Summary of knowledge about Mysql database

On June 8, 2017, the weather was cloudy. The mood is clear.

I have been dreaming for two consecutive nights. Last night, I dreamed of a junior high school classmate I had not contacted for a long time. I chatted with him on the way to work in the morning. The girl is currently on a business trip in Guizhou, a place with beautiful scenery. I said, "You are living a very prosperous life now." The girl said, "That's because you haven't seen my non-moisturizing side." Indeed, in life we ​​always show our bright side to others and "disguise" our downtrodden side.

Yesterday, a blogger asked me a question on the blog and even called me a "great god". I felt very panicked because I knew very well that I was far from this "great god", so I had to Redouble your efforts and move towards the goal of "greatness".

The question asked by the blogger is about the Mysql database, which involves data engine and repair and optimize data table statements. I have never considered these before. Problem, never used. So I searched for some information to understand it, and now I summarize the information I found as follows.

1.Mysql database storage engine

Concept:

The storage engine is actually how to store data, how to index the stored data and how to Methods for updating, querying data and other technical implementations. Data in MySQL is stored in files (or in memory) using a variety of different technologies, each of these technologies uses different storage mechanisms, indexing techniques, locking levels and ultimately provides a wide range of different functionality and capabilities. In MySQL, these different technologies and supporting related functions are called storage engines.

(1) innoDB storage engine

①The mysql table of the innodb storage engine provides transaction, rollback and system crash repair capabilities and transaction security of multi-version concurrency control.

②innodb supports auto-increment columns (auto_increment). The value of an auto-increment column cannot be empty. If it is empty when used, how can it automatically grow from the existing value? If there is, it is larger than the current value. If it is still large, this value will be saved.

③The innodb storage engine supports foreign keys. The table where the foreign key is located is called the child table (or slave table), and the table it depends on is called the parent table (or main table).

④The most important thing about the innodb storage engine is that it supports transactions and transaction-related functions.

⑤The innodb storage engine supports mvcc row-level locks.

⑥The innodb storage engine index uses B+Tree.

(2) MyISAM storage engine

1) MyISAM This storage engine does not support transactions, does not support row-level locks, and only supports concurrently inserted table locks. It is mainly used for high-load select .

2) MyISAM type tables support three different storage structures: static, dynamic, and compressed.

①Static type: means that the size of the defined table column is fixed (that is, it does not contain variable-length data types such as xblob, xtext, varchar, etc.), so that mysql will automatically use it Static myisam format.

The performance of tables using static formats is relatively high because the overhead required to store data in a predetermined format during maintenance and access is very low. But this high performance is obtained in exchange for space, because it is fixed when it is defined, so no matter how big the value in the column is, the maximum value will prevail and occupy the entire space.

②Dynamic type: If the column (even if there is only one column) is defined as dynamic (xblob, xtext, varchar and other data types), then myisam will automatically use the dynamic type.

Although dynamic tables occupy less space than static tables, they reduce performance because if the content of a field changes, its position is likely to need to be moved, which will leading to the generation of fragments. As the data changes more, fragmentation will increase, and data access performance will decrease accordingly.

There are two solutions for reducing data accessibility due to fragmentation:

a. Use static data types as much as possible.

b. Frequently use the optimize table statement, which will defragment the table and recover space lost due to table updates and deletions.

If the storage engine does not support optimize table, you can dump and reload the data, which can also reduce fragmentation.

③Compressed type: If a table is created in this database that is read-only during the entire life cycle, in this case, myisam's compressed table is used to reduce the space occupied. .

3) MyISAM also uses B+tree index, but it is slightly different from Innodb in specific implementation.

(3) MEMORY storage engine

①The memory storage engine is a little different from some previous storage engines. It uses data stored in memory to create tables, and all data is also stored in memory.

②Each table based on the memory storage engine actually corresponds to a disk file. The file name and table name of the file are the same, and the type is .frm. This file only stores the structure of the table, and its data files are stored in memory, which is conducive to rapid processing of data and improves the processing capabilities of the entire table.

③The memory storage engine uses hash (HASH) index by default, which is faster than using B+Tree type. If readers want to use B-tree type, they can reference it when creating.

④Memory storage engine file data are stored in memory. If an exception occurs in the mysqld process, these data will disappear if the machine is restarted or shut down. Therefore, the life cycle of tables in the memory storage engine is very short and is generally only used once.

(4) BlackHole storage engine (black hole engine)

supports transactions and supports mvcc row-level locks. It is mainly used for logging or synchronized archiving. Unless this storage engine has a special purpose, Otherwise it is not suitable for use!

2. Optimize tables and repair tables under Mysql

With the long-term use of mysql, you can optimize it by repairing the table. During optimization, you can reduce the disk space occupied and facilitate backup.

REPAIR TABLE `table_name` Repair table OPTIMIZE TABLE `table_name` Optimize table

REPAIR TABLE - Used to repair damaged tables.
OPTIMIZE TABLE——Optimize table, used to reclaim idle database space. When the data rows on the table are deleted, the occupied disk space is not immediately reclaimed. After using the OPTIMIZE TABLE command, the space will be reclaimed and the data rows on the disk will be rearranged.

Note: It is on the disk, not the database.
Most of the time, there is no need to run OPTIMIZE TABLE. You only need to perform a data table optimization operation regularly (once a week or once a month) after deleting data rows in batches, and only run it on those specific tables. .

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