Azure SQL db/dw series (10) -- re understanding the query store (3) -- configuring the query store

This paper belongs to Azure SQL DB/DW series
Last article ：Azure SQL DB/DW series （9）—— Reunderstanding Query Store（2）—— working principle
In this paper, Query Store Configuration of , Include PaaS Platform and non cloud versions

default setting

   Just like other functions , To maximize its potential use , It needs to be configured on demand .Query Store In the non cloud version （ Except managed instances , Because managed instances are very close to non cloud versions ） Not on by default , If you need to use it , Of course, open it , You can use the following command to directly open .

ALTER DATABASE [ Database name ] SET QUERY_STORE = ON;

   If you have sufficient authority , I also hope that all user libraries are opened , Then you can use the following command to open except master and Tempdb The library outside ：

DECLARE @SQL NVARCHAR(MAX) = N'';
SELECT @SQL += REPLACE(N'ALTER DATABASE [{
    {DBNAME}}] SET QUERY_STORE=ON ',
      '{
    {DBName}}', [name])
FROM sys.databases
WHERE state_desc = 'ONLINE'
      AND [name] NOT IN ('master', 'tempdb')
ORDER BY [name];
EXEC (@SQL);

   As mentioned earlier , Not right master and TempDB Enable , But you can Model Enable , It doesn't really collect Model Running information of the library , But just like other configurations , Encapsulate this configuration , So that the newly created database will automatically have this attribute . about msdb, Just like the running mode of the user library .
   The following is a list of configuration items , Contains Azure SQL DB And non cloud versions ：

   The following describes the contents listed in the above table .

OPERATION_MODE

  Query Store Can run in read-only perhaps read and write In mode , In read-only mode , The data will still be Query Store, But new queries are no longer collected from the time this option is configured . This option is often used for Query Store When the space is full , But you need to analyze the stored data first . Generally speaking, it is recommended to use READ_WRITE：

ALTER DATABASE [<Database Name>] SET QUERY_STORE ( OPERATION_MODE = READ_WRITE );

   Query storage has the concept of actual state and desired state . If the desired status is READ_WRITE, But the actual status is READ_ONLY, In the catalog view sys.database_query_store_options In the column readonly_reason The default setting of the display reason is READ_WRITE.

CLEANUP_POLICY (STALE_QUERY_THRESHOLD_DAYS)

  Query Store Need to store data , The retention time of data is the same as SIZE_BASED_CLEAN_UP and QUERY_STORE_CAPTURE_MODE May affect the final data storage . The default value is 30 God .

ALTER DATABASE [<Database Name>] SET QUERY_STORE ( CLEANUP_POLICY = ( STALE_QUERY_THRESHOLD_DAYS = <Value> ) );

DATA_FLUSH_INTERVAL_SECONDS

   Due to performance ,Query Store The collected data will be stored in memory and then written to disk asynchronously according to the configured value . This time is data flush interval, This value is in seconds .
   This value needs to be configured carefully , Too soon will be right I/O Carry out pressure , It is too slow and easy to lose a large amount of data or the data is not updated in time when problems occur . Generally speaking, no change is required , Especially shorter .
   The default value is 900 Second is 15 minute , Modify the command to ：

ALTER DATABASE [<Database Name>] SET QUERY_STORE ( DATA_FLUSH_INTERVAL_SECONDS = <Value> );

MAX_STORAGE_SIZE_MB

   Namely Query Store Free space for , If space is running out , that Query Store The status of will automatically change from READ_WRITE become READ_ONLY, This is related to the database LDF The database becomes read-only when the file is full .
   Even if SIZE_BASED_CLEARUP_MODE Set to AUTO, In very high load environments , It is still possible to reach the space limit quickly .
   If you have a lot of space , Of course, it is good to set it larger , But consider STALE_QUERY_THRESHOLD_DAYS Value , Retention time is also a factor .

ALTER DATABASE [<Database Name>] SET QUERY_STORE ( MAX_STORAGE_SIZE_MB = <Value> );

QUERY_STORE_CAPTURE_MODE

   The default is ALL, The other is for NONE, Set up NONE Will make Query Store Stop collecting new queries , But will not stop collecting already in Query Store Runtime statistics for queries in . The third option is AUTO, tell SQL Server Do not capture queries that are resource intensive or infrequently executed .

ALTER DATABASE [<Database Name>] SET QUERY_STORE ( QUERY_STORE_CAPTURE_MODE = [<Value>] );

   about CUSTOM, There are three values to control Query Store The collection behavior of . I will not extend the introduction here . Examples are as follows ：

ALTER DATABASE [<Database Name>]
SET QUERY_STORE = ON
    (
      QUERY_CAPTURE_MODE = CUSTOM,
      QUERY_CAPTURE_POLICY = (
        STALE_CAPTURE_POLICY_THRESHOLD = 24 HOURS,
        EXECUTION_COUNT = 30,
        TOTAL_COMPILE_CPU_TIME_MS = 1000,
        TOTAL_EXECUTION_CPU_TIME_MS = 100
      )
    );

MAX_PLANS_PER_QUERY

   The default is 200 Execution plans , however 2016 I won't support it . The greater the numerical , The more space you take up , however 200 An execution plan is useful for analyzing the performance of a query , It can be said that it is just right . If you find not enough , You can check and expand with the following command ：

SELECT query_hash,
COUNT (DISTINCT query_plan_hash) distinct_plans
FROM sys.dm_exec_query_stats
GROUP BY query_hash
ORDER BY distinct_plans DESC;

ALTER DATABASE [<Database Name>] SET QUERY_STORE ( MAX_PLANS_PER_QUERY = <Value> );

WAIT_STATISTICS_CAPTURE_MODE

   The default is ON, It's also not suitable for 2016, Another value is OFF, This part is more important , It's not going to unfold here for the time being , It will be introduced later ：

ALTER DATABASE [<Database Name>] SET QUERY_STORE ( WAIT_STATISTICS_CAPTURE_MODE = <Value> );

   because GUI It's easier to use , Just know the configuration , I'm not going to do that here . however GUI Some values cannot be modified , Therefore, it is recommended to use the command to configure .
  sys.database_query_store_options The configuration values are stored inside , Can be used to check .

Best practices

   First configuration MAX_STORAGE_SIZE_MB, Make sure not too few , At the same time pay attention to 30 Whether the retention time of days is enough . General recommendations 2048MB As a starting point . Also note that this data is stored in PRIMARY Filegroups , Make sure there is enough space .
   The second is QUERY_STORE_CAPTURE_MODE, It is suggested to change to AUTO, Unless you have other reasons .
   SIZE_BASED_CLEANUP_MODE It should also be set to AUTO.
   Next is INTERVAL_LENGTH_MINUTES, The default is 60, If the system is not very busy , You can just change it to 15 second , If a very busy system , You can change to 60 above , But not too much .
   WAIT_STATISTICS_CAPTURE_MODE Set to ON, Later on .

   Next article ：Azure SQL DB/DW series （11）—— Reunderstanding Query Store（4）——Query Store maintain