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This article mainly explains the "summary of the basic knowledge points of Oracle". The content of the explanation in the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn the summary of basic knowledge points of Oracle.

First of all, I took an Oracle architecture diagram, and like everyone who saw it for the first time, my mind was a mess. Although I studied database in college, I had nothing to do with Oracle, and then I came into contact with Oracle by mistake.

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The Oracle server consists of two parts

Instance structure + database structure = Oracle server

The instance memory structure contains the global area of the SGA system, and the instance process contains some important background processes.

But Oracle memory structure includes not only SGA but also PGA, UGA and so on.

The process structure includes not only the background process, but also the server process Server Process, the user process User Process and so on.

Starting with SGA

There are Shared pool shared pool, Database Buffer Cache database cache and Redo Log Buffer Cache redo log cache in SGA.

Java Pool, Stream Pool and Large Pool. Of course, there are other areas of memory.

The Shared pool shared pool is mainly composed of two parts.

1. Library Cache library cache

As the name implies, the library stores things, including recently executed SQL statements and PL/SQL procedures, including parsing processes, execution plans, and execution results.

2. Data Dictionaray Cache data dictionary cache

Data dictionary is to store data files, tables, columns and other defined information as well as permission information.

Database Buffer Cache database cache

This area reads the recently accessed cache blocks from the data file, which greatly improves the performance of data queries and changes in the database.

The cache blocks in this are made up of the following

1. The cache block being used, usually the frequently accessed cache block, is not written to the data file

2. Clean cache block, used but written to the data file by the DBWR process

3. Empty, completely unused cache blocks function as clean cache blocks waiting to be written

4. Dirty cache blocks, in which is the cache block where the data has been changed

Redo Log Buffer Cache redo log buffer

This area is used to record the information of all the operations performed by the user on the database. For example, if the user changes a piece of data, the modified data and location are recorded in the redo log buffer and then written to the redo log file by the LGWR process. Generally, more than two redo log files are then written cyclically. For example, when the first file is full, it is written on the second, in archive mode. When switching the redo log file, all the data from the full file is mirrored to the archive log file, which is equivalent to backup again.

Important background processes

DBWR database write process

The function of this process has been seen above, which is to write the dirty data from the database cache into the data file and save it permanently.

Trigger condition

1. The dirty cache is full and has to be written

2. The time interval is up. Usually write once every three seconds.

3. When you need to cache data from a data file but have no free data blocks

4. When a checkpoint occurs, that is, when CKPT is triggered

5. When the tablespace is set to read-only or offline backup

6. Temporary table changes, such as deletion, etc.

The LGWR redo log writing process writes the operation records of the redo log cache to the redo log file.

Trigger condition

1. The time interval has arrived

2. Before the DBWR process writes

3. When the cache is full of 1MB or 1/3

4. When the user submits the COMMIT operation

The CKPT checkpoint process informs the DBWR process to work and writes records such as the system change number SCN to the header of the data file and the control file.

Synchronize data files, control files, and redo log files

Trigger condition

1. When redoing log switching

2. When normal transaction processing is completed, such as DBWR and LGWR finish the work.

3. When the system is shut down

The PMON process monitoring process monitors other processes, clears up failed user processes and releases resources that were in use at the time

The SMON system monitors the process in the event of an uncontrollable database crash, such as a power outage. The record of the redo log file will be automatically applied for instance recovery.

The ARCH system starts the process that occurs only under the archive, and copies the data content of the switched redo log file to the archive log file.

Database structure

1. Physical storage structure (file structure)

2. Logical storage structure

Physical storage structure

The database is physically stored in a physical file

1. The data file stores the system table information of the database and all the data information.

2. The control file parameter file Init.ora records the location of the control file, including the name of the database, checkpoint information, the internal timestamp created by the database and the backup information of all the file information, etc.

3. All operations of the redo log file to the database will be recorded in this file

4. Archive log file redo log file backup file

5. When starting the instance, the parameter file allocates memory and starts the background process for SGA according to the configuration in the parameter file

6. The password file stores the user information and authenticates the super user sys

Logical storage structure

Logically peel off the table space, segment, area, and block layer by layer

A tablespace can only belong to one database, but a database can have multiple tablespaces

A tablespace is logically composed of multiple segments and physically composed of one or more data files

A data file can only belong to one tablespace

Segments can be discontiguous, so it also means that segments can span data files.

An area consisting of consecutive blocks of data.

Block, data block, the smallest Imax O unit in Oracle

Thank you for your reading, the above is the "Oracle basic knowledge points summary" content, after the study of this article, I believe you have a deeper understanding of the Oracle basic knowledge points summary of this problem, the specific use of the need for you to practice and verify. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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