SQL optimization solution for millions of databases]

1. To optimize the query, try to avoid full table scanning. First, consider creating an index on the columns involved in where and order.

2. Try to avoid null value judgment on the field in the WHERE clause; otherwise Engine Abandon using indexes for full table scanning, for example:
Select ID from t where num is null

It can be on num. Set The default value is 0. Make sure that the num column in the table does not have a null value, and then query it as follows:
Select ID from t where num = 0

3. Try to avoid using it in the WHERE clause! = Or <> operator. Otherwise, the engine will discard the index for full table scanning.

4. Try to avoid using or in the WHERE clause to connect to the condition. Otherwise, the engine will discard the index and perform full table scanning, for example:
Select ID from t where num = 10 or num = 20

You can query it as follows:
Select ID from t where num = 10
Union all
Select ID from t where num = 20

5. Use in and not in with caution. Otherwise, a full table scan may occur, for example:
Select ID from t where num in (1, 2, 3)

For continuous values, you can use between instead of in:
Select ID from t where num between 1 and 3

6. The following query will also cause a full table scan:
Select ID from t where name like '% ABC %'
To improve efficiency, you can consider full-text search.

7. If a parameter is used in the WHERE clause, a full table scan is performed. Because SQL Only when running Analysis Local variable, but optimized Program You cannot set Access The plan must be postponed to running; Compile Select. However, if an access plan is created during compilation, the value of the variable is still unknown and thus cannot be used as an input for index selection. The following statement performs a full table scan:
Select ID from t where num = @ num

You can change it to force query to use the index:
Select ID from T with (index name) where num = @ num
. Avoid performing expression operations on fields in the WHERE clause whenever possible. This will cause the engine to discard the use of indexes for full table scanning. For example:
Select ID from t where num/2 = 100

Should be changed:
Select ID from t where num = 100*2

9. Avoid using fields in the WHERE clause whenever possible Function This will cause the engine to discard the index and perform a full table scan. For example:
Select ID from t where substring (name, 1, 3) = 'abc'-name ID starting with ABC
Select ID from t where datediff (day, createdate, '2017-11-30 ') = 0-'2017-11-30' generated ID

Should be changed:
Select ID from t where name like 'abc %'
Select ID from t where createdate> = '2014-11-30 'and createdate <'2014-12-1 ′

10. do not perform functions, arithmetic operations, or other expression operations on the left side of "=" in the WHERE clause. Otherwise System The index may not be correctly used.

11. when using an index field as a condition, if the index is a composite index, you must use the first field in the index as the condition to ensure that the system uses the index, otherwise, the index will not be used, and the field order should be consistent with the index order as much as possible.

12. Do not write meaningless queries. If you need to generate an empty table structure:
Select col1, col2 into # T from t where 1 = 0

This type Code If no result set is returned but system resources are consumed, change it to the following:
Create Table # T (...)

13. In many cases, replacing in with exists is a good choice:
Select num from a where num in (select num from B)

Replace the following statement:
Select num from a where exists (select 1 from B where num = A. Num)

14. Not all indexes are valid for queries. Data When there is a large amount of data repeat in the index column, the SQL query may not use the index. For example, if the table has sex fields, male and female are almost half of each other, then, even if an index is created on sex, the query efficiency will not be affected.

15. the more indexes, the better. Although the index can improve the efficiency of the SELECT statement, it also reduces the efficiency of insert and update, because the insert or update statements may recreate the index, therefore, you need to carefully consider how to create an index, depending on the actual situation. It is recommended that the number of indexes in a table be no more than six. If there are too many indexes, consider whether the indexes on some columns that are not frequently used are necessary.

16. update the clustered index data column should be avoided as much as possible, because the order of the clustered index data column is the physical storage order of the table records. Once the column value changes, the order of the entire table record will be adjusted, it will consume a considerable amount of resources. If Application If the system needs to frequently update the clustered index data column, you need to consider whether to create the index as a clustered index.

17. use numeric fields whenever possible. Do not use numeric fields only. Design Is Character This will reduce the query and Connection Performance And will increase the storage overhead. This is because the engine compares each character in the string one by one during query and connection processing, and only one comparison is required for the number type.

18. try to use varchar/nvarchar instead of Char/nchar, because the first step is to reduce the storage space of the variable-length field, which can save storage space. Secondly, for queries, searching in a relatively small field is obviously more efficient.

19. Do not use select * from t anywhere, replace "*" with a specific field list, and do not return any fields that are not used.

20. Try to use table variables instead of temporary tables. If the table variable contains a large amount of data, note that the index is very limited (only the primary key index ).

21. Avoid frequent creation and deletion of temporary tables to reduce the consumption of system table resources.

22. Temporary tables are not unavailable. Using them appropriately can make some routines more effective. For example, when you need to reference large tables or a data set in common tables repeatedly. However, it is best to use the export table for one-time events.

23. when creating a temporary table, if a large amount of data is inserted at one time, you can use select into instead of create table to avoid creating a large number of logs to increase the speed. If the data volume is small, to ease system table resources, create table first and then insert.

24. if a temporary table is used, you must explicitly delete all temporary tables at the end of the stored procedure. First truncate the table and then drop the table, so that the system table can be locked for a long time.

25. Avoid using a cursor whenever possible, because the efficiency of the cursor is poor. If the cursor operation has more than 10 thousand rows of data, you should consider rewriting.

26. before using the cursor-based or temporary table method, you should first find the set-based Solution Solution To solve the problem, the set-based method is generally more effective.

27. Like a temporary table, the cursor is not unavailable. Using a fast_forward cursor for a small dataset is usually better than other row-by-row processing methods, especially when several tables must be referenced to obtain the required data. A routine that includes "sum" in the result set is usually faster than a cursor. If Development Time permits. You can try both the cursor-based method and the set-based method to see which method works better.

28. Set nocount on at the beginning of all stored procedures and triggers, and set nocount off at the end. You do not need to go back to every statement in the stored procedure and trigger Client Send the done_in_proc message.

29. Avoid large transaction operations as much as possible to improve the system concurrency capability.

30. Avoid returning a large amount of data to the client as much as possible. If the data volume is too large, consider whether the corresponding requirements are reasonable.