"Talk about MySQL cardinality statistics"

Give me daydreams 2020-11-10 17:23:19
talk mysql cardinality statistics


**

Hi, Hello everyone ! I'm daydreaming .

The topic I want to share with you today is :“ What is the cardinal number ?”

https://mp.weixin.qq.com/s/FgxwAFQbEjv5i-TxjvLK6Q

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One 、 What's the base number ?

Cardinality Refers to MySQL The number of different values for a column in a table .

If this class is the only index , The cardinal number = Row number .

If this column is sex, Enumeration types are only male and female , Then it's the cardinal number 2

Cardinality The higher the , The more valuable columns become indexes .MySQL The implementation plan will also be based on Cardinality Choose the index .

You can see the cardinality of the columns in the table in the following way .

For example, this classic example :
There is a column for sex, The for sex The value stored in the column Either male or female , Its largest base is 2.
Then there is no need for sex Index . because , In order to improve you based on sex Query speed of ,MySQL A new index will be created for the new index you selected B+Tree. But you sex There are only two values , about MySQL Come on , Even if it creates B+Tree Indexes , When the query is actually executed , At most one binary query , The rest of the operation can only be traversal , So for sex It doesn't make much sense to create an index .

Two 、InnoDB When to update the base ?

Parameters :innodb_stats_auto_recalc control MySQL Whether to actively recalculate the persistent information . The default is 1 Express true,0 Express false.
By default, when rows in a table change more than 10% when , Recalculate the cardinality information .

3、 ... and 、 The base number is estimated

The cardinality is not updated in real time ! And it's an estimated value from sampling !

As for the formula of cardinal number , It may not matter .

It's important that you know , It's an estimate by randomly sampling data pages .

And the number of pages randomly sampled can be determined by the parameter innodb_stats_persistent_sample_pages Set up , The default value is 20.

That means The base value is not accurate , Even if the results of each calculation are still quite large .

Four 、 Persistence base

You can use the parameter innodb_stats_persistent Controls whether the cardinality is persisted , The default is off.

Of course, you can set it for a separate table STATS_PERSISTENT=1 So it's innodb_stats_persistent Will be automatically enabled .

The advantage of opening it is : restart MySQL It's not going to be repeated , Speed up the restart .

Four 、 How to actively update the base ?

Execute the following SQL Will trigger InnoDB Update base ( Even if you don't realize it updates the cardinality ).

So try to choose a business low peak period

  • analyze table tableName;

If the number of samples is too small , The base number of the calculation is too wrong . That's likely to lead to MySQL The optimizer for has selected the wrong index . This is that you can increase the value appropriately . But add Too much may lead to ANALYZE TABLE slow .

conversely , ANALYZE TABLE It's too slow . You can adjust the parameters moderately innodb_stats_persistent_sample_pages Value . But this may lead to inaccurate calculation of cardinality .

If there is no way to balance the two . Consider reducing the number of index columns in the table or limiting the number of partitions to reduce ANALYZE TABLE complexity . The number of columns in the primary key of a table is also important , Because primary key columns are attached to every non unique index .

Reference resources :

https://dev.mysql.com/doc/refman/5.7/en/innodb-persistent-stats.html

https://dev.mysql.com/doc/refman/5.7/en/innodb-analyze-table-complexity.html

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