Performance analysis of MySQL index

itread01 2021-01-21 23:27:24
performance analysis mysql index

# Why do you do performance analysis? Do you have such a situation . In front of a person you haven't written much about 、 Complex business , You've been thinking about it for a long time , Finally, I started to knock down the first code . The process of writing is vague , Sometimes I can make myself dizzy . But you finally finished it . But click to execute , Finished , Yes bug. What should I do? ?debug There are many ways , Console printing is a kind of . Information printed through the console , We can modify code based on feedback , Until the code runs properly . It's the same with building an index . Last post [《 Talking about sql Indexes 》](, It is said that the difficulty of index is to make the most suitable index for a specific table . Because it doesn't just depend on what's in your watch , It depends more on your business , What keywords do you often search for in your business . We can write code every day , Index can't be built every day . So most of the time , Because I'm not familiar with , The indexes we build in the beginning are often not the best , Only adjust the index according to the feedback , In order to make a most suitable index for this table . What I want to share today is how to understand this feedback , That is, how to do efficiency analysis .# How to do efficiency analysis EXPLAIN Keywords ! Use EXPLAIN Keywords can tell MySQL How to deal with you SQL Sentence of , Analyze the performance bottleneck of your query statement or table structure . but EXPLAIN It doesn't give us direct feedback on the quality of the index we built . The usage is like this , We built the index , Take the most common sentences in business SQL Statement EXPLAIN once , If the feedback works well , Then the index is the most suitable for this table , On the contrary, it needs to be improved . Or change the index , If you don't change SQL.# EXPLAIN How to play ### Sentence `explain To examine sql Sentence `( Horizontal table ) or `explain To examine sql Sentence \G`( Vertical table )### A concrete example ![]( The explanation of each field can be seen from the figure above ,explain The information that comes out has the following fields :|id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|| ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | Take a closer look , Is it a bit like a payslip ? The meaning of each field is like this :- id: View the reading order of the table . The example above only queries one table , But if it's a multi table joint query , Then there are : id In the same way , The order of execution is from top to bottom ; id Different words ,id The higher the value, the higher the priority ;- select_type: Show differential union queries 、 Subquery 、 General query, etc . Here are the possible values , And the corresponding information : SIMPLE -- Simple select Inquire about , Does not contain subqueries or union; PRIMARY -- If the query contains any complex sub parts , The outermost query is marked as PRIMARY; SUBQUERY -- stay select or where Include subqueries ; DERIVED -- stay from The list contains subqueries marked as DERIVED( derivative ),MySQL These subqueries are executed recursively , Put the results in a provisional form ; UNION -- If the second one select Appears in union After , Is marked as union; if union Included in from Clause in the subquery , The outer layer select Will be marked as :DERIVED; UNION RESULT -- From union Table to get the result select;- table: Display the table name .- type: Shows which type of query is used . Here are the possible values , And the corresponding information : system -- There is only one line in the table ( System table ), This is const A special case of type , It doesn't usually appear , You can ignore it . const -- It means that through the index, you can find ,const Used to compare primary key( Primary key ) perhaps unique( only ) Indexes . Because it only matches one line of data , So soon , If you set the primary key at where In the list ,mysql You can convert the query to a constant . eq_ref -- Unique scan index , For each index key , In the table ** It just happened to be ** There's a record that matches . Used for primary key or unique index scanning . ref -- Non unique scan index , Returns all rows that match a single value . above `eq_ref` The multivalued case of . Such as where age=1,age=1 There's just one line of information about , It shows eq_ref,age=1 How many lines of information are there , It shows ref. range -- Retrieve only rows in a given range , To select a line using a quotation . If type The value of the column is `range`,key Column will show which index is used . It's usually in where In the sentence between、<、>、in And so on . This kind of range scan query is better than full table scan . index -- Full Index Scan( Scan all indexes ),index And ALL The difference is index Type only traverses the index tree . It's all about reading the whole meter , however index It's reading from the index ,all It's read from the hard disk , And index files are usually smaller than data files . all -- Full Table Scan( Scan the whole table ), Traverse the entire table to find a matching row , The index is totally useless . From the best to the worst is :system>const>eq_ref>ref>range>index>ALL Details : 1. type yes ALL, When the data reaches more than one million, it must be optimized . 