Manage database concurrency
- Skill 3.1: Implement transactions
- Skill 3.2: Manage isolation levels
- Skill 3.3: Optimize concurrency and locking behavior
- Skill 3.4: Implement memory-optimized tables and native stored procedures
- Thought experiment
- Thought experiment answers
Skill 3.4: Implement memory-optimized tables and native stored procedures
The In-Memory OLTP feature built into SQL Server 2016 adds a new memory-optimized relational data management engine and a native stored procedure compiler to the platform that you can use to run transactional workloads with higher concurrency. A memory-optimized table is a highly optimized data structure that SQL Server uses to store data completely in memory without paging to disk. It uses hash and nonclustered ordered indexes to support faster data access from memory than traditional B-tree indexes. SQL Server maintains a copy of the data on disk to guarantee transaction durability and to reload tables during database recovery.
To further optimize query performance, you can implement natively compiled stored procedures as long as the stored procedure accesses memory-optimized tables only. A natively compiled stored procedure is a stored procedure compiled into machine language for faster execution, lower latency, and lower CPU utilization.
Define use cases for memory-optimized tables
You use memory-optimized tables when you need to improve the performance and scalability of existing tables, or when you have frequent bottlenecks caused by locking and latching or code execution. SQL Server uses optimistic concurrency management for memory-optimized tables, which eliminates the need for locks and latches and in results in faster operations. In addition, SQL Server uses algorithms that are specifically optimized to access data from memory and natively compiled stored procedures to execute code faster. Depending on the type of workload you run, you can achieve five to 20 times performance gains with higher throughput and lower latency after migrating an existing disk-based table to a memory-optimized table.
In general, OLTP workloads with the following characteristics benefit most from migration to memory-optimized tables: short transactions with fast response times, queries accessing a limited number of tables that contain small data sets, and high concurrency requirements. This type of workload could also require high transaction throughput and low latency at scale. In the exam, you must be able to recognize the following use cases for which memory-optimized tables are best suited:
High data ingestion rate The database engine must process a high number of inserts, either as a steady stream or in bursts. Bottlenecks from locking and latching are a common problem in this scenario. Furthermore, last-page contention can occur when many threads attempt to access the same page in a standard B-tree and indexes intended to improve query performance add overhead time to insert operations. Performance is often measured in terms of throughput rate or the number of rows loaded per second. A common scenario for this workload is the Internet of Things in which multiple sensors are sending data to SQL Server. Other examples include of applications producing data at a high rate include financial trading, manufacturing, telemetry, and security monitoring. Whereas disk-based tables can have difficulty managing the rate of inserts, memory-optimized tables can eliminate resource contention and reduce logging. In some cases, requirements permitting, you can further reduce transaction execution time by implementing delayed durability, which we describe in greater detail in the next section.
High volume, high performance data reads Bottlenecks from latching and locking, or from CPU utilization can occur when there are multiple concurrent read requests competing with periodic inserts and updates, particularly when small regions of data within a large table are accessed frequently. In addition, query execution time carries overhead related to parsing, optimizing, and compiling statements. Performance in this case often requires the overall time to complete a set of steps for a business transaction to be measured in milliseconds or a smaller unit of time. Industries with these requirements include retail, online banking, and online gaming, to name a few. The use of memory-optimized tables in this scenario eliminates contention between read and write operations and retrieves data with lower latency, while the use of natively compiled stored procedures enables code to execute faster.
Complex business logic in stored procedures When an application requires intensive data processing routines and performs high volume inserts, updates, and deletes, the database can experience significant read-write contention. In some scenarios, the workload must process and transform data before writing it to a table, as is common in Extract-Transform-Load (ETL) operations, which can be a time-consuming operation. In other scenarios, the workload must perform point lookups or minimal joins before performing update operations on a small number of rows. Industries with these types of high-volume, complex logic workloads include manufacturing supply chains and retailers maintaining online, real-time product catalogs. Memory-optimized tables can eliminate lock and latch contention and natively compiled stored procedures can reduce the execution time to enable higher throughput and lower latency. Another possibility is to use delayed durability to reduce transaction execution time, but only if the application requirements permit some potential data loss.
Real-time data access Several factors contribute to latency when accessing data in traditional disk-based tables, including the time required to parse, optimize, compile, and execute a statement, the time to write a transaction to the transaction log, and CPU utilization when the database is experiencing heavy workloads. Examples of industries requiring low latency execution include financial trading and online gaming. With memory-optimized tables, the database engine retrieves data more efficiently with reduced contention and natively compiled stored procedures execute code more efficiently and faster. In addition, point lookup queries execute faster due to the use of non-clustered hash indexes and minimal logging reduces overall transaction execution time. Altogether, these capabilities of memory-optimized tables enable significantly lower latency than disk-based tables.
