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  • What is a Gap Lock and what problem does it solve?
    What is a Gap Lock and what problem does it solve?
    The main reason for Gap locks is to prevent phantom reading and ensure data consistency of the database at the repeatable read isolation level. When performing a range query, such as SELECT...FORUPDATE, InnoDB will add a Gap lock to the index range, preventing other transactions from inserting new records into the range. 1. The Gap lock locks the "gap" between index records, not the specific row; 2. It is mainly used for range query, such as SELECT...FORUPDATE or SELECT...LOCKINSHAREMODE; 3. The Gap lock is released at the end of the transaction; 4. The Gap lock does not block read operations, but will prevent other transactions from inserting data into the locked range; 5. The Gap lock is sometimes combined with the record lock to form.
    Mysql Tutorial . Database 433 2025-06-17 09:35:20
  • How large should the innodb_buffer_pool_size be set to?
    How large should the innodb_buffer_pool_size be set to?
    Setting the ideal size of innodb_buffer_pool_size requires based on the dataset size, server memory and whether the service is exclusive. Usually for dedicated MySQL servers, it is recommended that the initial value is 70-80% of the system memory, such as 16GB server set to 12GB-14GB and 64GB set to 45GB-55GB; however, it is necessary to adjust the actual data volume and system load to avoid insufficient memory or use of swap partitions; evaluate the usage of the buffer pool by checking the .ibd file size and monitoring tools (such as SHOWENGINEINNODBSTATUS, performance_schema, etc.), and pay attention to signals such as high disk reading, low hit rate, or frequent page eviction; at the same time, note
    Mysql Tutorial . Database 560 2025-06-17 09:33:21
  • What does the (11) in INT(11) actually mean?
    What does the (11) in INT(11) actually mean?
    The numbers in INT(11) represent the display width, not the storage size or numerical range. Specifically: 1. The display width only works when combined with ZEROFILL. If INT(3) ZEROFILL insertion 7 will be displayed as 007; 2. The INT type always occupies 4 bytes, and the value range is fixed to -2,147,483,648 to 2,147,483,647 (signed) or 0 to 4,294,967,295 (unsigned); 3. INT(n) does not limit the number of digits inserted, which is different from CHAR(n); 4. Tools often generate INT(11) by default, especially for primary key ids, but have no impact on performance and data integrity; 5. Unless it depends on ZEROFILL formatted input
    Mysql Tutorial . Database 750 2025-06-17 09:32:50
  • How to create a new MySQL database and user?
    How to create a new MySQL database and user?
    To create a new MySQL database and user, first use the CREATEDATABASE command to create the database, for example: CREATEDATABASEmy_blog; then create the user and set the password, such as CREATEUSER'blog_user'@'localhost'IDENTIFIEDBY'StrongP@ssw0rd!'; then authorize the database permissions through GRANTALLPRIVILEGESONmy_blog.*TO'blog_user'@'localhost'; execute FLUSHPRIVILEGES; refresh the permissions, and finally verify whether you log in successfully and view the database
    Mysql Tutorial . Database 374 2025-06-17 09:24:41
  • Why is InnoDB the recommended storage engine now?
    Why is InnoDB the recommended storage engine now?
    InnoDB is MySQL's default storage engine because it outperforms other engines such as MyISAM in terms of reliability, concurrency performance and crash recovery. 1. It supports transaction processing, follows ACID principles, ensures data integrity, and is suitable for key data scenarios such as financial records or user accounts; 2. It adopts row-level locks instead of table-level locks to improve performance and throughput in high concurrent write environments; 3. It has a crash recovery mechanism and automatic repair function, and supports foreign key constraints to ensure data consistency and reference integrity, and prevent isolated records and data inconsistencies.
    Mysql Tutorial . Database 216 2025-06-17 09:18:21
  • What is the difference between UNION and UNION ALL?
    What is the difference between UNION and UNION ALL?
    ThemaindifferencebetweenUNIONandUNIONALLinSQListhatUNIONremovesduplicaterows,whileUNIONALLretainsallrowsincludingduplicates.1.UNIONperformsaDISTINCToperationacrossallcolumnsfrombothresultsets,whichinvolvessortingorhashingdatatoeliminateduplicates,mak
    Mysql Tutorial . Database 808 2025-06-14 00:37:21
  • How to find and optimize slow queries in MySQL?
    How to find and optimize slow queries in MySQL?
    Turning on slow query logs, using tool analysis, optimizing specific queries, and regular monitoring are four key steps in optimizing MySQL slow query. First, check and enable slow_query_log through SHOWVARIABLES to set the appropriate long_query_time threshold and log path; secondly, use mysqldumpslow or pt-query-digest to analyze the log location problem SQL; then use EXPLAIN to view the execution plan, focusing on optimizing queries such as missing indexes, large number of scan lines, and file sorting; finally establish a continuous monitoring mechanism and review the logs regularly, and ensure long-term effectiveness in combination with SQL audits before going online.
