[MySQL from introduction to proficiency] [advanced chapter] (x) MyISAM's indexing scheme & advantages and disadvantages of indexing

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1. brief introduction

The previous articles have been introduced InnoDB Index scheme of storage engine , This article then introduces MyISAM Index scheme of storage engine .
MyISAM and InnoDB The default indexes of the storage engine are B+Tree Indexes
MyISAM Engine USES B+Tree As an index structure , Leaf node data Domain stores The address of the data record .

2. Environmental Science

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MySQL	5.7

MyISAM The index principle of

We know from the previous study that InnoDB The engine is index, that is, data , That is, all complete user records have been included in the leaf node of the cluster index , The index and data are placed under the name Table name .ibd, For example, using InnoDB Engine index_demo The data and indexes of the table are stored in index_demo.ibd in ,
and MyISAM The engine stores the index and data in two separate files , For example, using MyISAM Engine engine_demo_table The index of the table is stored in engine_demo_table.MYI In file , Data is stored in engine_demo_table.MYD In the table .
MyISAM The indexing scheme of has the following characteristics ：

• Record the records in the table According to the insertion order of records Stored separately in a file , be called Data files . This file is not divided into several data pages , As many records as there are, just insert as many records into this file . Because when inserting data, there is no There is no deliberate sorting according to the size of the primary key , So we can't use dichotomy to find these data .
• Use MyISAM The table of the storage engine will store the index information in another file called index file （ Table name .MYI） In another file ,MyISAM A separate index is created for the primary key of the table , However, the leaf node of the index does not store complete user records , It is Primary key value + Data record address The combination of .
  
  There are three columns here , Suppose we use Col1 Primary key , The image above is a MyISAM Primary index of table （Primary key） schematic . It can be seen that MyISAM The index file of only holds the address of the data record . stay MyISAM in , Primary index and secondary index （Secondary key） There is no difference in structure . It's just a primary key index requirement key Is the only one. , And the secondary index key Can be repeated .

MyISAM And InnoDB Comparison of

MyISAM The indexing methods are " It's not clustering " Of , And InnoDB contain 1 Cluster indexes are different . The indexing methods of these two engines have the following differences ：

1. stay InnoDB In the storage engine , We just need to match... According to the primary key value Cluster index A search will find the corresponding record , And in the MyISAM But it needs to be done once Back to the table operation , signify MyISAM The indexes established in are equivalent to secondary indexes .
2. InnoDB The data file itself is the index file （ Index is data ）, and MyISAM Index files and data files are separate , Index file only holds the address of the data record .
3. InnoDB The nonclustered index of data The domain stores the corresponding records The value of the primary key , and MyISAM The index records the address of the data .
4. MyISAM The operation of returning table is very fast , Because taking the address offset directly to the file to get data , take the reverse into consideration InnoDB Back to the table is to find user records in the clustered index by obtaining the primary key value , Although it's not slow , It's better to visit the address directly than not .
5. InnoDB List of requirements Must have primary key （MyISAM There can be no ）, If no primary key is explicitly specified , be MySQL The system will automatically select a non empty and unique column representing data as the primary key , If there is no such column , be MySQL Automatically for InnoDB Table generates an implicit field as the primary key , The length of this field is 6 Bytes , Type is long .

PS:
6. Too long fields are not recommended as primary keys , Because all secondary indexes refer to primary key indexes , Too long primary key index will make the secondary index too large , And the size of each data page is 16KB, Too long primary key index will cause less data stored in each data page .
7. Using nonmonotonic fields as primary keys InnoDB It's not a good idea , because InnoDB The data file itself is a piece B+Tree, Non monotonic primary keys will cause new records to be inserted , Data files for maintenance B+Tree Frequent split adjustment . Very inefficient , While using Adding fields as primary keys is a good choice .

The advantages of indexing

1. Indexing can greatly improve the efficiency of data retrieval , Reduce the IO cost
2. By creating a unique index , It can ensure that every row in the database table The uniqueness of data .
3. In terms of reference integrity of data , Can speed up the connection between tables , let me put it another way , When the dependent child table and parent table are jointly queried , Can improve the query speed .
4. When using grouping and sorting clauses for data queries , Sure Significantly reduce the time of grouping and sorting in queries , Reduce CPU Consumption of

Disadvantages of indexes

The cost of space

Every time you build an index, you build a tree for it B+ Trees , Every tree B+ Each node of the tree is a data page , A data page is occupied by default 16KB Storage space , A big one B+ The tree consists of many data pages , That's a lot of storage space .

The cost of time

The data in the table is checked every time increase 、 Delete 、 Change In operation , All need to be modified B+ Tree index . And we talked about it B+ Each node of the tree is based on the value of the index column Sort from small to large And make up Double linked list . Whether it's a record in a leaf node , Or the records in the inner node form a one-way linked list according to the value of the index column from small to large . And increase 、 Delete 、 Changing operations may damage the sorting of nodes and records , So the storage engine needs extra time to do some Record shift , Page splitting , Page recycling And other operations to maintain the sorting of nodes and records . If we build many indexes , Each index corresponds to B+ Trees should be maintained accordingly , It will slow down performance .

summary

This article introduces in detail MyISAM The index scheme of ,MyISAM The engine and InnoDB The indexes used by the engine by default are B+Tree Indexes , The difference between them is MyISAM The index and data of are separated , The leaf node of the index will only store the address of the data , When searching data, you need to return to the table , and InnoDB The index of the engine is data , The leaf nodes of the clustered index store complete user records .