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Home

Technical Articles

Database

Mysql Tutorial

What is the performance impact of using SELECT *?

Using SELECT* will affect database performance and the required columns should be specified explicitly. First, it increases unnecessary data transmission and network load, especially when the table contains a large number of fields (such as TEXT or BLOB); second, it may cause index failure, trigger additional disk I/O operations, and reduce query efficiency; finally, if the table structure changes, SELECT* may cause application errors or unpredictable behavior, reducing maintainability.

Jun 19, 2025 am 12:58 AM

Performance impact SELECT performance

What is the core difference between TRUNCATE TABLE and DELETE FROM TABLE?

The core difference between TRUNCATETABLE and DELETEFROMTABLE lies in the data deletion method and its impact on the database. 1. In terms of log behavior, DELETE records transaction log line by line, supporting rollback and point-in-time recovery, while TRUNCATE only records page release, which is more efficient but has limited functions. 2. In terms of performance, TRUNCATE does not scan progressively, has less competition for locks, and has a small log space, so it is faster and lighter; DELETE is slower due to process progressively. 3. In terms of constraints and dependencies, TRUNCATE cannot be used when there is a foreign key reference (unless cascade truncation is enabled) and does not trigger the trigger; DELETE respects the integrity of the reference and can trigger the trigger. 4. In terms of transaction support, two

Jun 19, 2025 am 12:56 AM

Under what conditions will a MySQL index not be used?

MySQL index may not be used in the following situations: 1. The query conditions do not match the index column or do not start from the leftmost column of the joint index; 2. Perform function or expression operations on the index field; 3. Fuzzy queries starting with % of LIKE; 4. The query conditions do not match the index column data type; 5. The index selectivity is too low, causing the optimizer to give up on use. For example, if the joint index (name,age) is only queried, it will not take effect; using YEAR(create_time)=2023 will cause the index to be invalid; LIKE'?c' cannot go through the index and scan the full table; VARCHAR fields use numerical query to trigger implicit conversion; low-selective fields such as gender fields may be ignored by the optimizer. Mastering these situations helps

Jun 19, 2025 am 12:55 AM

mysql index Unused conditions

What's the difference between caching_sha2_password and mysql_native_password authentication?

caching_sha2_password is more secure and has caching function than mysql_native_password. First, caching_sha2_password uses SHA-256 encryption algorithm to provide stronger security, while mysql_native_password uses the vulnerable SHA-1 algorithm; second, caching_sha2_password supports cache authentication results, improving the performance of frequent connections, while mysql_native_password does not have this function; finally, mysql_native_password has better compatibility and is suitable for old systems, while c

Jun 19, 2025 am 12:52 AM

mysql Certification

What is the process for upgrading MySQL from version 5.7 to 8.0?

UpgradingMySQLfrom5.7to8.0requiresseveralkeysteps:first,checkcompatibilitybyreviewingdeprecatedfeaturesliketheutf8characterset,verifyingpluginsandstorageengines,andensuringapplication-levelcompatibilitywithORMs;second,backupalldatabasesusingmysqldump

Jun 19, 2025 am 12:46 AM

Mysql upgrade 5.7 to 8.0

How to troubleshoot 100% CPU usage by the MySQL server?

Common reasons why MySQL consumes 100% of CPU include slow queries, missing indexes, temporary tables or excessive sorting, and configuration issues. 1. First use SHOWPROCESSLIST to check active connections and resource-consuming operations; 2. Enable and analyze slow query log location historical issues; 3. Check Created_tmp_tables and Sort_merge_passes to judge temporary tables and sorting; 4. Use EXPLAIN to analyze SQL execution plans and optimize full table scanning and file sorting; 5. Check other factors such as connection count, statistical information updates and timing tasks. The above steps can gradually narrow the scope of problems and optimize performance bottlenecks.

Jun 19, 2025 am 12:45 AM

mysql CPU Usage

How to create a new user and grant it only SELECT and INSERT on a specific table?

