Determining when a query was last run

Running the T-SQL script given will identify the top 20 times a given query was run, as ordered by last_execution_time. You can identify the relevant query by supplying part of it as a filter via the WHERE clause.

For example, determine when a specific stored procedure was last time changed:

SELECT DISTINCT TOP 20
        DB_NAME(QT.dbid) AS DatabaseName
        , QT.text as sql_statement
        , QS.last_execution_time
FROM
        sys.dm_exec_query_stats QS
        CROSS APPLY sys.dm_exec_sql_text(QS.sql_handle) as QT
WHERE
        QT.dbid = DB_ID()               -- limit search on current database
        AND QT.text LIKE '%CREATE PROCEDURE%GetEmployees%'
ORDER BY QS.last_execution_time DESC;

Finding the queries that use the most I/O

Running the SQL script given in the following listing will identify the top 20 queries that use the most I/O.

SELECT TOP 20
        DB_NAME(QT.dbid) as database_name
        , QS.total_logical_reads + QS.total_logical_writes as total_io
        , QS.execution_count   
        , SUBSTRING(QT.text, (qs.statement_start_offset/2)+1,
                ((      CASE qs.statement_end_offset
                                WHEN -1 THEN DATALENGTH(QT.text)
                                ELSE qs.statement_end_offset
                        END - QS.statement_start_offset)/2) + 1) as statement_text
        , QT.text as parent_statement_text     
        , O.name
        , O.type_desc
        , QP.query_plan
FROM
        sys.dm_exec_query_stats QS
        CROSS APPLY sys.dm_exec_sql_text(QS.sql_handle) QT
        CROSS APPLY sys.dm_exec_query_plan(QS.plan_handle) QP
        LEFT JOIN sys.objects O ON O.object_id = QT.objectid
WHERE QT.dbid = DB_ID()
ORDER BY total_io DESC

Find missing indexes using DMVs

The following query determines which are the missing indexes and displays their column details based on dm_db_missing_index_group_stats DMV.

USE <DatabaseName>;

SELECT 
        REPLACE(ID.[Statement], '[' + DB_NAME(ID.database_id) + '].', '') as table_name,
        CAST(S.avg_total_user_cost * S.avg_user_impact * S.user_seeks as INT) as [score],
        ID.equality_columns, ID.inequality_columns, ID.included_columns,               
        'CREATE INDEX '
        + 'IX_' + OBJECT_NAME(ID.object_id) + '_' + CAST(ID.index_handle as VARCHAR)
        + ' ON ' + REPLACE(ID.[Statement], '[' + DB_NAME(ID.database_id) + '].', '')
        + ' (' + ISNULL (ID.equality_columns,'')
        + CASE
                WHEN ID.equality_columns IS NOT NULL AND ID.inequality_columns IS NOT NULL
                THEN ', ' + ID.inequality_columns
                ELSE ''
          END
        + ')'
        + ISNULL (' INCLUDE (' + ID.included_columns + ')', '') AS create_index_statement
FROM
        sys.dm_db_missing_index_details ID
        INNER JOIN sys.dm_db_missing_index_groups G
                ON ID.index_handle = G.index_handle
        INNER JOIN sys.dm_db_missing_index_group_stats S
                ON G.index_group_handle = S.group_handle
WHERE
        ID.database_id = DB_ID()
        AND OBJECTPROPERTY(ID.[object_id], 'IsMsShipped') = 0  
ORDER BY score DESC;

Performance Monitor counters to analyze disk bottleneck

Counter Description Expected Value
% Disk Time Percentage of time disk was busy with read/write activities. Average value < 85%
Current Disk Queue Length Number of outstanding disk requests at the time performance data is collected Average value < 2 per disk
Avg. Disk Queue Length Average number of queued disk requests during the sample interval Average value < 2 per disk
Disk Transfers/sec Rate of read/write operations on disk Maximum value < 400 per disk
Disk Bytes/sec The rate at which bytes are transferred to or from the disk during read or write operations.
If the amount of data transfer exceeds the capacity of the disk subsystem, then a backlog starts developing on the disk subsystem, as reflected by the Disk Queue Length counter.
Maximum value < 800MB per second
Avg. Disk Sec/Read Average time in ms to read from disk Average value < 10 ms
Avg. Disk Sec/Write Average time in ms to write to disk Average value < 10 ms

Performance Monitor counters to analyze memory pressure

Counter Description Expected Value
Memory
Available Bytes Free physical memory Should not be too low
Pages/sec Rate of hard page faults.

