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Statistics used in the cached execution plans

Page Contents

Statistics used in the cached execution plans – Stored Procedures


Summary

The query optimization process sometimes requires an understanding of how the SQL Server’s Query engine compiles, re-compiles, and executes SQL batches. Some of the most important elements used by the Query optimizer when constructing a good plan are the “Interesting statistics”. These are statistical information used by the Query optimizer when constructing a good enough query execution plan.
This blog attempts to explain what are the “interesting statistics”, when they are updated and how the statistical information relates to the query recompilation process. The topic is related to Temporary tables statistics when used in stored procedures.

Batch compilation and recompilation

To begin with, let’s analyze the batch compilation/recompilation diagram (By Arun Marathe, Jul 2004, Batch Compilation, Recompilation and Plan Caching Issues in SQL Server 2005). The idea is to create a set of experiments that will capture the behavior of a stored procedure through the different phases of the query compilation/recompilation process, particularly those related to the statistics that are used to generate the execution plan.


Figure 1, Batch Compilation/Recompilation diagram

I’ve used the AdventureWorks database to set up the test environment and MS Profiler to capture various Events relevant to the experiments.

MS Profiler events

    1. Attention (Errors and Warnings)
    2. Auto Stats (Performance)
    3. SP:CacheHit (Stored Procedures)
    4. SP:CacheInsert  (Stored Procedures)
    5. SP:CacheMiss  (Stored Procedures)
    6. SP:CacheRemove  (Stored Procedures)
    7. SP:Completed  (Stored Procedures)
    8. SP:Recompile  (Stored Procedures)
    9. SP:Starting  (Stored Procedures)
    10. RPC: Starting (Stored Procedures)*
    11. RPC:Completed (Stored Procedures)*
    12. SP:StmtStarting  (Stored Procedures)
  1. SQL:StmtRecompile (TSQL)
  2. SQL:StmtStarting  (TSQL)

Database Objects
Set AdventureWorks DB compatibility level to 140 – SQL Server 2017. The version provides easy access to the information about the interesting statistics saved with the query plan (SSMS – SELECT Plan Operator, Properties,OptimizerStatsUsage).

Below is the set of SQL Server object definitions used for the testing.

USE master
GO
ALTER DATABASE AdventureWorks
    SET COMPATIBILITY_LEVEL = 140;
GO

USE AdventureWorks
GO

/* dbo.SalesOrderDetail table */
DROP TABLE IF EXISTS dbo.SalesOrderDetail
GO

SELECT *
    INTO dbo.SalesOrderDetail
FROM Sales.SalesOrderDetail;
GO

--add primary, clustered key
ALTER TABLE dbo.SalesOrderDetail
    ADD CONSTRAINT PK_SalesOrderDetailID PRIMARY KEY CLUSTERED (SalesOrderDetailID);  
GO

--NCI on ProductID
CREATE NONCLUSTERED INDEX IX_SalesOrderDetail_ProductID 
    ON dbo.SalesOrderDetail (ProductID);
GO

/* dbo.Products table */
DROP TABLE IF EXISTS dbo.Products
GO
    
SELECT *
    INTO dbo.Products
FROM Production.Product
GO

--add primary, clustered key
ALTER TABLE dbo.Products
    ADD CONSTRAINT PK_ProductID PRIMARY KEY CLUSTERED(ProductID)
GO

--NCI on ListPrice
CREATE NONCLUSTERED INDEX NCI_Products_ListPrice
    ON dbo.Products (ListPrice)
        INCLUDE([Name],ProductNumber)
GO

/* dbo.TestQueryExectuion stored proc*/
DROP PROCEDURE IF EXISTS dbo.TestQueryExecution;
GO

CREATE PROCEDURE dbo.TestQueryExecution
          @ProductID INT
         ,@ProdName NVARCHAR(50)
         ,@MinLinePrice MONEY = $100
AS
BEGIN
    SET XACT_ABORT ON; 
    SET NOCOUNT ON

    --query1
    SELECT   d.CarrierTrackingNumber,
             d.OrderQty,
             d.UnitPrice,     
             d.ProductID,
             p.[Name]
    FROM    dbo.SalesOrderDetail d
        INNER JOIN dbo.Products p
            ON d.ProductID = p.ProductID
    WHERE d.ProductID = @ProductID 

    --query2
    SELECT [Name]
           ,ProductNumber
           ,ListPrice
           ,Color
    FROM dbo.Products
    WHERE ListPrice >= @MinLinePrice
        AND  [Name] LIKE (@ProdName +'%')

    RETURN;
END

Information about the statistics/indexes on the tables can be retrieved using the queries below.

