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Technical Articles

Database

What are materialized views in SQL and how do they differ from standard views?

Materializedviewsstoreprecomputedqueryresultsondiskforfasteraccess,unlikestandardviewsthatexecutequeriesondemand.1)Theyimproveperformanceforcomplexqueriesandrepetitivereadsbyphysicallysavingdata.2)Theyrequireperiodicrefreshingtostaycurrent,supporting

Aug 05, 2025 pm 02:55 PM

sql Materialized view

How to use the ON DUPLICATE KEY UPDATE statement in MySQL?

ONDUPLICATEKEYUPDATE is used to handle unique key or primary key conflicts. 1. When there is duplication of insertion data, no error is reported but the specified field is updated; 2. Applicable to counters, data synchronization and other scenarios; 3. The syntax is INSERTINTO...VALUES...ONDUPLICATEKEYUPDATEcolumn=VALUES(column); 4. VALUES(column) obtains the value inserted in the original plan; 5. Only trigger updates when a unique key conflict occurs; 6. Each duplicate record is processed independently; 7. Pay attention to unexpected updates of AUTO_INCREMENT and TIMESTAMP columns; 8. It can avoid complex logic of checking first and then inserting.

Aug 05, 2025 pm 02:34 PM

What is an Oracle SID?

TheOracleSIDuniquelyidentifiesadatabaseinstanceonasystem;1)Itdistinguishesinstancesbyassociatingthemwithspecificprocessesandmemoryareas;2)ItisusedbytheOS,OracleNet,andadministrativetoolstotargetthecorrectinstance;3)Unliketheservicename,whichrepresent

Aug 05, 2025 pm 02:14 PM

What is a correlated subquery in MySQL and what are its performance implications?

AcorrelatedsubqueryinMySQLdependsontheouterqueryandexecutesonceperouterrow,asshowninaquerycomparingeachemployee'ssalarytotheirdepartment'saverage;1.Itworksbyreferencingoutercolumns(e.g.,e1.department)andrecalculatingforeachrow;2.Itcanbeslowduetorepea

Aug 05, 2025 pm 01:57 PM

How to audit database activity in MySQL?

UseMySQLEnterpriseAuditPluginifonEnterpriseEditionbyenablingitinconfigurationwithserver-audit=FORCE_PLUS_PERMANENTandcustomizeeventsviaserver_audit_events;2.Forfreealternatives,usePerconaServerorMariaDBwiththeiropen-sourceauditpluginslikeaudit_log;3.

Aug 05, 2025 pm 01:34 PM

Securing Your MongoDB Database

Enableauthenticationwithstrongaccesscontrolbyconfiguringauthorization:enabledinmongod.confandcreatinguserswithleast-privilegerolesusingstrongpasswordsandcredentialrotation.2.SecurenetworkexposurebybindingMongoDBtolocalhostorspecificinternalIPs,usingf

Aug 05, 2025 pm 01:18 PM

How does the EXISTS operator compare to the IN operator in SQL?

UseEXISTSforexistencechecks,especiallywithlargeorcorrelatedsubqueriesandwhenNULLvaluesarepresent,asitstopsatthefirstmatchandhandlesNULLssafely;useINformembershipchecksagainstsmall,known,ornon-nullvaluesetswherereadabilitymattersandperformanceisnotcri

Aug 05, 2025 pm 01:08 PM

sql

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