Session state management Applications that require the storage of state information for stateless protocols, such as HTTP, often use a database to persist this information across multiple requests from a client. This type of workload is characterized by frequent inserts, updates, and point lookups. When running at scale with load balanced web servers, multiple servers can update data for the same session or perform point lookups, which results in increased contention for the same resources. This type of workload is characterized by frequently changes to a small amount of data and incurs a significant amount of locking and latching. Memory-optimized tables reduce contention, retrieve data effectively, and reduce or eliminate IO when using non-durable (SCHEMA_ONLY) tables, which we describe in the next section. On the other hand, the database engine does not resolve conflicts resulting from attempts by separate concurrent transactions to modify the same row, which is relatively rare in session state management. Nonetheless, the application should include logic to retry an operation if a transaction fails due to a write conflict.
Applications relying heavily on temporary tables, table variables, and table-valued parameters Many times an application needs a way to store intermediate results in a temporary location before applying additional logic to transform the data into its final state for loading into a target table. Temporary tables and table variables are different ways to fulfill this need. As an alternative, an application might use table-valued parameters to send multiple rows of data to a stored procedure or function and avoid the need for temporary tables or table variables. All three of these methods require writes to tempdb and potentially incur additional execution time due to the IO overhead. You can instead use memory-optimized temporary tables, table variables, and table-valued parameters to take advantage of the same optimizations available for memory-optimized tables. By doing so, you can eliminate both the tempdb contention and the IO overhead. You can achieve faster execution time when using these objects in a natively compiled stored procedure.
ETL operations ETL operations typically require the use of staging tables to copy data from source systems as a starting point, and might also use staging tables for intermediate steps necessary to transform data prior to loading the processed data into a target table. Although this workload does not usually suffer from bottlenecks related to concurrency problems, it can experience delays due to IO operations and the overhead associated with query processing. For applications requiring low latency in the target database, consider using memory-optimized tables for faster, more efficient access to data. To reduce or eliminate IO, use non-durable tables as we describe in the next section.
Optimize performance of in-memory tables
As we described in the previous section, there are many use cases for which migrating disk-based tables to memory-optimized tables improves overall performance. However, to ensure you get optimal performance, there are several tasks that you can perform.
Before we look at these tasks, let’s start by creating the data directory on the root drive to hold a new database, and then enabling in-memory OLTP in a new database as shown in Listing 3-10. Enabling a database for memory-optimized tables requires you to define the filegroup by using the CONTAINS MEMORY_OPTIMIZED_DATA option. SQL Server uses this filegroup container to store checkpoint files necessary for recovering memory-optimized tables.
LISTING 3-10 Enable in-memory OLTP in a new database
CREATE DATABASE ExamBook762Ch3_IMOLTP ON PRIMARY ( NAME = ExamBook762Ch3_IMOLTP_data, FILENAME = 'C:\data\ExamBook762Ch3_IMOLTP.mdf', size=500MB ), FILEGROUP ExamBook762Ch3_IMOLTP_FG CONTAINS MEMORY_OPTIMIZED_DATA ( NAME = ExamBook762Ch3_IMOLTP_FG_Container, FILENAME = 'C:\data\ExamBook762Ch3_IMOLTP_FG_Container' ) LOG ON ( NAME = ExamBook762Ch3_IMOLTP_log, FILENAME = 'C:\data\ExamBook762Ch3_IMOLTP_log.ldf', size=500MB ); GO
Now let’s create the Examples schema, and then add one memory-optimized table and one disk-based table for comparison, as shown in Listing 3-11. Notice the addition of the MEMORY_OPTIMIZED = ON clause, which instructs the database engine to create a table dynamic link library (DLL) file and load the table into memory. The database engine also generates and compiles DML routines for accessing data in the table and saves the routines as DLLs, which are called when data manipulation is requested. Unless you specify otherwise, as we describe later in the “Durabiity options” section, a memory-optimized table is durable in which case it must have a primary key defined. Furthermore, it must also contain at least one index, which is satisfied below by the specification of NONCLUSTERED on the primary key column. We discuss indexing options in greater detail later in the “Indexes” section.