USE AdventureWorks
go

--sys.stats
SELECT
     TableName = OBJECT_NAME(sp.object_id)
    ,[Statistic ID] = sp.stats_id -- If statistics correspond to an index, the stats_id value is the same as the index_id value in the sys.indexes.
    ,[Statistics] = s.[name] 
    ,[Last Updated] = sp.last_updated 
    ,sp.[rows]
    ,sp.rows_sampled
    ,sp.unfiltered_rows
    ,Modifications = sp.modification_counter
    ---
    ,s.object_id
FROM sys.stats AS s
OUTER APPLY sys.dm_db_stats_properties (s.object_id,s.stats_id) AS sp
WHERE s.object_id IN (OBJECT_ID(N'dbo.SalesOrderDetail'), (OBJECT_ID(N'dbo.Products')));

--sys.sysindexes
SELECT TableName = OBJECT_NAME(i.id)
      ,IndexName = i.[name]
      ,i.indid --If statistics correspond to an index, the stats_id value in the sys.stats is the same as the index_id
      ,IndexDesc = CASE 
                        WHEN i.indid = 0 THEN 'HEAP'
                        WHEN i.indid = 1 THEN 'CLUSTERED'
                        WHEN i.indid > 1 THEN 'NONCLUSTERED'
                   END
      ,i.rowcnt
      ,i.dpages
      ,i.rowmodctr
FROM sys.sysindexes i
WHERE i.id  IN (OBJECT_ID(N'dbo.SalesOrderDetail'), (OBJECT_ID(N'dbo.Products')));

The following examples assume the default settings for the Sql  Server’s options related to the statistics:
 AUTO_CREATE_STATISTICS ON
– AUTO_UPDATE_STATISTICS ON
AUTO_UPDATE_STATISTICS_ASYNC OFF 

A bit of theory first before proceeding with the tests. : )

colmodctr

colmodctr is an ever-increasing counter that tracks the changes made on tables (a counter per column excluding the non-persistent computed columns). colmodctr is not transactionally consistent which means that is not affected by the rolled-back changes i.e if a transaction inserts 10 rows in a table and then roll-back, the counter will still report 10 changes.
SQL Server Statistics (automatically/manually created/updated) on a column(s) will store the snapshot value of the colmodctr for the leftmost column in the stats blob.
The counter is very important since it’s one of the elements that drive the query recompilation decisions related to the statistics changed reasons. 

colmodctr counter can be accessed through the following system views.


Figure 2, colmodctr, system views – standard and hidden

One way  to access the hidden tables is to; Open a separate SSMS instance, close the object explorer, and create a single connection using the Server name: i.e ADMIN:(local)
NOTE: The structure of the hidden tables and the tables’ accessibility are not documented and may be changed in future versions.

Recompile thresholds (RT)

RT concept defines the number of changes on a table column needed to be done in order to indicate the statistical information of that column as stale. 
The changes include the column values changes through the DML operations such as INSERT, UPDATE, DELETE… i.e Inserting 10 new rows in a table is considered as 10 changes(identified by the colmodctr counters mentioned before).
If the table does not have statistical information i. e HEAP table with no indexes and no manually created statistics, and the query plans that reference the table does not load/automatically create interesting statistics, the only relevant change when performing the RT crossing test will be the change in the number of rows inserted and/or deleted.

colmodctr(current) – colmodctr(snapshot) |  >= RT

or

 | cardinality(current) – cardinality(snapshot) |  >= RT

current     – refers to the current value of the modification counter
snapshot – refers to the value of the mod. counter captured during the last plan compilation(re-compilation).
cardinality* – the number of rows in the table.

Cardinality

In mathematics, the cardinality of a set is defined as the number of elements in a SET.  A SET is an unordered collection of elements in which each element is unique.

In RDBMS – see RDBMS fundamentals), data is presented in a form of a Table. An RDBMS table has its roots in a structure called Relation (attributes ~ columns, tuple ~ row). The number of tuples is represented by the cardinality of the relation – a Relation does not have duplicate tuples.
However, the table structure deviates from the strict rules and allows, e.g., duplicate rows. This means, that we use cardinality to represent the number of rows in a table, only if the table has no duplicates.
Sometimes in the literature, we find that the cardinality of a table represents the number of unique rows out of the total number of rows. So, cardinality in this context represents uniqueness. 

