MySQL data directory (1) -- database structure (24)

In the last article, we said that both index space and time will be consumed , Don't build indexes .

When the column needs to be used as a condition , Sort , When grouping, you can choose the front row as the index .

It also depends on the cardinality at the forefront , The larger the base, the more suitable for indexing .

Avoid when querying % On the front .

When querying the range, only the leftmost side will trigger the index , Unless the precision value is specified .

The index exists independently , You can't calculate or calculate , This will not trigger the index .

Long string query , It costs a lot of time and space , At this time, you can create hash Index or select the first few digits of the string for index sorting , If you use string prefix as index , The sorting will be invalid , Sort by file .

Try to use overlay index , It can avoid the second query of the back table .

B+ Tree selection index (2)---mysql From entry to mastery （ 23 )

Mysql Data directory

We all know innoDB and myISAM Storage engines store data on disk , And the operating system manages the disk is the file system , So it's like nnoDB and myISAM Such storage engines store data in file systems . When we read data , Just read the data from the file system , When we write data , The storage engine will write data to the file system .

Data directory

Mysql At startup , Will download some files in a directory of the file system , Then the data generated during the operation will also be placed in this directory , It is called data directory . We installed mysql When it's time to specify mysql Installation directory , There are many executable files , The data directory we are talking about here must be separated from the installation directory , How do we look at ourselves mysql What about your data directory ？ The system variable corresponding to the data directory is datadir, As long as we input the current command, we can see ：

mysql> show variables like 'datadir';
+---------------+------------------------+
| Variable_name | Value                  |
+---------------+------------------------+
| datadir       | /usr/local/mysql/data/ |
+---------------+------------------------+
1 row in set (0.00 sec)

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The structure of the data directory

that mysql What data will be generated when running ？ Of course, the database we created , surface , View , trigger , Other user data , In addition to these data ,mysql For better operation , Increase of efficiency , There will also be some additional data .

The representation of a database in a file system

Create a table from the specified database , So let's start with creating a database , The representation of database in file system begins , We are create database ‘’, What happened ？

[email protected] mysql % cd data 
[email protected] data % ls
B-D226JG5H-0020.local.err	mysql
B-D226JG5H-0020.local.pid	mysqld.local.err
auto.cnf			mysqld.local.pid
dev				mysqld_safe.pid
ib_buffer_pool			performance_schema
ib_logfile0			sys
ib_logfile1			testmac
ibdata1				utf_8
ibtmp1

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View data files data Under the table of contents , We can see the various databases we have created , So when you run create database '' Of When ：

1） In the data directory , Create a subdirectory with the same name as the database .

2) Enter the specified database file , You can see the default created db.opt file , It stores the rules of the current database , features , Such as comparison rules , Character set, etc. .

Table's position in the file system

The table is divided into two parts ：1） Table structure file .2） Table data file .

When we create table '' A watch time , It will be in the database file directory , Create a table name .frm The file of . For example, we have index_page_tb, So in utf_8 There will be a in the directory of index_page_tb.frm The file records the structure of the current table , For example, row rules with tables , Character set , The data type of each column , Index, etc. . It is worth noting that innoDB and mISAM Will create .frm File to record the structure of the current table , But the stored data is different . In the next article, let's look at the storage file of data .（ Be careful ：.frm It's binary , If you open it directly, it will be garbled ）