    Mysql Tutorial . Database 480 2025-06-14 00:37:01
  • What are the most important parameters for mysqldump?
    What are the most important parameters for mysqldump?
    Thefiveessentialmysqldumpparametersforreliablebackupsare--single-transaction,--lock-tables,--routines--events--triggers,connectionoptionslike-h-u-p,and--add-drop-table/--add-drop-database.First,--single-transactionensuresaconsistentbackupwithoutlocki
    Mysql Tutorial . Database 882 2025-06-14 00:36:20
  • How to alter a large table without locking it (Online DDL)?
    How to alter a large table without locking it (Online DDL)?
    Toalteralargeproductiontablewithoutlonglocks,useonlineDDLtechniques.1)IdentifyifyourALTERoperationisfast(e.g.,adding/droppingcolumns,modifyingNULL/NOTNULL)orslow(e.g.,changingdatatypes,reorderingcolumns,addingindexesonlargedata).2)Usedatabase-specifi
    Mysql Tutorial . Database 665 2025-06-14 00:36:00
  • How does InnoDB implement Repeatable Read isolation level?
    How does InnoDB implement Repeatable Read isolation level?
    InnoDB implements repeatable reads through MVCC and gap lock. MVCC realizes consistent reading through snapshots, and the transaction query results remain unchanged after multiple transactions; gap lock prevents other transactions from inserting data and avoids phantom reading. For example, transaction A first query gets a value of 100, transaction B is modified to 200 and submitted, A is still 100 in query again; and when performing scope query, gap lock prevents other transactions from inserting records. In addition, non-unique index scans may add gap locks by default, and primary key or unique index equivalent queries may not be added, and gap locks can be cancelled by reducing isolation levels or explicit lock control.
    Mysql Tutorial . Database 696 2025-06-14 00:33:01
  • How does AUTO_INCREMENT work in MySQL?
    How does AUTO_INCREMENT work in MySQL?
    After setting the AUTO_INCREMENT column in MySQL, the database will add 1 to assign a new value to ensure uniqueness. For example, when there are IDs 1 to 5 in the table, the ID of the next inserted row is 6, and even if ID 5 is deleted, it will not be reused. If the table is empty, it starts from 1; if the specified value such as 100 is manually inserted, it starts from 101. This mechanism may cause numerical jumps due to failed inserts, transaction rollbacks, or batch operations, but does not affect performance and integrity. The starting value can be modified through ALTERTABLE, if set to 100, but conflicts with existing values ??must be avoided. In the master-master copy scenario, configure auto_increment_offset and auto_increment_i
    Mysql Tutorial . Database 339 2025-06-14 00:32:30
  • How to diagnose and resolve a deadlock in MySQL?
    How to diagnose and resolve a deadlock in MySQL?
    MySQL deadlock is caused by transaction loop waiting for resources, and can reduce the probability of occurrence by analyzing logs, unified access order, shortening transaction time, and optimizing indexes. 1. Use SHOWENGINEINNODBSTATUS\G to view the LATESTDETECTEDDEADLOCK section to obtain the transaction ID, hold lock, request lock and SQL statement. 2. Common reasons include inconsistent access order, too long transactions, and unreasonable indexes, resulting in too large lock range. 3. Solution strategies include unifying data access order, splitting transactions, optimizing SQL index hits, and adding retry mechanisms. 4. In actual example, two transactions update the same records in reverse order trigger deadlock, and the solution is to unify the operation order.
    Mysql Tutorial . Database 720 2025-06-14 00:32:11
  • How to optimize a query with too many OR conditions?
    How to optimize a query with too many OR conditions?
    Faced with SQL query performance problems that contain a large number of OR conditions, the answer is to optimize by reducing the number of ORs, using indexes reasonably, and adjusting the structure. Specific methods include: 1. Split the query into multiple subqueries and merge them with UNION or UNIONALL, so that each subquery can use the index independently; 2. Use IN to replace multiple OR conditions in the same field to improve readability and execution efficiency; 3. Create appropriate indexes, such as single column index, composite index or overlay index to accelerate data retrieval; 4. Optimize from the data modeling level, such as introducing a tag system, intermediate table or replacing OR conditions with JOIN, thereby fundamentally reducing the use of OR.
    Mysql Tutorial . Database 982 2025-06-14 00:31:00
  • How to fix the 'Too many connections' error?
    How to fix the 'Too many connections' error?
    When the "Toomyconnections" error occurs, it should be solved by adjusting the database configuration, optimizing application connection usage, cleaning up idle connections, and upgrading server configuration. 1. View and improve the max_connections value of MySQL and set it reasonably to match server performance. 2. The application side uses connection pools, optimizes slow queries, and releases connections in a timely manner to avoid wasting resources. 3. Check for idle or abnormal connections through SHOWPROCESSLIST, manually KILL invalid connection, and set wait_timeout to automatically disconnect idle connection. 4. If the problem persists, consider upgrading server resource configuration or introducing architectural optimization solutions such as read and write separation.
    Mysql Tutorial . Database 515 2025-06-14 00:23:31

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