MySQL learning

sql Optimize

It's best to optimize in the following order ：
1、SQL Statement and index optimization
2、 Database table structure optimization
3、 Optimization of system configuration
4、 Hardware optimization
details

The underlying implementation principle and optimization of index

B+ Trees , Optimized B+ Trees
It mainly adds a pointer to the next leaf node among all leaf nodes , therefore InnoDB build
It is recommended that most tables use the default self incrementing primary key as the primary index .

Binary tree 、 Balanced binary trees 、b Trees 、b+ Trees

B+ Trees , Optimized B+ Trees
It mainly adds a pointer to the next leaf node among all leaf nodes , therefore InnoDB build
It is recommended that most tables use the default self incrementing primary key as the primary index .

Detailed explanation

What are the principles for indexing data tables ？

In the most frequently used 、 To narrow the scope of the query to build an index on the field .
In frequent use 、 Index the fields that need to be sorted

When is it not appropriate to build an index ？

For the columns rarely involved in the query or the columns with more duplicate values , It's not good to index .
For some special data types , It's not good to index , For example, text field （text） etc.

What is a lock ？

answer ： Database is a shared resource used by multiple users . When multiple users access data concurrently , In number
Multiple transactions access the same data at the same time in the database . If you do not control concurrent operations, you can
Can read and store incorrect data , Breaking database consistency .
Locking is a very important technology to realize database concurrency control . When a transaction is processing a data object
Before the operation , Make a request to the system first , Lock it . After locking, the transaction has a certain effect on the data object
The control of , Before the transaction releases the lock , Other transactions cannot update this data object .
Basic lock type ： Locks include row level locks and table level locks

What is stored procedure ？ What to call ？

answer ： Stored procedure is a precompiled SQL sentence , The advantage is to allow modular design , That means just
To create a , Later in the program can be called many times . If an operation needs to be performed more than once SQL,
Using stored procedures is better than just SQL Statement execution should be fast . You can call a stored procedure with a command object

Say yes SQL What are the methods of statement optimization ？（ Choose a few ）

1、Where clause ：where Connections between tables must be written in other Where Before condition , Those can be
The condition to filter out the maximum number of records must be written in Where End of clause .HAVING Last .
2、 use EXISTS replace IN、 use NOT EXISTS replace NOT IN. 3、 Avoid using calculations on index columns
4、 Avoid using... On index columns IS NULL and IS NOT NULL
5、 Optimize the query , Try to avoid full scan , The first thing to consider is where And order by Involve
Index the columns of and .
6、 Should try to avoid in where Field in the clause null Value judgement , Otherwise, it will cause the engine to give up
Full table scan using index
7、 Should try to avoid in where The clause performs an expression operation on the field , This will cause the engine to be abandoned
Index and scan the whole table