begin tran select cont2 from testklup with ( HOLDLOCK ) where nlskey = 1 在默认的事务隔离级别下,开启事务,然后查询中使用HOLDLOCK提示。HOLDLOCK将共享锁保留到事务完成,而不是在相应的表、行或数据页不再需要时就立即释放锁。 然后使用DMV视图sys.
<span>begin</span> <span>tran</span> <span>select</span> cont2 <span>from</span> testklup <span>with</span>(<span>HOLDLOCK</span>) <span>where</span> nlskey<span>=</span><span>1</span>
在默认的事务隔离级别下,开启事务,然后查询中使用HOLDLOCK提示。HOLDLOCK将共享锁保留到事务完成,而不是在相应的表、行或数据页不再需要时就立即释放锁。
然后使用DMV视图sys.dm_tran_locks来查看持有锁的情况,查询结果如下:
从查询结果,可以得出申请了如下的锁:
1)在表object上申请了IS锁
2)在非聚集索引PAGE上申请了IS锁
3)在非聚集索引KEY上申请了S锁
4)在聚集索引的PAGE上申请了IS锁
5)在聚集索引的KEY上申请了S锁
查看锁申请和释放的顺序
我们使用SQL Profiler来跟踪锁申请和释放的事件。
备注:也可以在执行语句前,开启1200跟踪标记,使用语句dbcc traceon(1200,-1) (感谢CareySon)
模板选择TSQL-LOCKS,只跟踪运行语句的会话spid,同时事件增加Lock:Acquired和Lock:Released。执行查询跟踪结果如下:

MySQL functions can be used for data processing and calculation. 1. Basic usage includes string processing, date calculation and mathematical operations. 2. Advanced usage involves combining multiple functions to implement complex operations. 3. Performance optimization requires avoiding the use of functions in the WHERE clause and using GROUPBY and temporary tables.

Efficient methods for batch inserting data in MySQL include: 1. Using INSERTINTO...VALUES syntax, 2. Using LOADDATAINFILE command, 3. Using transaction processing, 4. Adjust batch size, 5. Disable indexing, 6. Using INSERTIGNORE or INSERT...ONDUPLICATEKEYUPDATE, these methods can significantly improve database operation efficiency.

In MySQL, add fields using ALTERTABLEtable_nameADDCOLUMNnew_columnVARCHAR(255)AFTERexisting_column, delete fields using ALTERTABLEtable_nameDROPCOLUMNcolumn_to_drop. When adding fields, you need to specify a location to optimize query performance and data structure; before deleting fields, you need to confirm that the operation is irreversible; modifying table structure using online DDL, backup data, test environment, and low-load time periods is performance optimization and best practice.

Use the EXPLAIN command to analyze the execution plan of MySQL queries. 1. The EXPLAIN command displays the execution plan of the query to help find performance bottlenecks. 2. The execution plan includes fields such as id, select_type, table, type, possible_keys, key, key_len, ref, rows and Extra. 3. According to the execution plan, you can optimize queries by adding indexes, avoiding full table scans, optimizing JOIN operations, and using overlay indexes.

Subqueries can improve the efficiency of MySQL query. 1) Subquery simplifies complex query logic, such as filtering data and calculating aggregated values. 2) MySQL optimizer may convert subqueries to JOIN operations to improve performance. 3) Using EXISTS instead of IN can avoid multiple rows returning errors. 4) Optimization strategies include avoiding related subqueries, using EXISTS, index optimization, and avoiding subquery nesting.

Methods for configuring character sets and collations in MySQL include: 1. Setting the character sets and collations at the server level: SETNAMES'utf8'; SETCHARACTERSETutf8; SETCOLLATION_CONNECTION='utf8_general_ci'; 2. Create a database that uses specific character sets and collations: CREATEDATABASEexample_dbCHARACTERSETutf8COLLATEutf8_general_ci; 3. Specify character sets and collations when creating a table: CREATETABLEexample_table(idINT

To safely and thoroughly uninstall MySQL and clean all residual files, follow the following steps: 1. Stop MySQL service; 2. Uninstall MySQL packages; 3. Clean configuration files and data directories; 4. Verify that the uninstallation is thorough.

Renaming a database in MySQL requires indirect methods. The steps are as follows: 1. Create a new database; 2. Use mysqldump to export the old database; 3. Import the data into the new database; 4. Delete the old database.


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