A page fault occurs when a process requires data that is not in its space in physical memory. If the faulted page is found elsewhere in physical memory, then it is called a soft page fault. A hard page fault occurs when a process requires data that must be retrieved from disk.
The speed of a disk access is in the order of milliseconds, whereas a memory access is in the order of nanoseconds.

Average Value < 50
Page Faults/sec Rate of total page faults (soft page faults plus hard page faults) Compare with its baseline value for trend analysis
SQLServer:Buffer Manager
Buffer cache hit ratio Percentage of requests served out of buffer cache.
A low value indicates that few requests could be served out of the buffer cache, with the rest of the requests being served from disk.
Average Value >= 90% in an OLTP system.
Page Life Expectancy Time page spends in buffer cache without being referenced. A low value means that pages are being removed from the buffer, lowering the efficiency of the cache and indicating the possibility of memory pressure.
Checkpoint Pages/sec Pages written to disk by a checkpoint operation.
A dirty page is one that is modified while in the buffer. When it’s modified, it’s marked as dirty and will get written back to the disk during the next checkpoint.
Average Value < 30
Lazy writes/sec Dirty aged pages flushed from buffer. Average Value < 20
SQLServer:Memory Manager
Memory Grants Pending Number of processes waiting for memory grant. If this counter value is high, then SQL Server is short of memory.
See sys.dm_ exec_query_memory_grants dmv.
Average value = 0.
Target Server Memory (KB) Indicates the total amount of dynamic memory SQL Server is willing to consume. Close to size of physical Memory
Total Server Memory (KB) Amount of physical memory currently assigned to SQL. Close to Target server Memory (KB)
Process
Private Bytes Size, in bytes, of memory that this process has allocated that can’t be shared with other processes

When you get out-of-memory issues, they are recorded in sys.dm_os_ring_buffers dmv:

SELECT *
FROM sys.dm_os_ring_buffers
WHERE ring_buffer_type='RING_BUFFER_OOM';

Get database restore history

Returns last 10 restores performed on your server:

SELECT TOP 10
        RH.destination_database_name AS [Database],
        RH.user_name AS [Restored By],
        RH.restore_date AS [Restore Started],
        BMF.physical_device_name AS [Restored From],
        RF.destination_phys_name AS [Restored To],
        RH.*
FROM
        msdb.dbo.restorehistory RH
        INNER JOIN msdb.dbo.backupset BS ON RH.backup_set_id = BS.backup_set_id
        INNER JOIN msdb.dbo.restorefile RF ON RH.restore_history_id = RF.restore_history_id
        INNER JOIN msdb.dbo.backupmediafamily BMF ON BMF.media_set_id = BS.media_set_id
--WHERE destination_database_name = '<DatabaseName>'
ORDER BY RH.restore_history_id DESC
GO

Execute a SSIS package using T-SQL

DECLARE @execution_id BIGINT;
DECLARE @use32bitruntime BIT = CAST(0 AS BIT);

-- A new execution operation is created.
EXEC catalog.create_execution
@folder_name = N'<FolderName>',
@project_name = N'<ProjectName>',
@package_name = N'<PackageName.dtsx>',
@use32bitruntime = @use32bitruntime,
@reference_id = NULL,
@execution_id = @execution_id OUTPUT;

-- Set execution properties
EXEC catalog.set_execution_parameter_value
@execution_id,
@object_type = 50,
@parameter_name = N'LOGGING_LEVEL',
@parameter_value = 1;

-- Execution is started asynchronous
EXEC catalog.start_execution @execution_id;
GO

Support for simplified pagination using OFFSET-FETCH filter

Starting with SQL Server 2012.

