A practical method to improve the efficiency of database query, foreign key about performance

1. To optimize the query, avoid full-table scanning as far as possible, and first consider establishing an index on the columns involved in the Where and order by.

2. Avoid null-valued fields in the WHERE clause, which will cause the engine to discard full-table scans using the index, such as:

Select ID from t where num is null

You can set the default value of 0 on NUM, make sure that the NUM column in the table does not have a null value, and then query:

Select ID from t where num=0

3. Try to avoid using the! = or <> operator in the WHERE clause, or discard the engine for a full table scan using the index.

4. You should try to avoid using or in the WHERE clause to join the condition, otherwise it will cause the engine to abandon using the index for a full table scan, such as:

Select ID from t where num=10 or num=20

You can query this:

Select ID from t where num=10

UNION ALL

Select ID from t where num=20

5.in and not in should also be used with caution, otherwise it will result in full table scans, such as:

Select ID from t where num in

For consecutive 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 be more efficient, consider full-text indexing.

7. If you use a parameter in the WHERE clause, it also causes a full table scan. Because SQL resolves local variables only at run time, the optimizer cannot defer the selection of access plans to run time; it must be selected at compile time. However, if an access plan is established at compile time, the value of the variable is still unknown and therefore cannot be selected as an input for the index. The following statement will perform a full table scan:

Select ID from t where [email protected]

You can force the query to use the index instead:

Select ID from T with (index name) where [email protected]

8. You should try to avoid expression operations on the fields in the WHERE clause, which will cause the engine to discard the full table scan using the index. Such as:

Select ID from t where num/2=100

should read:

Select ID from t where num=100*2

9. You should try to avoid function operations on the fields in the WHERE clause, which will cause the engine to discard the full table scan using the index. Such as:

Select ID from t where substring (name,1,3) = ' abc '--name ID starting with ABC

Select ID from t where DATEDIFF (day,createdate, ' 2005-11-30 ') =0--' 2005-11-30 ' generated ID

should read:

Select ID from t where name like ' abc% '

Select ID from t where createdate>= ' 2005-11-30 ' and createdate< ' 2005-12-1 '

10. Do not perform functions, arithmetic operations, or other expression operations on the left side of "=" in the WHERE clause, or the index may not be used correctly by the system.

11. When using an indexed field as a condition, if the index is a composite index, you must use the first field in the index as a condition to guarantee 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, such as the need to generate an empty table structure:

Select Col1,col2 into #t from T where 1=0

This type of code does not return any result sets, but consumes system resources and should be changed to this:

CREATE TABLE #t (...)

13. It is a good choice to replace in with exists in many cases:

Select num from a where num in (select num from B)

Replace with 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, SQL is query-optimized based on data in the table, and when there is a large amount of data duplication in the index columns, SQL queries may not take advantage of the index, as there are fields in the table Sex,male, female almost half, So even if you build an index on sex, it doesn't work for query efficiency.

15. The index is not the more the better, although the index can improve the efficiency of the corresponding select, but also reduce the efficiency of insert and UPDATE, because the INSERT or update when the index may be rebuilt, so how to build the index needs careful consideration, depending on the situation. The number of indexes on a table should not be more than 6, if too many you should consider whether some of the indexes that are not commonly used are necessary.

16. You should avoid updating clustered index data columns as much as possible, because the order of the clustered index data columns is the physical storage order of the table records, which can consume considerable resources once the column values change to the order in which the entire table is recorded. If your application needs to update clustered index data columns frequently, you need to consider whether the index should be built as a clustered index.

17. Use numeric fields as much as possible, if the field containing only numeric information should not be designed as a character type, which will reduce the performance of queries and connections and increase storage overhead. This is because the engine compares each character in a string one at a time while processing queries and joins, and it is sufficient for a numeric type to be compared only once.

18. Use Varchar/nvarchar instead of Char/nchar as much as possible, because the first variable length field storage space is small, can save storage space, second, for the query, in a relatively small field in the search efficiency is obviously higher.

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

20. Try to use table variables instead of temporary tables. If the table variable contains a large amount of data, be aware 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 unusable, and they can be used appropriately to make certain routines more efficient, for example, when you need to repeatedly reference a dataset in a large table or a common table. However, for one-time events, it is best to use an export table.

23. When creating a temporary table, if you insert a large amount of data at one time, you can use SELECT INTO instead of CREATE table to avoid causing a large number of logs to increase speed, and if the amount of data is small, create table to mitigate the resources of the system tables. Then insert.