LISTING 3-11 Create a new schema and add tables to the memory-optimized database
USE ExamBook762Ch3_IMOLTP; GO CREATE SCHEMA Examples; GO CREATE TABLE Examples.Order_Disk ( OrderId INT NOT NULL PRIMARY KEY NONCLUSTERED, OrderDate DATETIME NOT NULL, CustomerCode NVARCHAR(5) NOT NULL ); GO CREATE TABLE Examples.Order_IM ( OrderID INT NOT NULL PRIMARY KEY NONCLUSTERED, OrderDate DATETIME NOT NULL, CustomerCode NVARCHAR(5) NOT NULL ) WITH (MEMORY_OPTIMIZED = ON); GO
Natively compiled stored procedures
The first optimization task to perform is to create a natively compile stored procedure. Natively compiled stored procedures are compiled at the time of creation, unlike interpreted stored procedures that compile at first execution. Furthermore, natively compiled stored procedures can access memory-optimized tables only. Native compilation translates the stored procedure code first into C code, and then into machine language, which enables the business logic to both execute and access data faster and more efficiently. The machine language version of the code is stored as a dynamic link library (DLL) in the default data directory for your SQL Server instance.
Many of the limitations that existed in SQL Server 2014 for natively compiled stored procedures have been removed. However, the following limitations still remain in SQL Server 2016:
tempdb access You cannot create or access temporary tables, table variables, or table-valued functions in tempdb. Instead, you can create a non-durable memory-optimized table (described later in this section) or memory-optimized table types or table variables.
Cursors As an alternative, you can use set-based logic or a WHILE loop.
CASE statement To work around lack of support for the CASE statement, you can use a table variable to store the result set. The table variable includes a column to serve as a condition flag that you can then use as a filter.
MERGE statement You cannot use a memory-optimized table as the target of a MERGE statement. Therefore, you must use explicit INSERT, UPDATE, or DELETE statements instead.
SELECT INTO clause You cannot use an INTO clause with a SELECT statement. As an alternative, use INSERT INTO <table> SELECT syntax.
FROM clause You cannot use a FROM clause or subqueries in an UPDATE statement.
PERCENT or WITH TIES in TOP clause There are no alternatives for using these options in a natively compiled stored procedure.
DISTINCT with aggregate functions There is no alternative for using this option in a natively compiled stored procedure.
Operators: INTERSECT, EXCEPT, APPLY, PIVOT, UNPIVOT, LIKE, CONTAINS There are no alternatives for using these operators in a natively compiled stored procedure.
Common table expressions (CTEs) You must rewrite your query to reproduce the functionality of a CTE in a natively compiled stored procedure.
Multi-row INSERT statements You must instead use separate INSERT statements in a natively compiled stored procedure.
EXECUTE WITH RECOMPILE There is no alternative for using this option in a natively compiled stored procedure.
View You cannot reference a view in a natively compiled stored procedure. You must define your desired SELECT statement explicitly in the procedure code.
To observe the performance difference between an interpreted stored procedure and a natively compiled stored procedure, create two stored procedures as shown in Listing 3-12. In this example, the following portions of the code are specific to native compilation:
WITH NATIVE_COMPILATION This clause is required to create a natively compiled stored procedure.
SCHEMABINDING This option is required to bind the natively compiled stored procedure to the object that it references. Consequently, you cannot drop tables referenced in the procedure code. Furthermore, you cannot use the wildcard (*) operator, and instead must reference column names explicitly in a SELECT statement.
BEGIN ATOMIC...END You use this option to create an atomic block, which is a block of T-SQL statements that succeed or fail together. A natively compiled stored procedure can have only one atomic block. Starting an atomic block creates a transaction if one does not yet exist or creates a savepoint if there is an existing transaction. An atomic block must include options defining the isolation level and language like this: WITH (TRANSACTION ISOLATION LEVEL = SNAPSHOT, LANGUAGE = N’English’).
LISTING 3-12 Create stored procedures to test execution performance
USE ExamBook762Ch3_IMOLTP; GO -- Create natively compiled stored procedure CREATE PROCEDURE Examples.OrderInsert_NC @OrderID INT, @CustomerCode NVARCHAR(10) WITH NATIVE_COMPILATION, SCHEMABINDING AS BEGIN ATOMIC WITH (TRANSACTION ISOLATION LEVEL = SNAPSHOT, LANGUAGE = N'English') DECLARE @OrderDate DATETIME = getdate(); INSERT INTO Examples.Order_IM (OrderId, OrderDate, CustomerCode) VALUES (@OrderID, @OrderDate, @CustomerCode); END; GO -- Create interpreted stored procedure CREATE PROCEDURE Examples.OrderInsert_Interpreted @OrderID INT, @CustomerCode NVARCHAR(10), @TargetTable NVARCHAR(20) AS DECLARE @OrderDate DATETIME = getdate(); DECLARE @SQLQuery NVARCHAR(MAX); SET @SQLQuery = 'INSERT INTO ' + @TargetTable + ' (OrderId, OrderDate, CustomerCode) VALUES (' + CAST(@OrderID AS NVARCHAR(6)) + ',''' + CONVERT(NVARCHAR(20), @OrderDate, 101)+ ''',''' + @CustomerCode + ''')'; EXEC (@SQLQuery); GO
Next, run the statements at least twice in Listing 3-13 to compare the performance of each type of stored procedure. Ignore the results from the first execution because the duration is skewed due to memory allocation and other operations that SQL Server performs one time only. The code in Listing 3-13 first inserts 100,000 rows into a disk-based table using an interpreted stored procedure and measures the time required to perform the INSERT operation. Then the code inserts rows into a memory-optimized table using the same interpreted stored procedure and measures the processing time. Last, the code deletes rows from the memory-optimized table, resets the time measurement variables, and then inserts rows into the table by using a natively compiled stored procedure.