Cardinality may represent the uniqueness of data values in a particular column – the lower the cardinality the selectivity of a value decreases, and the more duplicated values in the column. It is a measure that defines the density of a column – column density is a reciprocal value of the column’s cardinality. So the more selective values the less density. Then there is a metric called Density Vector that consists of the densities of the individual columns… super interesting stuff, but not for this post 🙂 

In a different context, Cardinality is a way to define the relationship between two entities in a data model. It is also known as the degree of relationship i 1-1, 1-m, m-n.

 

The Threshold Crossing  Test evaluates to TRUE if the number of changes is greater than the predefined RT value (see Figure 3)

Recompilation thresholds(RT) for all the tables referenced in the query are stored along with the query plan.

RT depends on the table type(permanent vs temporary) and the number of rows in the table.


Figure 3, Recompile thresholds

Special case. RT = 1 if the table has 0 rows (with or without statistics)

NOTE: Starting from SQL Server 2008 R2 SP1, Microsoft introduced TF2371. The trace flag activates the dynamic recompile threshold calculation. The higher number of rows in a table, the lower the RT. The functionality is implemented to allow automatic statistics updates to kick off more frequently for the big tables. i.e RT for a 10,000-row table is 500 + 0.20*10,000 = 2,500 – the number of changes required to trigger query recompile. For a table with 100M rows, the RT is 20,000,500. For some applications the RT may be too high, resulting in sub-optimal plans due to the lack of query recompilation. Hence the TF2371.
Starting from SQL Server 2016, the TF2371 is turned on by default.

Here are a couple of examples to illustrate Figure 3.
If there is a table A that contains 230 rows, RT for the table will be set to 500. This means that if we i.e insert 500 rows, the total number of rows (c)  will change to 730 (c>=230+500) which is enough changes to make the table’s statistics stale.
The change itself does not mean much if there are no queries that reference the table : )
The query plans may or may not initiate the auto-statistics creation on the specific table columns. Also, the referenced tables may not have any statistical information i.e HEAP table with no non-clustered indexes.

Experiments

Experiment 1 (stats change before query execution)

In this experiment, we will make “enough” changes to the ListPrice column (dbo.Products table) BEFORE running the stored procedure for the first time, 
The column is a key column in NCI_Products_ListPrice, the non-clustered index, and has statistical information attached to it (the stats object name is the same as the NCI)

Let’s begin the experiment by creating the test objects and checking the statistical information on the tables.

Step 1, Check the initial stats/rowmodctr information

Figure 4, Initial rowmodctr information

Step 2, Check stats BLOB and make changes on dbo.Products table

Run the DBCC  command below before and after the UPDATE to confirm that there were no changes in the stats BLOB information.

DBCC SHOW_STATISTICS ('dbo.Products',NCI_Products_ListPrice) 
    WITH STAT_HEADER
BEGIN TRANSACTION  
    UPDATE dbo.Products  --504 changes
        SET ListPrice +=ListPrice *0.10 --add 10% to the price
    UPDATE TOP(106) dbo.Products --106 changes
        SET ListPrice +=$10.00 -- add $10 dollars to the prices
ROLLBACK TRANSACTION

NOTE: rowmodctr is not transactionally consistent.


Figure 5, stats BLOB information


Figure 6, rowmodctr after the initial dbo.Products update

The changes are detected and available through SQL Server’s metadata.

Step 3, Run the stored procedure and observe the captured events by the Profiler.

EXEC dbo.TestQueryExecution
       @ProductID =897
      ,@ProdName = N'G'
     ,@MinLinePrice = $0

Figure 7, Statistics refresh

Following the batch compilation diagram, we can identify the following steps.

  1. The Cache Lookup step resulted in the SP:CasheMiss event. dbo.TestQueryExecution stored proc. does not exist in the cache.
  2. Query Compilation Begins. SQL Server engine is about to load all of the  “interesting statistics”. The loaded statistics can be retrieved from the Actual Execution Plan, the SELECT physical operator – OptimiserStatsUsage property.
  3. The Query engine checks if any of the loaded interesting statistics are stale. If yes, the system stops the batch compilation process and refreshes the statistics. In our case the system has 
    • Identified the number of changes made on the ListPrice column. From the stats/index information gathered after the initial update, the number of changes (rowmodctr/Modifications) is 610
    • Performed RT crossing test.  The test passed since the number of changes(610) exceeded the RT for tables with a number of rows greater than 500. RT = 500 + 0.20 * 504 ~ 601, 601 < 610
    • Executed StatMan, an internal process that automatically maintains statistics. The process updated the stale statistics NCI_Products_ListPrice on dbo.Product table
      SELECT StatMan([SC0], [SC1]) --from MS Profiler
      FROM   (SELECT  TOP 100 PERCENT [ListPrice] AS [SC0]
                                     ,[ProductID] AS [SC1]
              FROM  [dbo].[Products] WITH (READUNCOMMITTED)
              ORDER BY [SC0],[SC1]) AS _MS_UPDSTATS_TBL
      OPTION (MAXDOP 16);

      If we check the stats blob from Step 2, we will see that the Updated column changed its value to the current date – the stats blob has been updated.
      The AutoStats event reported the UPDATE of the statistics with EventSubClass = 1 – Other. More on the event can be found here.

