is a data structure used in the database to accelerate the data retrieval operation.In the Informix database, the index is achieved through the B-Tree index.By using the index correctly, the performance and response time of query can be significantly improved.Query optimization means improving query performance and efficiency by adjusting the execution plan and optimizing query statements of the query. This article will introduce the working principles of indexes in the Informix database and the basic principles of querying optimization. 1. The working principle of indexing in the Informix database: 1. B-Tree index: B-Tree is a very common index structure, which is a balanced, multi-branch tree structure.In B-Tree, each node of the index can save multiple index keys and pointers to the next-level node.This structure makes the storage of data in the index order orderly and helps quickly retrieve data. In the Informix database, the B-Tree index is the default index type.It is stored in paging files and sorted according to the specified column or expression.The creation of the B-Tree index can be completed through the Create Index statement. 2. Selectivity of indexing: The selectivity of the index refers to the uniqueness and repetitiveness of the index keys.The higher the selectivity, the greater the role of indexing in terms of querying performance.Selective calculations can be performed through the general record number of different values / indexes of the formula: selectivity = index column.When creating indexes, the size of the selectivity and indexing needs to be considered. 3. Index coverage: Through index coverage query, you can avoid access to the actual data on the disk, thereby improving the query performance.Index coverage means that the columns required in the query statement are included in the index without accessing the actual data.In the Informix database, indexing can be achieved by selecting the appropriate index and optimizing query statements. 2. The principle of query optimization: 1. Avoid query full table: Avoid full table scanning is one of the important principles to improve the performance of query.Full table scan can cause a large number of disk IO operations to reduce the efficiency of query.By using the appropriate index, you can avoid the full table scan and retrieve only related records. 2. Table connection optimization: Table connection is one of the common operations in the database. Through the appropriate connection order and connection conditions, the query performance can be improved.In the Informix database, you can optimize table connection operations by selecting the correct connection type (such as internal connection, left connection, right connection, etc.), establishing appropriate associated conditions, and using indexes. 3. Query sentence optimization: Optimized query statements are the key to improving query performance.Through reasonable writing query statements to avoid unnecessary calculations and data retrieval, it can reduce data reading and calculation overhead and improve the efficiency of query.When writing query sentences, you should pay attention to avoid using Select *, the appropriate WHERE clause, and the appropriate sorting method. 4. Collection and update of statistical information: Statistical information is essential for database query optimization.It can provide information about tables and indexes for query optimizers, so as to choose the appropriate execution plan.In the Informix database, the query performance can be optimized by collecting and updating statistics. 5. Adjustment of database configuration: In addition to optimizing query statements and indexes, you can also improve the query performance by adjusting the database configuration.For example, parameters such as the size and maximum number of connections can be adjusted to adapt to the actual query load. 3. Example program and related configuration: Here are some common example programs and related configurations that use indexes and optimization query in the Informix database: 1. Create index: CREATE INDEX idx_employee_id ON employees (employee_id); CREATE INDEX idx_customer_name ON customers (last_name, first_name); 2. Query optimization configuration: In the Informix database, the query optimizer uses an automatic query optimization mechanism.You can change the query optimization method by setting the informix_query_mode environment variable.For example, it can be set to '1' to use earlier query optimizers, or set to '2' to use a newer query optimizer. In addition, in the query statement, you can use the Hint statement prompts to force the query optimizer to select a specific execution plan.For example: SELECT /*+ INDEX(tablename indexname) */ column1, column2 FROM tablename WHERE condition; Through the above understanding, we can better understand the principles of indexing and query optimization in the Informix database, and can write effective query statements and adjust database configurations according to the actual situation to obtain better query performance and response time.