24. If a temporary table is used, be sure to explicitly delete all temporary tables at the end of the stored procedure, TRUNCATE table first, and then drop table, which avoids longer locking of the system tables.

25. Avoid using cursors as much as possible, because cursors are inefficient and should be considered for overwriting if the cursor is manipulating more than 10,000 rows of data.

26. Before using a cursor-based method or temporal table method, you should first look for a set-based solution to solve the problem, and the set-based approach is generally more efficient.

27. As with temporary tables, cursors are not unusable. Using Fast_forward cursors on small datasets is often preferable to other progressive processing methods, especially if you must reference several tables to obtain the required data. Routines that include "totals" in the result set are typically faster than using cursors. If development time permits, a cursor-based approach and a set-based approach can all be tried to see which method works better.

28. Set NOCOUNT on at the beginning of all stored procedures and triggers, set NOCOUNT OFF at the end. You do not need to send a DONE_IN_PROC message to the client after each statement that executes the stored procedure and trigger.

29. Try to avoid large transaction operation and improve the system concurrency ability.

30. Try to avoid the return of large data to the client, if the amount of data is too large, should consider whether the corresponding demand is reasonable.
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QUOTE: Original post by newkid on 2010-6-10 22:48
Remember that I have been arguing with others about this problem more than once.
1. You may have a bug in your program, and you can judge it by your own judgment, and it will do it quickly and well.
Most people's programs do not consider concurrency issues. Once considered, it has to be manually locked, the efficiency is very low.
Data may be bypassed by your application into the database.
2. Performance issues: Don't you have the overhead of doing it yourself?
A foreign key judgment is apportioned to the transaction level, and the overhead can be ignored and the user is completely unaware.
If you are importing data in bulk, you can temporarily mask the foreign keys, then quickly recover with the novalidate option, provided your data is clean.
3. Take a look at the troublesome example?
FOREIGN KEY constraint is just to prevent you from messing up, this is to protect you.
How about a car seat belt trouble? Sometimes it can save your life.
4. Nyfor said the delay constraint could be used. But according to my experience there is no need. If there is a foreign key, father data must be. For example, your storage list, warehousing details need this number, then Father form (warehousing order) certainly to Mr. Cheng.
In addition, the foreign key will provide the CBO with important information to generate the optimal plan.
Opposition: representative of the character Qingyun. The main points are as follows
QUOTE: Original post by Qingyun on 2010-6-10 13:32
I don't really like foreign keys,
Reason:
1. Program logic, integrity, I will be in the stored procedures or packages and other places to do rigorous judgment;
2. Performance issues, this is the most I do not like to use the key reasons, such as a business flow table, frequently inserted data, if the table has 3 foreign keys, then each time you insert a bar, you must have 3 foreign keys corresponding to the 3 table to do the corresponding lookup to determine if there is no corresponding data, if the 3 tables are also large, The judgment time of the 3 tables is very often, although the foreign key points to the associated table of the field is definitely indexed, but I think a lot of times, such a judgment is in the program control, through the foreign key to judge once, is to reduce performance, and in fact, some places do not judge the matter, but with the foreign key, Must be time to judge, no matter how the Oracle internal optimization of foreign keys for data retrieval speed, it is always a small consumption;
3. Maintenance trouble, many of the company's software are customized, this customization of things, random relatively large, project development and implementation process, need to often tinkering with the table; there is the business logic has a bug or other circumstances, need to constantly manually maintain data, there is a complex foreign key association, it is troublesome;
4. Foreign key dead two tables of data between the generation of the relationship, the most common is the document master-slave table, sometimes, in the generation of documents, is the details of the gentleman, and then generate the main table; If the foreign key is nailed, this can not be achieved;
Of course some key business, do need foreign key;
Why say this topic, I put the project database established today, with a few foreign keys, around colleagues said data is not rigorous, need to complicate the same field name of the foreign key are established, this I am reluctant;
For example, this database altogether 100 tables, according to their idea, the foreign key may have 300; I faint, too dogmatic;
If I were to build 300 indexes, I'd be happy to, because to improve operational efficiency, the key is only detection rigor, the efficiency of the database operation is reduced, there is no possibility of improvement;
In fact, this is only a design habit of the problem, there are interested people casually talk about their habits.
There is a voice of the opposition has a unique perspective, put it together for your reference
QUOTE: Original post by Ruideliang on 2010-6-12 14:42
The foreign key is exposed, the program is closed, is also tested by the program and foreign key constraints, human factors resulting in the possibility of constraint failure who big who small, very obvious, so against the use of foreign keys, because of the conflict with the system high availability purposes