LISTING 3-13 Execute each stored procedure to compare performance
SET STATISTICS TIME OFF; SET NOCOUNT ON; DECLARE @starttime DATETIME = sysdatetime(); DECLARE @timems INT; DECLARE @i INT = 1; DECLARE @rowcount INT = 100000; DECLARE @CustomerCode NVARCHAR(10); --Reset disk-based table TRUNCATE TABLE Examples.Order_Disk; -- Disk-based table and interpreted stored procedure BEGIN TRAN; WHILE @i <= @rowcount BEGIN; SET @CustomerCode = 'cust' + CAST(@i as NVARCHAR(6)); EXEC Examples.OrderInsert_Interpreted @i, @CustomerCode, 'Examples.Order_Disk'; SET @i += 1; END; COMMIT; SET @timems = datediff(ms, @starttime, sysdatetime()); SELECT 'Disk-based table and interpreted stored procedure: ' AS [Description], CAST(@timems AS NVARCHAR(10)) + ' ms' AS Duration; -- Memory-based table and interpreted stored procedure SET @i = 1; SET @starttime = sysdatetime(); BEGIN TRAN; WHILE @i <= @rowcount BEGIN; SET @CustomerCode = 'cust' + CAST(@i AS NVARCHAR(6)); EXEC Examples.OrderInsert_Interpreted @i, @CustomerCode, 'Examples.Order_IM'; SET @i += 1; END; COMMIT; SET @timems = datediff(ms, @starttime, sysdatetime()); SELECT 'Memory-optimized table and interpreted stored procedure: ' AS [Description], CAST(@timems AS NVARCHAR(10)) + ' ms' AS Duration; -- Reset memory-optimized table DELETE FROM Examples.Order_IM; SET @i = 1; SET @starttime = sysdatetime(); BEGIN TRAN; WHILE @i <= @rowcount BEGIN; SET @CustomerCode = 'cust' + CAST(@i AS NVARCHAR(6)); EXEC Examples.OrderInsert_NC @i, @CustomerCode; SET @i += 1; END; COMMIT; SET @timems = datediff(ms, @starttime, sysdatetime()); SELECT 'Memory-optimized table and natively compiled stored procedure:' AS [Description], CAST(@timems AS NVARCHAR(10)) + ' ms' AS Duration; GO
Your results vary from the results shown in the following example due to differences in hardware and memory configuration. However, your results should similarly reflect a variance in duration between the types of tables and stored procedures such that the memory-optimized table and natively compiled stored procedure performing inserts is considerably faster than the other two options:
Description Duration --------------------------------------------------- --------- Disk-based table and interpreted stored procedure: 10440 ms Description Duration --------------------------------------------------------- --------- Memory-optimized table and interpreted stored procedure: 10041 ms Description Duration --------------------------------------------------------------- --------- Memory-optimized table and natively compiled stored procedure: 1885 ms
A memory-optimized table can have up to eight non-clustered indexes, all of which are covering indexes. That is, they include all columns in the table. Unlike a traditional B-tree index for a disk-based table, an index for a memory-optimized table exists only in memory and does not contain data. Instead, an index points to a row in memory and is recreated during database recovery. In addition, updates to an indexed memory-optimized table do not get logged.
An index for a memory-optimized table can be one of the following three types:
Hash You use a nonclustered hash index when you have many queries that perform point lookups, also known as equi-joins. When you specify the index type, as shown below, you must include a bucket count. The bucket count value should be between one to two times the expected number of distinct values in the indexed column. It is better to have a bucket count that is too high rather than set it too low because it is more likely to retrieve data faster, although it consumes more memory.