  4. Query Optimiser starts to generate the query plan – a plan for each query statement.
    • The second query in the batch has a predicate on the Name column of the dbo.Products table. In an attempt to make better cardinality estimates on the rows that need to be processed, Query Optimiser decided to automatically create statistical information on the column.
      The system stops the batch compilation process and again executes the StatsMan process to create the new statistics.
      SELECT StatMan([SC0])
      FROM  ( SELECT TOP 100 PERCENT [Name] AS [SC0]
              FROM  [dbo].[Products] WITH (READUNCOMMITTED)
              ORDER BY [SC0]) AS _MS_UPDSTATS_TBL
      OPTION (MAXDOP 16);

      After creating the stats, QO decided not to use it  : (
      Below is the list of the “interesting statistics” loaded during the Query compilation process. The list does not include automatically created stats on the Name column.

      As a result of the updated statistics on the ListPrice column , the rowmodctr for the column was reset. 

    • QO sets the new recompilation thresholds(RT) for all tables used in the queries.
      1. RT(dbo. SalesOrderDetail) = 500 + 0.20(121317) =24763.4 (~24764)
      2. RT(dbo.Products) = 500 + 0.20(504)= 600.8(~601)
        This means that QO will initiate query recompile due to the “Statistics changed” reason if
        1. dbo. SalesOrderDetail
          1. 24764 or more inserted/deleted rows
          2. 24764 or more changes on SalesOrderDetailID, ProductID columns
        2. dbo.Products
          1. 601 or more inserted rows
          2. 601 or more changes on ProductID, ListPrice, and Name columns
  5. The query execution starts. The query plans are constructed and cached. SP:CacheInsert event reported that the stored procedure has been cached.

Experiment 2 (stats change during the query execution)

In this experiment, we will make “enough” changes to the Name column (dbo.Products table) HALFWAY THROUGH the stored procedure execution.

Step 1 Set up the environment

  • Run the script to reset the test environment
  • Add a WAITFOR statement between the two queries in the stored procedure
... 
SELECT   d.CarrierTrackingNumber,--query1
             d.OrderQty,
             d.UnitPrice,     
             d.ProductID,
             p.[Name]
    FROM    dbo.SalesOrderDetail d
        INNER JOIN dbo.Products p
            ON d.ProductID = p.ProductID
    WHERE d.ProductID = @ProductID 

    WAITFOR DELAY '00:00:06' -- buy some time to change statistisc
    
    SELECT [Name]--query2
           ,ProductNumber
           ,ListPrice
           ,Color
    FROM dbo.Products
    WHERE ListPrice >= @MinLinePrice
        AND  [Name] LIKE (@ProdName +'%')
...
  • Use PowerShell to execute the stored procedure. Add HostName property. Use the HostName to capture only the events related to the PS call. This will prevent MS Profiler from capturing events related to the UPDATE statement that will run in parallel.
    PS C:\Users\dean.mincic> Invoke-Sqlcmd -ServerInstance "." `
                                           -Database "AdventureWorks" `
                                           -HostName "experiment" `
                                           -Query "EXEC dbo.TestQueryExecution `
                                                      @ProductID =897 `
                                                      ,@ProdName = N'G' `
                                                      ,@MinLinePrice = 0.00;"
  • Add an ApplicationName filter to the Profiler trace (ApplicationName LIKE experiment)

Step 2, Run the PowerShell cmdlet, switch to SSMS, and run the UPDATE query below. The queries will generate enough changes to make the automatically created statistics on the Name column stale.

