MySQL index test

1 Index type

1 primary key （PRIMARY KEY）

name	effect
primary key （PRIMARY KEY）	A primary key index is an index created specifically for primary key fields , Duplicate or empty values are not allowed , And a table can only have one primary key index , It can improve query efficiency , And provide uniqueness constraints .

name	effect
id	Yes means that the query is executed select The order of clauses or operation tables .ID identical （ It can be considered the same group ） From top to bottom , Subquery ID The value is incremented ,ID The higher the value, the earlier it is executed .
select_type	Represents each... In the query select The type of clause （ Simple OR complex ）
table	indicate
partitions
type	all < index < range < ref < eq_ref < const/system < null
possible_keys	Pointed out that MySQL Which index can be used to find rows in the table , If there is an index on the field involved in the query , Then the index will be listed , But not necessarily used by queries .
key	Show MySQL The index actually used in the query , If you don't use an index , Is shown as NULL.
key_len	Represents the number of bytes used in the index , You can use this column to calculate the length of the index used in the query .
ref	Indicates the join matching condition of the above table , That is, which columns or constants are used to find values on index columns .
rows	Express MySQL According to table statistics and index selection , Estimated number of rows to read to find the required record .
filtered
Extra	Contains additional information that is not suitable for display in other columns but is important

select_type	effect
SIMPLE	The query does not contain subqueries or UNION.
PRIMARY	If the query contains any complex sub parts , The outermost query is marked ：PRIMARY .
SUBQUERY	stay SELECT or WHERE The list contains subqueries , The subquery is marked as ：SUBQUERY.
DERIVED	stay FROM The subqueries contained in the list are marked with ：DERIVED（ derivative ）.
DERIVED	If the second SELECT Appear in the UNION after , Is marked as UNION; if UNION Included in FROM Clause , Outer layer SELECT Will be marked as ：DERIVED.
UNION RESULT	from UNION Table to get the result SELECT Marked as ：UNION RESULT

type	effect
all	Full Table Scan, MySQL Will traverse the entire table to find the matching rows .
index	Full Index Scan,index And ALL The difference for index Type only traverses the index tree
range	Index range scan , The scan of the index starts at a certain point , Return the row matching the value field , Common in between、<、> And so on
ref	Non unique index scan , Returns all rows that match a single value . It is commonly used in searches using non unique indexes, that is, non unique prefixes of unique indexes
eq_ref	Unique index scan , For each index key , Only one record in the table matches it . Common in primary key or unique index scan
const/system	When MySQL Optimize some part of the query , And convert to a constant , Use these types to access . For example, place the primary key in where In the list ,MySQL You can convert the query to a constant
null	MySQL Decompose statements during optimization , Execute without even accessing tables or indexes

Extra	effect
Using index	The value represents the corresponding select The override index is used in the operation （Covering Index）.
Using where	Express MySQL The server does it after the storage engine receives the record “ Post filtration ”（Post-filter）, If the query fails to use the index ,Using where Our role is just to remind us MySQL Will use where Clause to filter the result set .
Using temporary	Express MySQL You need to use temporary tables to store result sets , Common in sorting and grouping queries .

1 Add index

alter table `student` add primary key `id` (`id`);

explain select * from student where id = 1;

id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	student		ALL	PRIMARY				18	10	Using where

2 Delete index

#  If the primary key is self incrementing, you need to delete the self incrementing （ Modify field type ）
alter table `student` drop primary key;

explain select * from student where id = 1;

id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	student		ALL					18	10	Using where

2 General index （INDEX/KEY）

name	effect
General index （INDEX/KEY）	General index is MySQL The most basic index type in , It has no data type restrictions , Allows you to insert duplicate and null values in a column that defines an index , It can speed up the system's access to data .

