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Technical Articles

Database

What are gap locks and next-key locks in InnoDB for MySQL?

Gaplockslockindexgapstopreventinsertions,next-keylockscombinerecordandgaplockstoblockinsertsandmodifications;1.GaplocksapplytorangesbetweenindexvaluesandpreventphantomreadsbyblockinginsertsinREPEATABLEREAD;2.Next-keylockslockbotharecordandthegapbefor

Aug 07, 2025 pm 04:54 PM

How to grant privileges to a user in Oracle

UseGRANTprivilege_nameTOusernameforsystemprivilegeslikeCREATESESSIONorCREATETABLE.2.UseGRANTobject_privilegeONschema.objectTOusernameforobjectprivilegessuchasSELECTorINSERTonspecifictables.3.GrantroleswithGRANTrole_nameTOusernametosimplifyprivilegema

Aug 07, 2025 pm 04:39 PM

What are the BLOB and CLOB data types in SQL?

BLOBstoresbinarydatalikeimages,audio,orPDFsasrawbyteswithoutcharacterencoding,whileCLOBstoreslargetextsuchasarticlesorJSONusingcharacterencodinglikeUTF-8andsupportsstringoperations;2.Bothcanhandleuptogigabytesofdatadependingonthedatabase,butperforman

Aug 07, 2025 pm 04:22 PM

sql

How to get a random row from a table in MySQL efficiently?

ORDERBYRAND() should be avoided for large tables, 1. Small table (=(SELECTFLOOR(RAND()(MAX(id)-MIN(id) 1)) MIN(id)FROMyour_table)ORDERBYidLIMIT1; 3. The better solution is to generate a random ID and find the first record greater than or equal to the ID, and cooperate with the retry mechanism to deal with the ID gap; 4. Pre-computed random columns with indexes can be added in high-performance scenarios, such as ALTERTABLEyour_tableADDCOLUMNrand_colFLOAT, CREATEINDEXidx_randONyour_table(rand)

Aug 07, 2025 pm 04:00 PM

How to empty a table in phpMyAdmin

ToemptyatableinphpMyAdmin,usethe"Empty"(TRUNCATE)optionbyselectingthetable,goingtothe"Operations"tab,andclicking"Go"inthe"Emptythetable(TRUNCATE)"section;2.Alternatively,runacommandintheSQLtabbyenteringTRUNCATE

Aug 07, 2025 pm 03:23 PM

How to increase the max_connections value in MySQL

Tosafelyincreasemax_connectionsinMySQL,firstcheckthecurrentvalueusingSHOWVARIABLESLIKE'max_connections';thentemporarilysetanewvaluewithSETGLOBALmax_connections=500;fortesting.Tomakeitpermanent,edittheMySQLconfigfile(e.g.,/etc/mysql/my.cnformy.ini)und

Aug 07, 2025 pm 01:31 PM

How to set the default character set for a database in MySQL

SetthedefaultcharactersetduringdatabasecreationusingCREATEDATABASEmydbDEFAULTCHARACTERSETutf8mb4COLLATEutf8mb4_unicode_ci.2.Forexistingdatabases,useALTERDATABASEmydbCHARACTERSETutf8mb4COLLATEutf8mb4_unicode_ci,notingthisonlyaffectsnewtables.3.Configu

Aug 07, 2025 pm 01:04 PM

How to Choose a MongoDB Driver

Select the official MongoDB driver that matches the main and backend languages; 2. Check the maturity and community activity of the driver, and give priority to those that are supported and maintained frequently; 3. Select synchronous or asynchronous drivers based on the application architecture, and decide whether to use ORM as needed to balance development speed and performance; 4. Consider team experience, familiarity with ORM can improve efficiency, but it is recommended to directly connect to native drivers in high-performance scenarios.

Aug 07, 2025 pm 12:54 PM

選擇驅(qū)動

How to call a function in MySQL

In MySQL, functions are called by referencing their names in SQL statements, rather than using CALL commands; 1. When using built-in functions, write the function name directly with brackets in clauses such as SELECT, WHERE, etc.; 2. When calling user-defined functions, they are also referenced in SELECT statements, such as SELECTCalculateTax(100); 3. The CALL statement is only used for stored procedures and cannot be used for functions. Incorrectly using CALL to call functions will lead to syntax errors. The correct way is to always use the function as a value generator in expressions or queries.

Aug 07, 2025 pm 12:51 PM

mysql function call

What are the performance benefits of using Hashes for small objects?

Using hashing to store small objects can improve performance because it reduces memory overhead and speeds up access. When storing large amounts of small data, use hash to merge fields to a single key, thereby sharing metadata, reducing the overhead of each field. For example, using 100 values as a separate key requires 100 complete object headers and hash only requires one key header; in addition, hash supports reading and writing by field without loading the entire object, such as Redis, HGET commands that can obtain a certain field individually and reduce network bandwidth consumption; finally, many systems provide atomic operations such as HINCRBY for safe incremental counters to ensure reliability in concurrent environments.

Aug 07, 2025 pm 12:47 PM

How to Use User-Defined Functions (UDFs) in MySQL?

MySQL supports creating SQL storage functions through CREATEFUNCTION, which can be used to return single values and can be called in the query; 1. Use DELIMITER to define the function, including RETURNS to specify the return type, READSSQLDATA declares data reading, and DETERMINISTIC indicates determinism; 2. Variables, IF/CASE conditions and loops can be used in the body of the function; 3. Can be deleted through DROPFUNCTION or redefined by CREATEORREPLACEFUNCTION; 4. Limitations include the inability to execute dynamic SQL, the inability to return result sets, and the data cannot be modified usually; 5. Real C/C UDF exists but needs to be compiled and has security risks.

Aug 07, 2025 am 11:32 AM

How to create a user in phpMyAdmin

The most reliable way to create a user is to use the SQL tab of phpMyAdmin, execute the CREATEUSER command and grant the corresponding permissions; 2. If the interface provides the "User" tab, you can also add the user and assign permissions through the interface; 3. After creation, you need to execute FLUSHPRIVILEGES and verify whether the user has successfully added it, and the operation requires corresponding permissions. It is recommended to use 'username'@'localhost' to enhance security.

Aug 07, 2025 am 11:27 AM

用戶創(chuàng)建

How to handle JSON data in Oracle?

Oracle database supports JSON natively since 12c. You can store JSON data by 1. Using VARCHAR2, CLOB or BLOB columns and adding ISJSON constraints; 2. Using JSON_VALUE to extract scalar values, JSON_QUERY to extract complex objects or arrays, and JSON_TABLE to flatten nested structures into relationship rows; 3. Filter data according to paths or conditions through JSON_EXISTS; 4. Create function-based index or OracleText search index to improve query performance; 5. Use JSON_MERGEPATCH or JSON_TRANSFORM after 19c; 6. Combine ISJSON

Aug 07, 2025 am 11:24 AM

Monitoring Your MongoDB Instance

UseMongoDB’sbuilt-intoolslikemongostatforreal-timeperformancestatsandmongotoptotrackcollection-levelread/writetimes.2.Enabledatabaseprofilingwithdb.setProfilingLevel(1,{slowms:100})tologslowqueriesandidentifyperformanceissueslikeCOLLSCANormissinginde

Aug 07, 2025 am 09:15 AM