BEGIN TRANSACTION  
    UPDATE dbo.Products  --504 changes (all rows)
        SET [Name] +='_ABC' 
    UPDATE TOP(106) dbo.Products --106 changes (random rows)
        SET [Name] +='_ABC'
COMMIT TRANSACTION

Step 3. Analyze the captured MS Profiler trace.
Figure 8, Query recompile

  • The first thing that is different from the last run is the SP:CacheHit event. The event shows that our stored procedure was found in the Plan cache. The previously set RTs and the interesting statistics are part of the cached information.
    NOTE: Auto-created statistic on the Name column was not used during the initial query compilation – the stats are not part of the interesting stats.
  • This time there were no changes on the columns that would initiate statistics updates, no new auto-created stats and the existing cached query plan does not need to be recompiled due to “statistic changed” reasons. The process proceeds with the query execution.
  • The first query is successfully executed followed by the  WAITFOR statement. During the statement execution (6 seconds delay) a separate query has made enough changes on the Name column(dbo.Products) to pass the RT crossing test for the table and flag the auto-created statistics on the column as stale. Even if not used by QO during the plan generation, the stats are marked as stale.
  • (1) The query execution stops at the  “Any stats stale?”  step. The System initiates the query recompile process – SP: Recompile due to 2 – Statistics changed reason. The event is followed by the statement level SQL:StmtRecompile event which indicates that only the second query needs to be recompiled.
  • (2) Again, the StatsMan process kicks in and updates the stale statistics. The RTs are set (in this case the number of rows has not changed, hence the RTs stayed the same). Rowmodctr value for the Name column is reset. to 0 
  • (3) The AutoStats event reported Statistics Update  having EventSubClass = 1 – Other
  • (4) The SP:StmtStarting event reports that the second query has been recompiled and the batch execution continues.

Experiment 3 (tables with no stats on columns)

The experiment demonstrates how queries get recompiled when referencing tables with no statistics. The recompiles due to the “statistics changed” reasons are initiated by the RT-table cardinality crossing test results only.
As previously mentioned, the cardinality-based RT crossing test is defined as

 | cardinality(current) – cardinality(snapshot) |  >= RT

Let’s create a test table and a stored procedure to perform the above experiment.

Step 1, set up the test environment

--a heap table - no statistics
DROP TABLE IF EXISTS dbo.testRecomp;
GO
CREATE TABLE dbo.testRecomp(id INT
                            ,filler CHAR(100)
                            );
GO
--test sp
DROP PROCEDURE IF EXISTS dbo.testRecompile;
GO
CREATE PROCEDURE dbo.testRecompile
AS
BEGIN
    SET XACT_ABORT ON;
    SET NOCOUNT ON;
  
    SELECT TOP(100) Average = AVG(t.id)
                   ,NoOfRows =  COUNT(t1.id)
    FROM dbo.testRecomp t
       CROSS JOIN dbo.testRecomp t1 
   RETURN;
END 
GO

.. and insert some data into the table…

--add 230 rows, RT will be set to 500
INSERT INTO dbo.testRecomp(id,filler)
    SELECT TOP(230) c.column_id,c.[name]
    FROM master.sys.columns c1 ,master.sys.columns  c;
GO

The initial statistical information looks like this (find how to retrieve the metadata related to the statistical information at the beginning of the post)


Figure 9, rowmodctr with no statistical information

Step Run the stored proc for the first time. The RT is set to 500.

EXEC dbo.testRecompile;

Step 3 Make enough changes to the table to pass the cardinality crossing test. Insert 500 rows. Do not use explicit transaction yet.

--BEGIN TRANSACTION
    INSERT INTO dbo.testRecomp 
        SELECT TOP(500)ABS(CHECKSUM(NEWID())) % 500,  b.[name] + '_' + cast (ROW_NUMBER() OVER(ORDER BY (SELECT NULL )) AS varchar(5))
        FROM sys.columns a, sys.columns b
--COMMIT TRANSACTION; --ROLLBACK TRANSACTION;

Step 3 Run the stored procedure again and observe the query execution behavior in Profiler.

Figure 10, Query recompile, table cardinality change – no stats

  • The new rowmodctr information looks like

    The new number of rows (rowcnt) is recorded along with the number of changes, rowmodctr=730. In this case, the rowmodctr value is not relevant since the RT crossing test depends only on changes in the table cardinality. This will be more visible if we ROLLBACK the row insertion operation which will be covered later.
  • The second execution followed the “Cashe lookup = Success” path (see the batch compilation diagram) and failed to pass the very last step  “Any stats stale?“.
  • At this stage, the system has detected that the RT cardinality crossing test has passed due to the number of changes(new rows) inserted in the table.
  • The system stopped the execution process and initiated the stored proc/statement recompile – SP:Recompile, SQL:StmtRecompile.  As in the previous examples, the reason for the recompile was 2 – Statistics changed.
    NOTE: The recompile process is not followed by the StatMan process since the query does not have any statsBlob information to be refreshed/created.