1 Single index

Using short index can not only improve query speed , And it can save disk operation and I/O operation , Such as address by varchar(200) If it is often stored as 100 Then set the index 100 Better than default and 200

alter table `student` add index `address` (`address`(100)) using btree;

2 Composite index

Composite index has the leftmost principle

alter table `student` add index `mult` (`age`,`name`) using btree;

3 unique index （UNIQUE KEY）

name	effect
unique index （UNIQUE KEY）	The value of a unique index column must be unique , Null value allowed . If it's a composite index , The combination of column values must be unique , The purpose of unique index is not to improve access speed , It's to avoid duplication of data .

alter table `student` add unique index `phone` (`phone`);

4 Full-text index （FULL TEXT）

name	effect
Full-text index （FULL TEXT）	Full text indexing allows the insertion of duplicate and null values in the indexed columns , Full text index is mainly used to find keywords in text , Only in CHAR、VARCHAR or TEXT Create on column of type , stay MySQL There are only MyISAM The storage engine supports full-text indexing .

1 How it was created

alter table `student` add fulltext index `name` (`name`);

2 Usage mode

select * from `student` where match(`name`) against(' Zhang San ')

5 Spatial index （SPATIAL）

name	effect
Spatial index （SPATIAL）	A spatial index is an index of a field of a spatial data type , Use SPATIAL Keyword extension , Columns that create spatial indexes must be declared as NOT NULL, Spatial index can only be used when the storage engine is MyISAM Create... In the table of , Spatial index is mainly used for geospatial data types GEOMETRY. This kind of index is rarely used .

alter table `student` add spatial index `map` (`map`);

2 Index method

1 BTREE

BTREE The index is stored in a tree structure , Usually used in like "=,>,>=,<,<=、BETWEEN、Like" The query efficiency of equal operator is higher .
B-Tree Is the most common type of index , All worth （ Indexed columns ） It's all sorted out , The distance from each leaf node to the following node is equal . therefore B-Tree It is suitable for finding data in a certain range , And it can directly support data sorting （ORDER BY）
B-Tree stay MyISAM The form and Innodb not quite the same

MySQL engine	Data files
MyISAM	MyISAM The table data file and the index file are separate , The index file only stores the disk address of the data record .
InnoDB	InnoDB The table data file itself is the primary index , Leaf nodes data Domain holds complete data records .

2 HASH

1 Support only “=”,“IN” and “<=>” Precise query , Can't use range query

because Hash Index comparison is to carry out Hash After the operation Hash value , So it can only be used for equivalent filtering , Can't be used for range based filtering , Because after the corresponding Hash After algorithm processing Hash

2 Sort is not supported

because Hash What the index stores is the process Hash After calculation Hash value , and Hash The magnitude of the value does not necessarily relate to Hash The key value before the operation is exactly the same , So the database can't use the index data to avoid any sort operation

3 Table scanning cannot be avoided at any time

because Hash Index comparison is to carry out Hash After the operation Hash value , So even if you meet someone Hash Number of records of key value data , I can't go from Hash Direct query in index , You still need to access the actual data in the table for corresponding comparison , And get the corresponding result

4 High retrieval efficiency

The index can be retrieved at one time , Unlike B-Tree The index needs to be from the root node to the branch node , Finally, you can access the page node for many times IO visit , therefore Hash The query efficiency of index is much higher than B-Tree Indexes

5 Only Memory The engine supports explicit Hash Indexes

But it's Hash yes nonunique Of , Too many conflicts will also affect the search performance .Memory The default index type of the engine is Hash Indexes , Although it also supports B-Tree Indexes

3 RTREE

R-Tree stay MySQL Rarely used , Support only geometry data type , The only storage engine that supports this type is MyISAM、BDb、InnoDb、NDb、Archive several .

3 Index usage considerations

1 Do not use NULL

As long as the column contains NULL Values will not be included in the index , A composite index has only one column containing NULL value , Then this column is invalid for the composite index , Therefore, when designing the database, we'd better not let the default value of the field be NULL;

2 Use short index

Using short index can not only improve query speed , And it can save disk operation and I/O operation .

3 Index column sort

MySQL Only one index will be used when querying , So if where If the clause has used an index , that order by The columns in will not use indexes , therefore order by Try not to include sorting of multiple columns , If you have to sort multiple columns , It's best to use a composite index .

4 Like sentence

In general, the use of like, If it is not used, you need to pay attention like ‘% Zhang San %’ No index ; but like ‘ Zhang San %’ To use the index .

5 Don't use NOT IN and <> operation