Causes and solutions for slow SQL Server database query speed

There are many reasons for slow query speed in SQL Server databases. The following are common causes:

1. No index or no index is used (this is the most common problem of slow query and is a defect in programming)

2. Low I/O throughput, resulting in a bottleneck effect.

3. the query is not optimized because no computing column is created.

4. Insufficient memory

5. slow network speed

6. The queried data volume is too large (you can use multiple queries to reduce the data volume in other ways)

7. Lock or deadlock (this is also the most common problem of slow query and is a defect in programming)

8. sp_lock and sp_who are active users. The reason is that they read and write competing resources.

9. Unnecessary rows and columns are returned.

10. The query statement is not good and is not optimized.

● You can optimize the query using the following methods:

1. Place data, logs, and indexes on different I/O devices to increase the reading speed. In the past, tempdb can be placed on raid0, which is not supported by SQL2000. The larger the data size (size), the more important it is to increase I/O.

2. vertically and horizontally split the table to reduce the table size (sp_spaceuse)

3. upgrade hardware

4. Create an index based on the query conditions, optimize the index, optimize the access mode, and limit the data volume of the result set. Note that the fill factor should be appropriate (preferably the default value 0 ). The index should be as small as possible. Use a column with a small number of bytes to create an index (refer to the index creation). Do not create a single index for fields with a limited number of values, such as gender fields.

5. Increase the network speed.

6. Expand the server memory. Windows 2000 and SQL Server 2000 support 4-8 GB memory.

Configure virtual memory: the virtual memory size should be configured based on services running concurrently on the computer. Run Microsoft SQL
Server? 2000, you can consider setting the virtual memory size to 1.5 times the physical memory installed on your computer. If you have installed the full-text search feature and intend to run Microsoft Search
To perform full-text indexing and query, you can consider setting the virtual memory size to at least three times the physical memory installed on your computer. Set SQL Server Max Server
The memory server configuration option is 1.5 times the physical memory (half of the virtual memory size ).

7. Increase the number of server CPUs. However, you must understand that resources such as memory are more required for concurrent processing of serial processing. Whether to use parallelism or serial travel is automatically evaluated and selected by MSSQL. A single task is divided
Multiple tasks can be run on the processor. For example, delayed query sorting, connection, scanning, and group by statement execution, SQL
The server determines the optimal parallel level based on the system load. complex queries that consume a large amount of CPU are most suitable for parallel processing. But update operations
Update, insert, and delete cannot be processed in parallel.

8. If you use like for query, you cannot simply use index, but the full-text index consumes space. Like 'a % ''use index like
''' % A' is not used for indexing. Use like ''' % A %''
The query time is proportional to the total length of the field value. Therefore, the char type cannot be used, but varchar is not used. Create a full-text index for a long field value.

9. Separate DB server and application server; Separate OLTP and OLAP

10. Distributed partition view can be used to implement Database Server consortium.

A consortium is a group of separately managed servers, but they collaborate to share the processing load of the system. This mechanism of forming Database Server consortium through partition data can expand a group of servers to support the processing needs of large multi-layer Web sites. For more information, see designing a database federation server.

A. before implementing the partition view, a horizontal partition table must be created.

B. After creating a member table, define a distributed partition view on each Member Server, and each view has the same name. In this way, queries that reference the view name of a distributed partition can be found in any
Member Servers. System operations are the same as if each member server has a copy of the original table, but in fact each server has only one member table and a distributed partition view. Data Location for application
The program is transparent.

11. Rebuild the index DBCC reindex, dbccindexdefrag, shrink data and log DBCC shrinkdb, and DBCC shrinkfile. set automatic log shrinking. for large databases, do not set Automatic database growth, which will reduce the server performance.

The writing of T-SQL is very important. The following lists common points: first, the process of DBMS processing the query plan is as follows:

1. query statement lexical and syntax check

2. submit the statement to the query optimizer of the DBMS.

3. optimizer performs algebra optimization and access path optimization

4. A query plan is generated by the Pre-compilation module.

5. Then, submit it to the system for processing and execution at the appropriate time.

6. Finally, return the execution result to the user.

Next, let's take a look at the data storage structure of SQL server: the size of a page is 8 K (8060) bytes, and the size of the eight pages is a disk area, which is stored according to the B tree.

12. Difference Between commit and rollback: Roll back all things. Commit: Submit the current transaction, there is no need to write in dynamic SQL
If you want to write data, write it out, for example, begin Tran exec (@ s) Commit trans, or write dynamic SQL as a function or stored procedure.

13. Use the WHERE clause in the SELECT statement to limit the number of returned rows to avoid table scanning. If unnecessary data is returned, the server's I/O resources are wasted, this increases the burden on the network and reduces performance. If the table is large, the table is locked during the table scan and other connections are prohibited from accessing the table. The consequence is serious.

14. SQL statement comments have no impact on execution

15. Try not to use the cursor. It occupies a large amount of resources. If you need row-by-row execution, try to use non-cursor technology, such as loop on the client, using temporary tables, table variables, subqueries, and case statements.

The cursor can be classified according to the extraction options it supports: only the rows must be extracted from the first row to the last row. Fetch next is the only allowed extraction operation and is also the default method. You can extract arbitrary rows randomly anywhere in the cursor. The cursor technology becomes very powerful in SQL2000, and its purpose is to support loops.

There are four concurrent options read_only: update cannot be located through the cursor, and there is no lock in the rows that make up the result set.

Optimistic
Values: Optimistic Concurrency Control is a standard part of transaction control theory. Optimistic Concurrency control is used in this situation, that is, in the interval between opening the cursor and updating the row, there is only a small chance for the second user to update
A row. When a cursor is opened with this option, there is no lock to control the rows, which will help maximize its processing capability. If you try to modify a row, the current value of the row is obtained
Compare the value. If any value changes, the server will know that the other person has updated the row and will return an error. If the value is the same, the server executes the modification.