CREATE TABLE Examples.Order_IM_Hash ( OrderID INT NOT NULL PRIMARY KEY NONCLUSTERED HASH WITH (BUCKET_COUNT = 1000000), OrderDate DATETIME NOT NULL, CustomerCode NVARCHAR(5) NOT NULL INDEX ix_CustomerCode HASH WITH (BUCKET_COUNT = 1000000) ) WITH (MEMORY_OPTIMIZED = ON);
Columnstore A new feature in SQL Server 2016 is the ability to add a columnstore index to a memory-optimized table. This type of index, which we cover in greater detail in Chapter 1, “Design and implement database objects,” is best when your queries perform large scans of a table because it can process data by using batch execution mode. Rather than read data row by row, it can process chunks of data in batches and thereby reduce query execution time and CPU utilization. Consider this type of index for single-threaded queries, sort operations (such as ORDER BY), and T-SQL window functions.
CREATE TABLE Examples.Order_IM_CCI ( OrderID INT NOT NULL PRIMARY KEY NONCLUSTERED, OrderDate DATETIME NOT NULL, CustomerCode NVARCHAR(5) NOT NULL, INDEX ix_CustomerCode_cci CLUSTERED COLUMNSTORE) WITH (MEMORY_OPTIMIZED = ON);
Nonclustered B-tree You use a memory-optimized nonclustered B-tree index when your queries have an ORDER BY clause on an indexed column, or when your queries return a few records by performing range selections against an index column using the greater than (>) or less than (<) operators, or testing an indexed column for inequality. You also can consider using a nonclustered index in combination with a columnstore index when your queries perform point lookups or need to join together two fact tables in a data warehouse.
CREATE TABLE Examples.Order_IM_NC ( OrderID INT NOT NULL PRIMARY KEY NONCLUSTERED, OrderDate DATETIME NOT NULL, CustomerCode NVARCHAR(5) NOT NULL INDEX ix_CustomerCode NONCLUSTERED ) WITH (MEMORY_OPTIMIZED = ON);
A new feature in SQL Server 2016 is the ability to add or drop indexes, or change the bucket count for an index in a memory-optimized table. To do this, you use the ALTER TABLE statement only, as shown in Listing 3-14. The CREATE INDEX, DROP INDEX, and ALTER INDEX statements are invalid for memory-optimized tables.
LISTING 3-14 Use the ALTER TABLE statement to add, modify, or drop an index
USE ExamBook762Ch3_IMOLTP; GO -- Add a column and an index ALTER TABLE Examples.Order_IM ADD Quantity INT NULL, INDEX ix_OrderDate(OrderDate); -- Alter an index by changing the bucket count ALTER TABLE Examples.Order_IM_Hash ALTER INDEX ix_CustomerCode REBUILD WITH ( BUCKET_COUNT = 2000000); -- Drop an index ALTER TABLE Examples.Order_IM DROP INDEX ix_OrderDate;
Offlload analytics to readable secondary
The ability to use both columnstore and nonclustered indexes in memory-optimized tables makes it much easier to support both OLTP and analytics workloads in the same database. However, sometimes analytics queries require considerable CPU, IO, and memory resources that might have an adverse impact on OLTP performance. If you need to support both OLTP and analytics workloads, consider an Always On configuration to offload analytics workloads to a readable secondary.
When you create a memory-optimized table, you must decide how SQL Server should manage durability of the data. You can choose one of the following two types:
Durable With this type, SQL Server guarantees full durability just as if the table were disk-based. If you do not specify the durability option explicitly when you create a memory-optimized table, it is durable by default. To explicitly define a durable table, use the SCHEMA_AND_DATA durability option like this:
CREATE TABLE Examples.Order_IM_Durable ( OrderID INT NOT NULL PRIMARY KEY NONCLUSTERED, OrderDate DATETIME NOT NULL, CustomerCode NVARCHAR(5) NOT NULL ) WITH (MEMORY_OPTIMIZED = ON, DURABILITY=SCHEMA_AND_DATA); GO
Non-durable By choosing this type of durability, you instruct SQL Server to persist only the table schema, but not the data. This option is most appropriate for use cases in which data is transient, such as an application’s session state management, or ETL staging. SQL Server never writes a non-durable table’s data changes to the transaction log. To define a non-durable table, use the SCHEMA_ONLY durability option like this:
CREATE TABLE Examples.Order_IM_Nondurable ( OrderID INT NOT NULL PRIMARY KEY NONCLUSTERED, OrderDate DATETIME NOT NULL, CustomerCode NVARCHAR(5) NOT NULL ) WITH (MEMORY_OPTIMIZED = ON, DURABILITY=SCHEMA_ONLY); GO
Because non-durable memory-optimized tables do not incur logging overhead, transactions writing to them run faster than write operations on durable tables. However, to optimize performance of durable memory-optimized tables, configure delayed durability at the database or transaction level. Just as with disk-based tables, delayed durability for a memory-optimized table reduces the frequency with which SQL Server flushes log records to disk and enables SQL Server to commit transactions before writing log records to disk.