SELECT orderId, orderDate, customerId, empId
FROM sales.orders
ORDER BY orderDate
OFFSET 50 ROWS FETCH NEXT 25 ROWS ONLY;

Improve performance using filtered indexes

A filtered index is a non-clustered index that contains only a subset of the number of rows contained in a table. You add a WHERE clause to reduce the number of rows that are stored at the leaf level.

CREATE NONCLUSTERED INDEX <Index_Name>
ON <Table Name>(<Column(s)>)
WHERE <Filter Condition>

A good real example it is use is on Status column, as you might have a large number of rows with Closed status that are not part of your current searches (the business wants to query for Open, Processing, Invoicing, etc)

CREATE NONCLUSTERED INDEX [IX_Orders_IdStatus_Filterered]
ON [dbo].[Orders] ([IdStatus])
WHERE ([IdStatus] IN ('O', 'P', 'I'))

Another well-suited scenarios is where you frequently have to filter out NULL values.

Pros:

  • By having fewer rows in an index, less I/O is done when that index is used.
  • Index size is dramatically reduced.
  • The statistics on these filtered indexes are filtered as well, which typically results in them being more accurate.
  • Cons:

  • Use certain expressions, such as BETWEEN, NOT IN, or a CASE statement.
  • Parameterized queries doesn't take advantage of filtered indexes.
  • Index design recommendation - how to choose optimal indexes

    Indexes should be considered on all columns that are frequently accessed by the WHERE, ORDER BY, GROUP BY, TOP, and DISTINCT clauses.

    Clustered index recommendations
    - It is highly recommended that every table should have at least a clustered index. However, there is also a very interesting topic to be taken into account on use-the-index-luke.com.
    - Consider having a clustered indexes when the data retrieval needs to be sorted.
    - Avoid creating a clustered index on columns that are highly updatable as row locator of all the non clustered indexes will be updated accordingly.
    - Create the clustered index first. In case the clustered index is created later => all non clustered indexes need to be rebuilt.
    - Keep clustered indexes narrow as this directly impacts index size on disk. This is very important as all non clustered indexes store the clustered keys as their row locator, or simpler said:
    clustered index row width = non clustered index column width + clustered index column width

    Nonclustered Index Recommendations
    - A nonclustered index is most useful when all you want to do is retrieve a small number of rows and columns from a large table.
    - Consider using INCLUDE clause in order to create covering indexes.
    - To improve the performance of a query, SQL Server can use multiple indexes on a table. Therefore, instead of creating wide index keys, consider creating multiple narrow indexes.
    - Foreign keys columns are good index candidates.

    Disk consideration
    - Place the table and index on separate disks.
    - Take into account index compression i.e. fewer pages and fewer index levels are needed to store the index.

    Column order matters in a composite index
    Using the most selective column first will help filter the index rows more efficiently.

    Avoid indexes on columns with lower selectivity
    If selectivity ratio > 0.85 => a table scan will be preferred.

    Take into account column uniqueness
    Creating an index on columns with a very low range of possible unique values (such as gender) will not benefit performance, because the query optimizer will not be able to use the index to effectively narrow down the rows to be returned.

    The data type of an index matters
    Unless they are absolutely necessary, minimize the use of wide data type columns with large sizes in an index. A large index key size increases the number of index pages, thereby increasing the amount of memory and disk activities required for the index.

    Indexed or materialized views
    - A database view can be materialized on the disk by creating a unique clustered index on the view. After a unique clustered index is created on the view, the view’s result set is materialized immediately and persisted in physical storage in the database, saving the overhead of performing costly operations during query execution.
    - Aggregations can be precomputed and persisted in the indexed view to minimize expensive computations during query execution.
    - Available only in Enterprise Edition.

    Other aspects to be taken into account
    - Considering using the Database Engine Tuning Advisor tool provided by SQL Server that helps you determine the correct indexes in a database for a given SQL workload ( a trace file or a table or, new with SQL Server 2012, you can use the queries that exist in the plan cache).
    - ColumnStore indexes. Used to index information by columns rather than by rows.
    - Spatial indexes.
    - XML indexes.
    - Index computed columns.
    - Consider using filtered indexes.
    - Resolve Key or RID lookups by using a clustered index or by using a covering index or by using a index join (i.e. an index intersection between two or more indexes to fully cover a query).

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