Experiment 3.1 (rowmodcnt not in use)

The next example shows that the RT cardinality crossing test is not related to rowmodctr as it may seem from the previous example where the number of changes followed table cardinality changes.

  • Follow the steps from the previous example.
  • Execute the INSERT query  from Step 3 within an explicit transaction
BEGIN TRANSACTION
    INSERT INTO dbo.testRecomp 
        SELECT TOP(500)ABS(CHECKSUM(NEWID())) % 500,  b.[name] + '_' + cast (ROW_NUMBER() OVER(ORDER BY (SELECT NULL )) AS varchar(5))
        FROM sys.columns a, sys.columns b
ROLLBACK TRANSACTION;
  • Observe that there are no query recompiles due to “statistic change since there were no table cardinality changes – the ROLLBACK “canceled” row insertions.
  • The statistical information shows that the rowmodctr= 720.

Conclusion

Query compilation, execution, and recompilation sequence among other steps include; loading interesting statistics – the statistical information on different table columns that Query Optimiser may find useful when creating a good plan and auto-creating statistical information on the columns that participate in i.e WHERE filter, GROUP BY ..etc. 
SQL Server query engine also checks the validity of the loaded statistical information during the initial stored procedure compilation and again during the stored procedure execution phase. If the loaded statistics are found to be stale, the former pauses stored procedure compilation, refreshes(re-samples/refreshes) the loaded statistical information, and continues the compilation process. If the Query engine detects stale loaded statistics during the execution phase,  the process stops refreshes(re-samples/updates) statistics and restarts the compilation process – query recompilation. The re-compiles are done per query, not per batch.
The examples in this blog showed that the statistical information can be automatically maintained by the queries that use them. Statistics can be also maintained manually.
To mark statistics as “Stale”, QO uses the Recompile Threshold(RT) crossing test. The test tracks the number of changes on the significant(leftmost) columns within the statistic BLOBs. The information is stored in an ever-increasing, non-transactionally consistent counter – “rowmodctr”.  The RTs are stored per table and within the compiled query.
The RT crossing test can be based only on the changes in the number of rows in a table.

 

Thanks for reading.

Dean Mincic

Read Only Routing in Sql Server

In this blog, I wanted to share a recent experience of implementing Read Only Routing functionality on the existing HA/AG Sql Server Cluster. The technology significantly offloaded the primary production server and enhanced  the read queries performances.

The Sql Server HA/AG environment
Windows Failover Cluster

The main business application uses Sql Server set of databases hosted by a WSFC(Windows Failover Cluster). The cluster has two nodes, NODE1 and NODE2. Both nodes have Windows 2012 R2 Server OS and Sql Server 2014 SP1 database platform. The failover cluster has VNN(Virtual Network Name) CLUST1.

Availability group

The system has an Availability Group AG1 (and consequently a WSFC resource group). The AG1 group members are the main business application’s backend databases.
The two availability replicas host the AG1 availability group databases. Each replica exists on a different node, primary on the NODE1, and secondary on the NODE2.
The primary replica is read/write. The secondary replica, in addition to providing the HA/DR for AG1, is set to allow Read Only access for the clients. The readable secondary replica is set to “YES” which allows read-only workloads even without  ApplicationIntent parameter in the connection string definition.

The AG1 availability mode is set to : Synchronous-commit mode
The AG1 failover type is : Automatic failover

The business databases usage pattern

Because the way the cluster was set up the clients were connecting to the production databases through WSFC – CLUST1. All the requests were directed to the current primary replica on the NODE1. The secondary replica on the NODE2, even if set up to allow read-only connections, was never used for that purpose. It was possible to connect to the current secondary replica directly through NODE2 though.
The clients who use the databases are:

  1. ~200 concurrent business application users. They perform read/write queries through the app’s UI

  2. ~10 scheduled jobs that runs regularly during the business hours. The tasks mostly extract data from the business databases (read only operations) in order to provide the required business information for a number of different applications on a number of different platforms i.e Windows Active Directory, AWS, AZURE, Linux etc.

  3. An “unknown”, or should I say, NULL 🙂  number of users who regularly run the ad-hoc, read-only queries through a number of different applications e.g Excel Power Queries, Tableau, Crystal Reports and who knows what..

  4. A number of custom, in-house build, LAMP model applications (only in this case M stands for Ms Sql Server..) that also perform read only operations.

Even if the high-end hardware keeps the cluster’s performance on the acceptable level, there is a lot of room for a number of improvements. One of these improvements, the Read-Only Routing  is described in this blog post.

