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Technical Articles

Database

Leveraging MySQL Invisible Indexes for Performance Testing

MySQL 8.0 introduces InvisibleIndexes, allowing temporary hiding of indexes without deleting. 1. Set invisible when creating: Use CREATEINDEX...INVISIBLE; 2. Modify the existing index to invisible: ALTERINDEX...INVISIBLE; 3. Restore visible: ALTERINDEX...VISIBLE. The invisible index is still maintained, but does not participate in the implementation plan generation. It is suitable for testing the effect of new indexes and avoiding the risk of direct online launch. Backup and copy will retain their status, and FORCEINDEX cannot bypass invisibility, and is suitable for performance tuning and indexing strategy adjustment without affecting online services.

Aug 06, 2025 pm 03:20 PM

How to change the owner of a database in SQL?

InSQLServer,useALTERAUTHORIZATIONONDATABASE::[DatabaseName]TO[NewOwnerLogin]orthedeprecatedsp_changedbownertochangeownership,ensuringthenewownerisavalidlogin.2.InPostgreSQL,executeALTERDATABASEdatabase_nameOWNERTOnew_owner,requiringsuperuserorcurrent

Aug 06, 2025 pm 03:17 PM

How to create and drop a database in MySQL?

To create and delete a MySQL database, use the CREATEDATABASE and DROPDATABASE commands. 1. Use CREATEDATABASEdatabase_name to create the database; if repeated errors are avoided, you can add IFNOTEXISTS, such as CREATEDATABASEIFNOTEXISTSschool; you can also specify character sets and proofreading rules, such as CHARACTERSETutf8mb4COLLATEutf8mb4_unicode_ci. 2. Use DROPDATABASEdatabase_name to delete the database; to prevent the database from reporting errors, IF can be added

Aug 06, 2025 pm 03:11 PM

Migrating Legacy Applications to MySQL 8.0

TomigrateolderapplicationstoMySQL8.0successfully,firstcheckapplicationcompatibility,thencarefullymigrateandconvertdata,updateconfigurationandsecuritysettings,andmonitorperformancepost-migration.1.Checkapplicationcompatibilitybyupdatingdatabasedrivers

Aug 06, 2025 pm 03:07 PM

應(yīng)用遷移

What is the difference between TRUNCATE and DELETE in Oracle?

DELETEisaDMLcommandthatremovesrowsoneatatimewithfulllogging,supportsWHEREclauses,firestriggers,andallowsrollback;2.TRUNCATEisaDDLcommandthatdeallocatesdatablocks,performsminimallogging,removesallrowswithoutaWHEREclause,doesnotfiretriggers,andcannotbe

Aug 06, 2025 pm 02:41 PM

oracle sql

Retail Analytics with SQL: Sales and Inventory Optimization

SQL can effectively improve retail sales and inventory efficiency. 1. When analyzing sales trends, count sales and order counts according to time dimensions (such as monthly), identify peaks and troughs, and group them into products or stores to find hot-selling categories; 2. By calculating inventory turnover rate (sales cost/average inventory), identify unsold products (large inventory and no sales in the past three months); 3. Forecast demand based on historical sales volume, and obtain replenishment suggestions based on current inventory; 4. Compare the sales performance and inventory turnover of different stores, discover operational shortcomings and optimize them in a targeted manner. By mastering these methods, you can use SQL to quickly mine the value of retail data.

Aug 06, 2025 pm 02:23 PM

ORA-00942: table or view does not exist

ORA-00942 errors are usually caused by insufficient permissions, wrong spelling of table names, no prefixes across schemas, uncreated tables, synonyms pointing to invalid objects or DBLink problems; 1. Check whether the table name is correct and uses uppercase or matches uppercase and uppercase; 2. Confirm whether the schema prefix is required such as SCOTT.EMP; 3. Verify whether the current user has access permissions, and if not, permissions need to be granted; 4. Confirm whether the table really exists, you can query it through all_tables or user_tables; 5. If you use synonyms, check whether the object it points to exist and is valid; 6. If you access it through DBLink, you need to confirm that the remote table exists and the permissions are correct, and finally ensure that every step is correct to ensure that the problem can be solved after troubleshooting.

