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Technical Articles

Database

What is the wait_timeout variable in MySQL and how to configure it?

Thewait_timeoutvariableinMySQLspecifieshowlongaserverthreadwaitsforactivityonanon-interactiveconnectionbeforeclosingit;whenaconnectionremainsidlelongerthanthisvalue,MySQLterminatesittofreeresources.1.Itappliestonon-interactiveconnectionslikethosefrom

Aug 05, 2025 pm 12:52 PM

What are temporary tables in SQL and when should you use them?

Usetemporarytablestobreakdowncomplexqueriesbystoringintermediateresultsforclarityandperformance.2.UsethemtoimproveperformanceonrepeatedoperationsbyreducingredundantI/Oonlargetables.3.Usethemforsession-specificdataprocessingsuchasinstoredproceduresorE

Aug 05, 2025 pm 12:44 PM

Understanding MySQL Storage Requirements and Capacity Planning

MySQL capacity planning requires estimating the data volume, selecting the storage engine, and formulating monitoring and expansion strategies. 1. Data volume estimation: Calculate the total space based on the field size and estimated records of each table, and consider overheads such as index (extra 20%-30%), undolog, redolog, etc.; 2. Storage engine selection: Priority is used for InnoDB, which supports transactions and is suitable for high concurrency scenarios. Enable compression function to save space when necessary. Select UTF8MB4 or utf8/latin1 according to the requirements of the character set; 3. Capacity monitoring and expansion: Check the table size and disk usage regularly, set alarm thresholds, and expand the capacity can be vertically upgraded or horizontally split, and clean up historical data in combination with the business to free up space.

Aug 05, 2025 pm 12:33 PM

Oracle SQL Trace and TKPROF for Performance Analysis

How to enable SQLTrace? 1. Enable for the current session: Use ALTERSESSIONSETSQL_TRACE=TRUE; 2. Enable for other sessions: Specify sid and serial_num through DBMS_SESSION.SET_SQL_TRACE_FOR_SESSION; 3. Global Enable: Modify the initialization parameter file to set SQL_TRACE=TRUE, but it is not recommended. Trace needs to be turned off after use. TKPROF is used to convert the original trace file generated by SQLTrace into more readable text output. Common commands such as tkproftracefile.trcoutput.txt.

Aug 05, 2025 pm 12:01 PM

How does a client discover the new master after a Sentinel failover?

TofindthenewmasterafteraRedisSentinelfailover,clientsmustuseaSentinel-awarelibrary,provideSentineladdressesandthemastergroupname,detectconnectionbreakstore-querySentinels,optionallylistentopub/subeventslike switch-master,andcarefullymanageDNSorproxyl

Aug 05, 2025 am 11:07 AM

sentinel failover

How to repair a table in phpMyAdmin

First,checkifthetableneedsrepairbyrunningCHECKTABLEyour_table_name;inphpMyAdmin—ifthestatusisnot'OK',thetableiscorrupted.2.Torepair,selectthetableandchoose"Repairtable"fromthe"Withselected:"dropdown,orrunREPAIRTABLEyour_table_name

Aug 05, 2025 am 11:01 AM

修復(fù)表格

What is fragmentation in SQL indexes and how do you fix it?

Index fragmentation refers to the inconsistent order of the indexes in SQLServer with the physical storage order or the unused space in the page, resulting in a degradation of query performance; it is divided into internal fragments (the page is not filled, adding I/O) and external fragments (the logic does not match the physical order, resulting in random read); indexes with fragmentation rate greater than 10% and page count exceeding 1000 can be detected through the sys.dm_db_index_physical_stats function; 1. There is no need to process the fragmentation rate below 10%; 2.10%-30% are used to organize online; 3. More than 30% are used to recreate the index using ALTERINDEXREBUILD to obtain the optimal structure; the best

Aug 05, 2025 am 10:49 AM

How do you add or delete a column in an existing SQL table?

To add or delete a column in a table, use the ALTERTABLE statement; 1. Use ALTERTABLEtable_nameADDCOLUMNcolumn_namedata_type[constraints] when adding a column, for example, add an email column and set the default value: ALTERTABLEemployeesADDCOLUMNstatusVARCHAR(20)DEFAULT'active'; 2. Use ALTERTABLEtable_nameDROPCOLUMNcolumn_name when deleting a column, for example, delete an email column: ALTERTABLEemployeesADDCOLUMNcolumn_name; 2. Use ALTERTABLEtable_nameDROPCOLUMNcolumn_name when deleting a column, for example, delete an email column: ALTERTABLEemployeesADDCOLUMNcolumn_name.

Aug 05, 2025 am 10:35 AM

How to create a trigger in Oracle?

To create an Oracle trigger, you must first clarify the trigger timing, table name and execution logic. 1. Use CREATEORREPLACETRIGGER to define the trigger name; 2. Specify the BEFORE, AFTER or INSTEADOF triggering timing; 3. Declare the INSERT, UPDATE or DELETE events; 4. Specify the associated table name; 5. Add FOREACHROW to implement row-level triggering; 6. Optional WHEN clause to set the trigger conditions; 7. Write PL/SQL logic between BEGIN and END; for example, use NEW and :OLD to reference new and old values to avoid mutating table errors, and finally use user_triggers to

Aug 05, 2025 am 09:54 AM

What are table-valued functions in SQL?

Table-valuedfunctions(TVFs)returnatableandcomeintwotypes:inline,whichusesasingleSELECTstatement,andmulti-statement,whichallowsmultipleSQLoperationstobuildacustomresultset.2.Theyofferreusability,parameterization,andseamlessintegrationintoqueriesliketa

Aug 05, 2025 am 09:44 AM

What is the difference between TRUNCATE, DELETE, and DROP in MySQL?

DELETEremovesspecificorallrows,keepstablestructure,allowsrollbackandtriggers,anddoesnotresetauto-increment;2.TRUNCATEquicklyremovesallrows,resetsauto-increment,cannotberolledbackinmostcases,doesnotfiretriggers,andkeepstablestructure;3.DROPremovesthee

Aug 05, 2025 am 09:39 AM

mysql Database operations

How to manage user accounts in phpMyAdmin

AccesstheUseraccountstabinphpMyAdminusinganadminaccounttomanageMySQLusers.2.Createanewuserbyclicking"Adduseraccount",settingausername,host,strongpassword,andassigningminimalnecessaryprivileges.3.Editexistingusersbyselecting"Editprivile

Aug 05, 2025 am 09:22 AM

User Management

How does SQL handle NULL in a comparison (e.g., WHERE column = NULL)?

Special attention is required when handling NULL values in SQL, because NULL represents unknown or missing values and does not follow conventional comparison logic. 1. Use ISNULL to check the NULL value, because =NULL will not take effect, it will return UNKNOWN; 2. Be careful when using NOTIN. If the subquery contains NULL value, the entire condition may return UNKNOWN. Use NOTEXISTS instead or filter NULL; 3. Aggregation functions such as COUNT, SUM, and AVG will ignore the NULL value, COUNT(*) counts all rows, and COUNT(column) only counts non-NULL values. Understanding these rules helps avoid common pitfalls in query results.

Aug 05, 2025 am 08:49 AM

Best Practices for Connection Pool Management in MongoDB Applications

UnderstandhowconnectionpoolsworkbyrecognizingthatMongoDBdriversreuseconnectionstoreduceoverhead,limitconcurrentoperationsviapoolsize,andrequirepropermanagementtoavoidtimeoutsorresourceexhaustion.2.Tuneconnectionpoolsettingsbasedonworkloadbyconfigurin

Aug 05, 2025 am 08:46 AM