To create a new user and grant only SELECT and INSERT permissions to a specific table, first create the user using the CREATEUSER statement in MySQL, then authorize it through the GRANT statement, and execute FLUSHPRIVILEGES refresh permissions; in PostgreSQL, also first create the user using CREATEUSER, and then grant the permissions through the GRANTSELECT and INSERTONTABLE statements. 1. Create user: MySQL syntax is CREATEUSER'new_user'@'host'IDENTIFIEDBY'password'; PostgreSQL is CREATEUSE

Jun 19, 2025 am 12:45 AM

mysql User rights

What are the most common string functions in MySQL?

The most commonly used string functions in MySQL include: 1. CONCAT() is used to concatenate strings, such as merging names or URLs; 2. SUBSTRING() extracts substrings by position and length, suitable for obtaining file extensions, etc.; 3. UPPER() and LOWER() are case-based to standardize comparison or output; 4. TRIM(), LTRIM() and RTRIM() remove spaces to clean up user input data. These functions can efficiently handle the formatting, splicing and cleaning tasks of text data.

Jun 19, 2025 am 12:43 AM

mysql String functions

Where is the MySQL configuration file my.cnf (or my.ini) located?

MySQL configuration files are usually located in standard paths, such as Linux in /etc/my.cnf or /etc/mysql/my.cnf, macOS (Homebrew) in /usr/local/etc/my.cnf, Windows in the installation directory or Windows directory. You can confirm the specific path by command mysql--help|grep"Defaultoptions" or in MySQL shell; if it is not found, you can manually create and set basic content, pay attention to permission issues and rings.

Jun 19, 2025 am 12:39 AM

mysql Configuration file

What is a Primary Key and what is its purpose?

Aprimarykeyensuresuniqueidentificationofrecords,supportstablerelationships,andimprovesqueryperformance.Ituniquelyidentifieseachrowinatableusingasinglecolumnorcompositekey,disallowingduplicatesandNULLvalues.1.Itenforcesdataintegritybypreventingduplica

Jun 19, 2025 am 12:37 AM

database primary key

Why is it recommended to use the utf8mb4 character set?

MySQL recommends using utf8mb4 character set because it can fully support four-byte characters such as emojis. Traditional utf8 only supports three-byte characters, which will cause errors or garbled codes when storing Emoji or special text. Therefore, if the application involves user input emojis or minority texts, utf8mb4 must be used to ensure the data is stored correctly. For example, the VARCHAR (255) field can store 255 emoji characters normally under utf8mb4. The content will not be lost. It is recommended to set the database table and field character set to utf8mb4. At the same time, the connection layer should also set charset=utf8mb4 to fully support more Unicode characters, including ancient characters, mathematical symbols, musical symbols and rare Chinese characters, but you need to pay attention to utf8mb4.

Jun 19, 2025 am 12:35 AM

What are Generated Columns and what are their use cases?

The generated columns are used in the database to automatically calculate values ??based on other list expressions, simplifying queries and improving performance. They avoid repeated complex calculations, such as automatically generating total_price through unit_price and quantity; they can improve the efficiency of querying, such as pre-calculating order_year to accelerate annual filtering; ensure logical consistency between multiple applications, such as unified calculation of after-tax prices; they are divided into two types: virtual (calculating when reading) and storage (calculating when writing), and should be selected based on the use case.

Jun 18, 2025 am 12:31 AM

How do COMMIT and ROLLBACK work?

COMMITpermanentlysaveschangesmadeduringatransaction,whileROLLBACKundoesthem.AtransactionisasequenceofSQLoperationstreatedasasingleunittoensuredataintegrity,followingACIDproperties.Forexample,inamoneytransfer,ifoneaccountisdebitedbuttheotherisn'tcredi

Jun 18, 2025 am 12:28 AM

commit ROLLBACK

What does LIMIT 10, 5 mean in a MySQL query?

LIMIT10,5meansskipthefirst10rowsandreturnthenext5rows.Thissyntaxisusedforpaginationwherethefirstnumberistheoffset(rowstoskip)andthesecondisthecount(rowstoreturn).ItisusefulfordisplayingdatainpagessuchasPage1:LIMIT0,5,Page2:LIMIT5,5,andsoon.Commonusec

Jun 18, 2025 am 12:28 AM

mysql limit