Aug 06, 2025 pm 02:06 PM

How to open phpMyAdmin in XAMPP

EnsureApacheandMySQLarerunninginXAMPPControlPanel,asbotharerequiredforphpMyAdmintofunction.2.Openabrowserandvisithttp://localhost/phpmyadminorhttp://127.0.0.1/phpmyadmintoaccesstheloginpage.3.Ifissuesoccur,resolveportconflictsbychangingApache’sportto

Aug 06, 2025 pm 01:57 PM

xampp

What is the difference between COUNT(*) and COUNT(column_name) in SQL?

COUNT()countsallrowsincludingthosewithNULLvalues,whileCOUNT(column_name)onlycountsrowswherethespecifiedcolumnisnotNULL;forexample,inatablewith5rows,COUNT()returns5,COUNT(name)returns4ifonenameisNULL,andCOUNT(salary)returns2ifonlytwosalariesarenon-NUL

Aug 06, 2025 pm 01:50 PM

How do you sort results in ascending or descending order using SQL?

To sort SQL results in ascending or descending order, you need to use the ORDERBY clause: 1. Arrange in ascending order by default, and ASC keywords can be omitted; 2. Use DESC keywords to achieve descending order; 3. You can sort by multiple columns, separating each column and sorting direction with commas; 4. ORDERBY is at the end of the SELECT statement, after WHERE, GROUPBY and HAVING; 5. Support sorting by column names, alias or expressions; 6. Sorting of strings is affected by database sorting rules and may not be case-sensitive; 7. NULL values are ranked first in ascending order, and after descending order, the specific behavior depends on the database system; 8. Common usages include sorting by name letters, the latest in date, or the calculated field, using OR

Aug 06, 2025 pm 12:28 PM

The Ultimate SQL Cheat Sheet for Data Analysts and Developers

SQLisamust-haveskillfordataanalystsanddevelopers,andmasteringcorecommandsenablesefficientquerying,transformation,andreporting.1.UseSELECTtoretrievespecificcolumnsandavoidSELECT*inproductiontoimproveperformance.2.SortresultswithORDERBY(afterWHERE)andl

Aug 06, 2025 pm 12:05 PM

A Practical Guide to SQL Performance Tuning and Best Practices

Write efficient queries, avoid SELECT*, use WHERE filtering as soon as possible, use functions with caution, and use LIMIT reasonably; 2. Use indexes effectively, establish composite indexes for commonly used columns of WHERE, JOIN, and ORDERBY, avoid over-index, use over-index and maintain regularly; 3. Optimize JOIN and subqueries, give priority to INNERJOIN instead of related subqueries, ensure that the JOIN field is indexed, and avoid Cartesian product; 4. Monitor and analyze query performance, use EXPLAIN to view execution plans, enable slow query logs, and test performance under real data volume; 5. Apply schema and configuration best practices, reasonably normalize and anti-normalize, use appropriate data types, partition large tables, and maintain statistical information

Aug 06, 2025 pm 12:02 PM

Performance tuning sql optimization

MongoDB Schema Design Best Practices

Schema is designed according to the query pattern. Data that is checked at high frequency is embedded, and multiple are used for independent updates are updated; 2. Embedding is suitable for a small pair, and references are suitable for big data or frequent updates; 3. Control the document size to avoid expansion and move, and build a collection of growing data separately; 4. Accurate index design, analyze slow queries, and make good use of composite and overlay indexes; 5. Unify the style and type of field naming to avoid chaos in later maintenance. These practices help you leverage MongoDB performance advantages and make the structure efficient and easy to maintain.

Aug 06, 2025 am 11:59 AM

Understanding MySQL InnoDB Storage Engine Architecture

InnoDB's architecture design includes logical storage structure, memory structure, transaction and logging system, and optimization features. 1. The logical storage structure is divided into tablespace, segments, zones and pages, supporting transactions and efficient data management. 2. The buffer pool in the memory structure caches data, reduces disk I/O, and manages page exchange in and out through the LRU algorithm. 3. The transaction mechanism depends on RedoLog and UndoLog, which is used for crash recovery, and UndoLog is used for rollback and MVCC. 4. Insert buffer optimization non-unique secondary index insertion, adaptive hash index accelerates equivalent query, and is enabled by default to improve performance.

Aug 06, 2025 am 11:42 AM