Oracle Database Programming: basic concepts of Oracle databases

Oracle Database Programming: basic concepts of Oracle databases 1. Basic concepts of Oracle databases: Oracle memory structure: the Oracle memory structure can be divided into the shared memory area and non-shared memory area, the shared memory zone is mainly composed of SGA (System Global Area), and the non-shared zone is mainly composed of PGA (Program Global Area. SGA: the global area of the system. SGA is a memory area shared by user processes. Therefore, it is also referred to as the shared memory area, several important regions of SGA include the database cache area, redo log cache area, shared pool, and other regions. PGA users store server process data and control information, which is a memory area independent of SGA. When a user process is connected to the Oracle server, the Oracle server allocates the corresponding PGA to each server process. PGA consists of four parts: sorting Area, Session Information, Cursor State, and Stack Space. Oracle background process: the Data Writing Process (DBWR) is responsible for writing the data modified by the Database Buffer Cache memory to the data file on the hard disk. Log write process (LGWR): writes changes in the redo log buffer to the redo log file. System Monitoring (SMON): When Oracle data is not properly shut down, SMON will perform necessary database repair operations for the next database activation. Process Monitoring (PMON): clears resources when an Oracle process fails. Faulty process Checkpoint Process (CKPT): updates the database status information of control files and data files whenever changes in the buffer cache are permanently recorded in the database. Logical Structure of the Oracle database: Tablespace: the Oracle database is divided into five data table spaces by default. Segment: we may need to divide different areas in the tablespace to store different data. We call these areas segments ). ZONE: Oracle will first configure a series of data blocks, knowing that a series of data blocks will be configured next time after all these data blocks are full. These continuous data blocks are called zones. Block: The smallest storage unit in Oracle is block. VIEW: a view is a virtual table generated from one or more tables using query statements. a view can be considered as the result of a query statement, it is only stored in the database in the form of representation. Synonym: the alias of the database object. Sequence: Sequence is a special object in Qracle. It is used to achieve automatic growth of primary key columns in database tables. Virtual table: DUAL is a special table with only one row and one column. The column name is virtual and the data type is CHAR (1 ). This table is used when we want to quickly obtain information irrelevant to a specific table but related to external sources or functions.