Read Only Routing

Simply speaking, with Read Only Routing in place, within the connection string, we can specify the client app. connection intention. The intention will later decide on which server the request will be routed. The intention can be:

  • Read only – Client connection needs only to read data
  • Read and write (Default value) – Client connection needs to read and write data.

This means that if we know that an application needs only to read data, we can use a connection string property ApplicationIntent with the value of READONLY to “tell the cluster” to redirect the requests to the secondary, read-only replica. This will offload the NODE1 resources(assuming that the current server role is the primary replica) and will “employ” the NODE2 resources in order to execute the queries.
The connection string property ApplicatoinIntent is supported by all SQL Server 2014 Clients such as ADO.NET(SqlClient),  Sql Server Native Client 11.0 ODBC driver,  SQLNCLI11 OLEDB Driver.
To make the cluster be able to recognise and redirect such a connections, we need to set up:

  • Availability group Listener for the same subnet ( in this case both replicas are on the same subnet)
  • Read-Only routing list
Availability group listener

The availability group listener is a WSFC resource that is logically bound to the AG through the corresponding WSFC resource group . It’s a VNN(virtual network name) with one or many IP addresses attached to it. The Listener runs on a single node(always on the current primary replica) and just handles the connection forwarding. The listener endpoint is always the primary replica.

It is possible to create multiple listeners for an availability group. Only the first listener can be created through tsql. The others can be created using WSFC Failover Cluster Manager or PowerShell script.
Since I was setting up the first listener for AG1, I was able to use tsql.

To set up an AG Listener we need to:

  1. Set up a DNS entry, preferably using a static IP address. In this case the DNS name is AG1-Listener. *
  2. Add the listener to the existing availability group, AG1.

*NOTE: If you decide to use SSMS to create a new listener, SSMS (through WSFC) will create a DNS entry for you. In this case you’ll need to provide the required AD credentials to the WSFC to be able to create the entry – Create Computer Objects permission.

Add the listener to the existing availability group

USE [master]
GO
ALTER AVAILABILITY GROUP AG1
    ADD LISTENER 'AG1-Listener' (
        WITH IP(('10.XX.XX.XX','255.XX.XX.XX'))
       ,PORT = 1433)

Check the listener’s metadata:

SELECT gl.dns_name
      ,gl.port
      --,gl.is_conformant
      ,gl.ip_configuration_string_from_cluster
      ,lip.ip_address
      ,lip.ip_subnet_mask
      ,lip.is_dhcp
      ,lip.network_subnet_ip
      ,lip.network_subnet_prefix_length
      ,lip.network_subnet_ipv4_mask
      ,lip.state_desc
FROM sys.availability_group_listeners gl
INNER JOIN sys.availability_group_listener_ip_addresses lip
    ON gl.listener_id = lip.listener_id

Set up Read-only routing lists

Read-Only routing URL is a pointer that determines which replica will response to the client driver(provider) Read-Only request.

Read-Only routing mechanism performs the following sequence of events when allocating a secondary replica to route the clients requests to.

  1. Client requests a connection to AG1-listener specifying ApplicationIntent=ReadOnly, Server=’AG1-Listener’, Database=’prod_db’.  The request is routed to the NODE1(primary replica).
  2. NODE1 checks the incoming connection, ApplicationIntent parameter value and checks if the specified database is in the availability group.
  3. The server enumerates and checks the replicas defined in the Read-Only routing list.
  4. The routing target is the first readable secondary replica that accepts ALL or Read-Only connections from the list.
  5. The server sends the secondary replica’s Read-only routing URL to the client.
  6. The client then redirects the connection request to the secondary replica following the provided URL.

To set up Read-only routing list we need to assign the Read-only routing URLs for both of the replicas since the replicas may switch roles during the failover. The URLs is used to tell the client where to redirect the Read-only requests.
The URL contains network protocol, system address and the port number.