If you set delayed durability at the database level, every transaction that commits on the database is delayed durable by default, although you can override this behavior at the transaction level. Similarly, if the database is durable, you can configure the database to allow delayed durable transactions and then explicit define a transaction as delayed durable. If you prefer, you can disable delayed durability and prevent delayed durable transactions entirely regardless of the transaction’s commit level. You can also specify delayed durability for a natively compiled stored procedure. Listing 3-15 includes examples of these various settings.
LISTING 3-15 Configure delayed durability
--Set at database level only, all transactions commit as delayed durable ALTER DATABASE ExamBook762Ch3_IMOLTP SET DELAYED_DURABILITY = FORCED; --Override database delayed durability at commit for durable transaction BEGIN TRANSACTION; INSERT INTO Examples.Order_IM_Hash (OrderId, OrderDate, CustomerCode) VALUES (1, getdate(), 'cust1'); COMMIT TRANSACTION WITH (DELAYED_DURABILITY = OFF); GO --Set at transaction level only ALTER DATABASE ExamBook762Ch3_IMOLTP SET DELAYED_DURABILITY = ALLOWED; BEGIN TRANSACTION; INSERT INTO Examples.Order_IM_Hash (OrderId, OrderDate, CustomerCode) VALUES (2, getdate(), 'cust2'); COMMIT TRANSACTION WITH (DELAYED_DURABILITY = ON); --Set within a natively compiled stored procedure CREATE PROCEDURE Examples.OrderInsert_NC_DD @OrderID INT, @CustomerCode NVARCHAR(10) WITH NATIVE_COMPILATION, SCHEMABINDING AS BEGIN ATOMIC WITH (DELAYED_DURABILITY = ON, TRANSACTION ISOLATION LEVEL = SNAPSHOT, LANGUAGE = N'English') DECLARE @OrderDate DATETIME = getdate(); INSERT INTO Examples.Order_IM (OrderId, OrderDate, CustomerCode) VALUES (@OrderID, @OrderDate, @CustomerCode); END; GO --Disable delayed durability completely for all transactions -- and natively compiled stored procedures ALTER DATABASE ExamBook762Ch3_IMOLTP SET DELAYED_DURABILITY = DISABLED;
Determine best case usage scenarios for natively compiled stored procedures
In SQL Server 2016, you can use natively compiled stored procedures to get better performance when operating on memory-optimized tables. You use them for:
Applications for which obtaining the best possible performance is a requirement
Queries that execute frequently
Tasks that must perform as fast as possible
If you have a lot of rows to process and a lot of logic to apply, the natively compiled stored procedure performs faster than an interpreted stored procedure. It is also good when you need to perform any of the following tasks:
Nested loop join
Multi-statement SELECT, INSERT, UPDATE, or DELETE operations
Procedural logic, such as conditional statements and loops
It is not typically the best option when you need to process only a single row.
Enable collection of execution statistics for natively compiled stored procedures
The goal of using memory-optimized tables is to execute processes as quickly as possible. Consequently, you could be surprised that some statistics, such as worker_time and elapsed_time, do not get collected by DMVs such as sys.dm_exec_query_stats and sys.dm_exec_procedure_stats. In fact, these DMVs include no information about natively compiled stored procedures.
Instead, you need to specifically enable the collection of execution statistics by using one of the following system stored procedures:
sys.sp_xtp_control_proc_exec_stats Use this system stored procedure to enable statistics collection for your SQL Server instance at the procedure level.
sys.sp_xtp_control_query_exec_stats Use this system stored procedure to enable statistics collection at the query level for selected natively compiled stored procedures.
Use the sys.sp_xtp_control_proc_exec_stats system stored procedure to enable and disable procedure-level statistics collection on your SQL Server instance, as shown in Listing 3-16. When SQL Server or a database starts, statistics collection is automatically disabled. Note that you must be a member of the sysadmin role to execute this stored procedure.
LISTING 3-16 Enable and disable statistics collection at the procedure level
--Enable statistics collection at the procedure level EXEC sys.sp_xtp_control_proc_exec_stats @new_collection_value = 1; --Check the current status of procedure-level statistics collection DECLARE @c BIT; EXEC sys.sp_xtp_control_proc_exec_stats @old_collection_value=@c output SELECT @c AS 'Current collection status'; --Disable statistics collection at the procedure level EXEC sys.sp_xtp_control_proc_exec_stats @new_collection_value = 0;
Listing 3-17 shows an example of using the sys.sp_xtp_control_query_exec_stats system procedure to enable and disable query-level statistics collection. You can even use it to enable statistics collection for a specific natively compiled stored procedure, but it must have been executed at least once before you enable statistics collection. When SQL Server starts, query-level statistics collection is automatically disabled. Note that disabling statistics collection at the procedure level does not disable any statistics collection that you have configured at the query level. As with the previous system stored procedure, you must be a member of the sysadmin role to execute sys.sp_xtp_control_query_exec_stats.
