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[TOC] I. model definition Appearance mode: appearance mode is to provide a unified interface to access a group of interfaces of subsystem. The facade pattern defines a high-level interface that makes subsystems easier to use. Appearance mode, also known as facade mode, is an object structural design mode. Insert picture descri ...

PostgreSQL uses the process mode. For each client, it will Fork a background process to respond to the client's request. This section describes the data structure that each background process has in shared memory: PGPROC. I. data structure Macro definition /* * Note: MAX_BACKENDS is limited to 2^18-1 because that's the width reser ...

Monitor whether the master-slave synchronization of MySQL is abnormal. If not, send SMS or email to the administrator. 1) develop a daemons script to detect every 30 seconds. 2) if the following error number (11581159100810071062) occurs during synchronization, the error will be skipped. Stage 3:3) if the IO and SQL threads are abnormal, notif ...

Abstract: Three different data De duplication methods will be shared belowData De duplication: it needs to be defined according to a certain field. When there is more than one line of records in this field, we define that there are duplicates in this line of data. Data De duplication method 1: When the maximum SN is in the table, we can get ...

Node connects to mysql In this note, the connection between node and mysql will be introduced, and the encapsulated code will be shared, which can be used directly in the project. Install mysql module npm install mysql Connect mysql const mysql = require('mysql'); let connection = mysql.createConnection({ host : 'localhost', user : 'r ...

SELECT and SET can be used to assign values to variables in SQL SERVER, but their usage and effect are different in some details. 1. In terms of variable assignment, SET is the ANSI standard assignment method, while SELECT is not. This is one of the reasons why SET is recommended. 2. SELECT can assign multiple variables at one time, while SET c ...

Objectives: Clarify the process of mybatis loading and parsing mapper file; Clarify the process of executing SQL by mybatis. When analyzing the source code of mybatis loading configuration, the previous article mentioned org.apache.ibatis.builder.xml.xmlconfigbuilder ා parseconfiguration method, and now continues to analyze mapperElement meth ...

In many cases, in order to quickly retrieve the deleted tables, you can use the recycle bin to find and rebuild the deleted tables.(1)Whether the deleted table can be found in the recycle bin has a lot to do with the reservation policy and space size of undo. Timeout or insufficient space will cause the table to fail to flash back and can only ...

Adding, deleting and modifying data tables can never avoid the problem of fragmentation. After Oracle introduced table space local management and ASSM, in extreme cases, it is clear that the utilization rate of table space is not high, and the database objects that need to be put into storage are not large, but the error is reported ORA-01653: ...

In MySQL, the CONCAT() function is used to connect multiple strings into one string. With this function, we can splice the sql that we couldn't get in one step. It may be more convenient in our work. Here are some common scenarios. Note: it may be slightly different for lower version 5.7. 1. Query all users by splicing SELECT DISTINCT CONC ...