/*
Define Read-Only URL on the current primary replica.
The URL will be used when the replica switch role and become secondary rep.
*/
ALTER AVAILABILITY GROUP AG1
	MODIFY REPLICA ON N'NODE1'
	WITH (SECONDARY_ROLE(READ_ONLY_ROUTING_URL=N'tcp://NODE1.myDomain.au:1433'))
GO

/*
Define Read-Only URL on the current secondary readable replica.
*/
ALTER AVAILABILITY GROUP AG1
	MODIFY REPLICA ON N'NODE2'
	WITH (SECONDARY_ROLE(READ_ONLY_ROUTING_URL=N'tcp://NODE2.myDomain.au:1433'))
GO

Check the Read-Only URLs

SELECT replica_server_name 
      ,[endpoint_url]           AS WSFC_NodeEndpointURL
      ,availability_mode_desc   AS AvailabilityMode
      ,failover_mode_desc       AS FailoverType
      ,primary_role_allow_connections_desc AS AllowedConnectionsWhenPrimary
      ,secondary_role_allow_connections_desc AS AllowedConnectonsWhenSecondary
      ,read_only_routing_url 
FROM sys.availability_replicas
WHERE read_only_routing_url IS NOT NULL

..and finally create the Read-Only routing list

--when a client read-only request hits the primary replica, the server will response with 
--the secondary replica (NODE2) URL since the replica is the first on the list
ALTER AVAILABILITY GROUP AG1
    MODIFY REPLICA ON N'NODE1'
    WITH (PRIMARY_ROLE(READ_ONLY_ROUTING_LIST=(N'NODE2',N'NODE1')))
GO

-- and if the replicas swith the places, the system will response with the 
-- current secondary replica( now NODE1) URL
ALTER AVAILABILITY GROUP AG1
    MODIFY REPLICA ON N'NODE2'
    WITH (PRIMARY_ROLE(READ_ONLY_ROUTING_LIST=(N'NODE1',N'NODE2')))
GO

Check the read-only routing lists

-- https://blogs.msdn.microsoft.com/alwaysonpro/2014/01/22/modifying-alwayson-read-only-routing-lists/
SELECT  ag.name AS [Availability Group]
       ,ar.replica_server_name AS [When Primary Replica Is]
       ,rl.routing_priority AS [Routing Priority]
       ,CASE 
            WHEN ar.replica_server_name = ar2.replica_server_name 
                THEN 'Read-Only on primary' + '('+ ar.replica_server_name + ')'
            ELSE ar2.replica_server_name
        END AS [Read-Only Routed To]
       ,ar.secondary_role_allow_connections_desc
       ,ar2.read_only_routing_url
FROM    sys.availability_read_only_routing_lists rl
        INNER JOIN sys.availability_replicas ar 
            ON rl.replica_id = ar.replica_id
        INNER JOIN sys.availability_replicas ar2 
            ON rl.read_only_replica_id = ar2.replica_id
        INNER JOIN sys.availability_groups ag 
            ON ar.group_id = ag.group_id
ORDER BY ag.name
       ,ar.replica_server_name
       ,rl.routing_priority
A few more things about the readable, secondary replicas

There is a certain data latency between primary and secondary replica. The data movement process usually takes a few seconds and follows the pseudo-sequence below.

  • User make a change on  AG databases (Begin transaction)
  • Primary replica sends a log information which describes the actions to be performed on the secondary replica(s).
  • The secondary replica(s) runs a dedicated “Redo” process that commits the changes on the secondary replica. At this point in time, the changes are not visible by the read-only queries that runs on the secondary. The readers are not blocked by the redo actions* as they read the previous version of the data that’s being changed.
  • The change is committed on the primary replica (Commit transaction ). The changes are visible on both replicas.

*All read-only queries that runs on the secondary databases are automatically executed within the SNAPSHOT TRANSACTION ISOLATION LEVEL. All locking hints e.g (tablock,xlock,..etc) are ignored.

The DDL actions on the primary replica(applied through the log records redo actions) and conversely, the READ actions on the secondary replica can block each other.  Secondary replica’s “SELECT” queries require Sch-S locks that guarantee the schema stability of the objects in use  e.g During the execution of.. Select * from tab1, the lock manager has to prevent other sessions from changing the tab1 table structure during the select operation. On the other hand, the redo DDL actions require Sch-M (Schema modification locks) that prevent concurrent access to the locked resource e.g (TRUNCATE TABLE tab1 ). Schema locks are acquired regardless of transaction isolation level. Sch-S is compatible with all other locks except Sch-M (and  Sch-M is incompatible with all lock modes). It’s probably not a good idea to perform read operations on the secondary replica if the main system performs frequent DDLs.

Conclusion

Always On Availability group  active, secondary replicas support the Read-Only access. The client requests for read-only access can be automatically redirected to the secondary replica. This will help to conserve primary replica’s resources for the mission critical workloads. The read-only queries will run faster since the workloads ,by default, use row versioning (snapshot isolation level) to remove read/write blocking. The read-only access will have some data latency and that needs to be considered when deciding to go with this technology.

Thanks for reading.

Dean Mincic