LISTING 3-17 Enable and disable statistics collection at the query level
--Enable statistics collection at the query level EXEC sys.sp_xtp_control_query_exec_stats @new_collection_value = 1; --Check the current status of query-level statistics collection DECLARE @c BIT; EXEC sys.sp_xtp_control_query_exec_stats @old_collection_value=@c output; SELECT @c AS 'Current collection status'; --Disable statistics collection at the query level EXEC sys.sp_xtp_control_query_exec_stats @new_collection_value = 0; --Enable statistics collection at the query level for a specific --natively compiled stored procedure DECLARE @ncspid int; DECLARE @dbid int; SET @ncspid = OBJECT_ID(N'Examples.OrderInsert_NC'); SET @dbid = DB_ID(N'ExamBook762Ch3_IMOLTP') EXEC [sys].[sp_xtp_control_query_exec_stats] @new_collection_value = 1, @database_id = @dbid, @xtp_object_id = @ncspid; --Check the current status of query-level statistics collection for a specific --natively compiled stored procedure DECLARE @c bit; DECLARE @ncspid int; DECLARE @dbid int; SET @ncspid = OBJECT_ID(N'Examples.OrderInsert_NC'); SET @dbid = DB_ID(N'ExamBook762Ch3_IMOLTP') EXEC sp_xtp_control_query_exec_stats @database_id = @dbid, @xtp_object_id = @ncspid, @old_collection_value=@c output; SELECT @c AS 'Current collection status'; --Disable statistics collection at the query level for a specific --natively compiled stored procedure DECLARE @ncspid int; DECLARE @dbid int; EXEC sys.sp_xtp_control_query_exec_stats @new_collection_value = 0, @database_id = @dbid, @xtp_object_id = @ncspid;
Statistics collection queries
After enabling statistics collections at the procedure level, you can query the sys.dm_exec_procedure_stats DMV to review the results. Listing 3-19 illustrates an example query that filters for natively compiled stored procedures. This query returns results for the time during which statistics collection was enabled and remains available after you disable statistics collection at the procedure level.
LISTING 3-18 Get procedure-level statistics
SELECT OBJECT_NAME(PS.object_id) AS obj_name, cached_time as cached_tm, last_execution_time as last_exec_tm, execution_count as ex_cnt, total_worker_time as wrkr_tm, total_elapsed_time as elpsd_tm FROM sys.dm_exec_procedure_stats PS INNER JOIN sys.all_sql_modules SM ON SM.object_id = PS.object_id WHERE SM.uses_native_compilation = 1;
Here is an example of the results from the query in Listing 3-18:
obj_name cached_tm last_exec_tm ex_cnt wrkr_tm elpsd_tm --------- ----------------------- ----------------------- ------ -------- -------- OrderInsert_NC 2016-10-15 20:44:33.917 2016-10-15 20:44:35.273 100000 376987 383365
You can also review the statistics collection at the query level by executing a query against the sys.dm_exec_query_stats DMV, as shown in Listing 3-19.
LISTING 3-19 Get query-level statistics
SELECT st.objectid as obj_id, OBJECT_NAME(st.objectid) AS obj_nm, SUBSTRING(st.text, (QS.statement_start_offset / 2 ) + 1, ((QS.statement_end_offset - QS.statement_start_offset) / 2) + 1) AS 'Query', QS.last_execution_time as last_exec_tm, QS.execution_count as ex_cnt FROM sys.dm_exec_query_stats QS CROSS APPLY sys.dm_exec_sql_text(sql_handle) st INNER JOIN sys.all_sql_modules SM ON SM.object_id = st.objectid WHERE SM.uses_native_compilation = 1
The information available in the query results from Listing 3-19 is similar to the procedure-level statistics, but includes a row for each statement in the natively compiled stored procedure and includes the query text for each statement. Note that total_worker_time and total_elapsed_time were excluded from this example to restrict the width of the query results.
obj_id obj_name Query last_exec_tm ex_cnt ------- -------------- ---------------------- ----------------------- ------- 981578535 OrderInsert_NC INSERT INTO 2016-10-15 21:09:25.877 100000 Examples.Order_IM (OrderId, OrderDate, CustomerCode) VALUES (@OrderID, @OrderDate, @CustomerCode)
Transaction management is the key to the SQL Server support of ACID. ACID properties determine whether a set of statements are handled individually or as an indivisible unit of work, whether a transaction violates database rules, whether one transaction can see the effects of other transactions, and whether a statement persists after an unexpected shutdown.