2. In general , If you want to optimize the query to at least range Level , It's better to achieve ref.- possible_keys and key possible_key: Show the indexes that may be applied to this table , One or more . Indexes that could theoretically be used , But it doesn't have to be actually used by queries . key: Index actually used , If it is null, No index is used . There are four situations in these two columns : 1. possible_key Valuable ,key Valuable : normal , Sometimes the former has multiple values, but only one of the latter is normal . 2. possible_key Valuable ,key It's not worth it :** Index failed , There's a problem **. 3. possible_key It's not worth it ,key Valuable : Condition query ( Such as where) No index or no condition query is used , But the columns of the query (select The fields at the back ) Just in sequence 、 The quantity is consistent with the index . 4. possible_key It's not worth it ,key It's not worth it : normal , You didn't build the index .- key_len: Represents the number of bits used in the index . This column can be used to calculate 【 The length of the index used in the query 】, With the same query results , The smaller the value, the better . key_len The value displayed is the maximum possible length of the index field , Instead of the actual length , It is calculated by table definition , It's not a search in a table . Suppose you build a composite index (col1,col2), If 【 Through col1 Condition query 】 and 【 Through col1 and col2 Condition query 】 The result is the same , Then the former is better , Because you only need to use one field ,key_len The value of will be smaller , As mentioned above, it is determined by the length of the table definition key_len Value .- ref: Display key The value of the index reference in the column . There are two possible values , And the corresponding information : Library name . Table name . Field name -- Indicates which column of which table in which library the index refers to ; const -- The value representing the index reference is a constant , It's usually where id=1 That's how it comes out ;- rows: According to table statistics and index selection , Roughly estimate the number of rows to read to find the record to query .- Extra: Very important additional information . Here are the possible values , And the corresponding information : Using filesort -- Explain mysql Completely or partially not sorted according to the index you built , It needs to be optimized .MySQL A sort operation that cannot be done with an index is called “ File sorting ”; Using temporary-- Temporary tables are used to store intermediate results ,mysql Use temporary tables when sorting query results . It also needs to be optimized , Because the creation and deletion of temporary tables are cost-effective , Common in order by and group by; Using index -- It means corresponding select Overlay index used in operation , Avoid accessing the columns of the table , It's efficient . If it appears at the same time Using where, Indicates that the index is used to perform a query of the key value of the index , If not , Indicates that the index is used to read data rather than perform query actions ; Using where -- Used where Filtration ; Using join buffer -- Using wire caching , If this field always appears , You can increase this value appropriately in the configuration file ; Impossible where -- where The value of clause is always false, Cannot be used to get any tuples ; Select tables optimized away -- In the absence of group by In the case of clause , Based on index optimization MIN/MAX To operate or respond to MyISAM Storage engine optimization count(*) operation , You don't have to wait for the execution phase to calculate , The query execution plan generation stage completes the optimization ; distinct -- Optimization distinct operation , Stop looking for the same value immediately after finding the first set of matching tuples . Be careful : 1. Use group by When sorting , If it's useful to index , It's best to follow the order of the index , such as , There is a composite index (col1,col2), When sorting, if you skip col1, Use it directly col2 Sort , It can lead to Using filesort、Using temporary And other serious problems . 2. Try to use an overlay index ,select The following column names are exactly in the order of the index created 、 The number is the same . In this way, the index can be used to read data directly , Avoid reading table columns .# Finally, a simple case , I'll put the topic first , Let's think again , Finally, put the answer .### The title is to write SQL The order of execution .![]( Ideas 1. First of all to see id Column ,id The bigger, the higher the priority , Index from id For 4 I'm going to start looking at , In this line of work table For t2, The query is t2, So the first query is the last part `select name,id from t2`.2. id For 3 My line of work ,table yes t1, So what we're looking for is `select id,name from t1 where other_column=''`; At the end of the line Extra Column `Using where` It also proves this point .3. id For 2 My line of work ,table yes t3, So what we're looking for is `select id from t3`; in addition ,key The column is primary The primary key is used as index ,Extra Column `Using Index` Indicates that the overlay index is used ( That is to say, the reference is in select Behind ).4. id For 1 My line of work ,

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