SQL Server guarantees ACID by managing the effects of a transaction’s success or failure through committing or rolling back a transaction, using a default isolation to prevent changes made by one transaction from impacting other transactions, and relying on a transaction log for durability.
Implicit transactions start automatically for specific DML statements, but require an explicit COMMIT TRANSACTION or ROLLBACK TRANSACTION statement to end. Before using implicit transactions, you must enable the implicit transaction mode.
Explicit transactions require a BEGIN TRANSACTION statement to start and a COMMIT TRANSACTION or ROLLBACK TRANSACTION to end. You should incorporate error handling and include logic to avoid nesting transactions for more complete control over transaction behavior.
Savepoints allow you to partially rollback a transaction to a named location. Neither the SAVE TRANSACTION nor the ROLLBACK TRANSACTION statements have an effect on the @@TRANCOUNT variable (as long as the transaction rolls back to a specific savepoint rather than completely).
A high concurrency database can suffer from data integrity issues when a process attempts to modify data while other simultaneous processes are trying to read or modify the data. Potential side effects include dirty reads, non-repeatable reads, phantom reads, and lost updates.
SQL Server uses resource locks to enable high-concurrency while maintaining ACID properties for a transaction. SQL Server uses a lock hierarchy on resources to protect transactions and the types of locks that SQL Server can acquire on resources. SQL Server’s response to a request for a new lock when a lock already exists depends on the compatibility between the requested and existing lock modes.
SQL Server uses isolation levels to control the degree to which one transaction has visibility into the changes made by other transactions. Each of the following isolation levels has potential side effects on data integrity and on concurrency: READ COMMITTED, READ UNCOMMITTED, REPEATABLE READ, SERIALIZABLE, SNAPSHOT, and READ_COMMITTED_SNAPSHOT.
You can change the isolation level at the session level by using the SET TRANSACTION ISOLATION LEVEL statement or at statement level by using a table hint to raise concurrency at the risk of introducing potential side effects.
Because SQL Server acquires different types of locks for each isolation level, raising or lowering isolation levels have varying effects on transaction performance.
The SNAPSHOT and READ_COMMITTED_SNAPSHOT isolation levels both create copies of data and require more CPU and memory than other isolation levels. In addition, they both require adequate space in tempdb, although of the two isolation levels, READ_COMMITTED_SNAPSHOT requires less space.
Use the system DMVs sys.dm_tran_locks and sys.dm_os_wait_stats to find locked resources, understand why they are locked, and identify the lock mode acquired for the locked resources.
SQL Server uses lock escalation to more effectively manage locks, but as a result can result in more blocking of transactions. Use the sys.dm_os_wait_stats DMV to monitor lock escalation events and look for ways to tune queries if performance begins to degrade due to more blocking issues.
A deadlock graph provides you with insight into the objects involved in a deadlock and identifies the terminated process. You can capture a deadlock graph by using either SQL Server Profiler to later review deadlock events that have yet to occur or by using Extended Events to review deadlock events that have already occurred.
Enclosing a transaction in a TRY/CATCH block to retry it is usually the best way to resolve a deadlock. Alternative methods have varying trade-offs and include using the SNAPSHOT or READ_COMMITTED_SNAPSHOT isolation levels, using the NOLOCK, HOLDLOCK, or UPDLOCK query hints, or adding a new covering nonclustered index.
Memory-optimized tables are well-suited for specific OLTP scenarios: high data ingestion rate; high volume, high performance data reads; complex business logic in stored procedures; real-time data access; session state management; applications relying heavily on temporary tables, table variables, and table-valued parameters; and ETL operations.
Besides implementing memory-optimized tables to improve an application’s performance, you can also consider the following techniques to optimize performance even more: natively compiled stored procedures, the addition of indexes to the memory-optimized tables, the use of a readable secondary in an Always On configuration to which you can offload analytics workloads, non-durable tables, or delayed durability for transactions.
Natively compiled stored procedures typically execute faster and are best suited for applications requiring high performance, queries that execute frequently, and tasks that must perform extremely fast. You experience better performance gains over an interpreted stored procedure when a natively compiled stored procedure must process many rows of data and apply complex logic.
Use the system stored procedures sys.sp_xtp_control_proc_exec_stats and sys.sp_xtp_control_query_exec_stats to enable or disable the collection of execution statistics for natively compiled stored procedures at the procedure level or query level, respectively. After enabling statistics collection, use the sys.dm_exec_procedure_stats and sys.dm_exec_query_stats DMVs to review the statistics.