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Simple SQL Server

I started out working with Microsoft Access and SQL Server back in 2000 as the only employee doing IT full-time, and worked most of my career where “big fish in a little pond” was an overstatement. Learning is scarce when you do everything and don’t work with anyone who knows more than you. In 2010 I was plunged into the Ocean and grabbed onto anything I could find to stay afloat. I wasn’t going to simply run scripts I didn’t understand, so I learned the DMVs and system tables in the scripts I found and rewrote them all. Now, I know enough where I can start giving back to a community that saved me from drowning.

Query Stats

DBAs rarely use the full potential of sys.dm_exec_query_stats.  It’s common to see the queries for looking at the most expensive queries according to any of the stats contained within the current cache, which is great to see.  However, if you grab snapshots of this information the proper way then you can get more detail that will really make a difference in your job.  Capturing this every 4 hours is probably enough, and here’s what it’s going to do for you.

First, using it the way most DBAs do and look at the most expensive queries according to a specific metric.  It works the same for them all, so we’ll just say we’re interested in physical reads in this case because our PLE isn’t what we’re hoping it could be.  You’ve probably seen the queries that directly query sys.dm_exec_query_stats to give you this similar to the one on MSDN for this DMV.  That will give you everything that’s in the cache right now, which could contain queries that recompile every hour in combination with other queries that have remained in the cache since the server was rebooted two weeks ago.  This means that when you’re querying to see what you can do to improve your PLE during business hours you’re getting anything that happens to be there now, not what ran in the time window you’re looking to improve.

Now look at it from the point of view that you capture this information every 4 hours.  You can get the total number of physical reads grouped by sql_handle where the interval_end time is in the last month, on weekdays, between 9:00 AM and 5:00 PM.  This is both a more complete and more accurate picture of what you care about, and you’re more likely to be focusing on the queries you should be tuning from the start.

Once you do start tuning, you want to know what effect you really had, and not just rely on the couple sets of test parameters you used in your NonProd environments.  A couple days after your changes made it to production, it’s a good idea to make sure you had the desired effect.  Query the data you have saved off for the last three days and for the same days last week.

What I went over so far is just realizing how this takes the traditional use of this to a new level, but there’s more functionality here now that it’s being saved off.  Now this is extremely useful in incident response.  Before you could see if anything was hogging resources as seen in my Recently Recompiled Resource Hogs post, but now you take it a step further by being able to verify if what’s running long now has always run that way or if something is different.  Also, with having the plan_handles saved off you can even see if it’s getting a different execution plan, which will help you determine if this was a change in data or if it’s a bad execution plan.

Not to shoot myself in the foot, but this isn’t perfect.  If you capture this data every 4 hours starting at 12:00 and it recompiles at 3:59 then you’ll only capture one minute for that interval.  Also, if you’re dealing with a query that can’t be cached then it won’t be found here at all; this is why you’re also running traces to find the big queries and watching sys.dm_os_performance_counters to know when there are a lot of compiles and recompiles.

To start capturing this, here are the tables you’ll need to capture everything.

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

SET ANSI_PADDING ON
GO

/*
IF OBJECT_ID('QueryStats') IS NOT NULL BEGIN
    DROP TABLE QueryStats
END

IF OBJECT_ID('QueryStatsLast') IS NOT NULL BEGIN
    DROP TABLE QueryStatsLast
END

IF OBJECT_ID('QueryText') IS NOT NULL BEGIN
    DROP TABLE QueryText
END
--*/

CREATE TABLE [dbo].[QueryStats](
    [interval_start] [smalldatetime] NOT NULL,
    [interval_end] [smalldatetime] NOT NULL,
    [sql_handle] [varbinary](64) NOT NULL,
    [plan_handle] [varbinary](64) NOT NULL,
    [statement_start_offset] [int] NOT NULL,
    [statement_end_offset] [int] NOT NULL,
    [execution_count] [int] NOT NULL,
    [total_elapsed_time_ms] [int] NOT NULL,
    [min_elapsed_time_ms] [int] NOT NULL,
    [max_elapsed_time_ms] [int] NOT NULL,
    [total_worker_time_ms] [int] NOT NULL,
    [min_worker_time_ms] [int] NOT NULL,
    [max_worker_time_ms] [int] NOT NULL,
    [total_logical_reads] [int] NOT NULL,
    [min_logical_reads] [int] NOT NULL,
    [max_logical_reads] [int] NOT NULL,
    [total_physical_reads] [int] NOT NULL,
    [min_physical_reads] [int] NOT NULL,
    [max_physical_reads] [int] NOT NULL,
    [total_logical_writes] [int] NOT NULL,
    [min_logical_writes] [int] NOT NULL,
    [max_logical_writes] [int] NOT NULL,
    [creation_time] [smalldatetime] NOT NULL,
    [last_execution_time] [smalldatetime] NOT NULL,
) ON [PRIMARY]
GO

--Wide is bad, if you have a lot of nonclustered indexes.  
--This is a high-writes, low-reads table, so a single clustered index is probably all I want.
--With Interval_End being the first key field, they're inserted in order and eliminate mass fragmentation
  -- as well as bad write performance.
CREATE UNIQUE CLUSTERED INDEX IX_QueryStats_intervalend_sqlhandle_statementstartoffset_planhandle_U_C ON QueryStats
(
    interval_end
    , sql_handle
    , statement_start_offset
    , plan_handle
)

CREATE TABLE [dbo].[QueryStatsLast](
    [sql_handle] [varbinary](64) NOT NULL,
    [plan_handle] [varbinary](64) NOT NULL,
    [statement_start_offset] [int] NOT NULL,
    [statement_end_offset] [int] NOT NULL,
    [objtype] [nvarchar](20) NOT NULL,
    [execution_count] [bigint] NOT NULL,
    [total_elapsed_time_ms] [bigint] NOT NULL,
    [min_elapsed_time_ms] [bigint] NOT NULL,
    [max_elapsed_time_ms] [bigint] NOT NULL,
    [total_worker_time_ms] [bigint] NOT NULL,
    [min_worker_time_ms] [bigint] NOT NULL,
    [max_worker_time_ms] [bigint] NOT NULL,
    [total_logical_reads] [bigint] NOT NULL,
    [min_logical_reads] [bigint] NOT NULL,
    [max_logical_reads] [bigint] NOT NULL,
    [total_physical_reads] [bigint] NOT NULL,
    [min_physical_reads] [bigint] NOT NULL,
    [max_physical_reads] [bigint] NOT NULL,
    [total_logical_writes] [bigint] NOT NULL,
    [min_logical_writes] [bigint] NOT NULL,
    [max_logical_writes] [bigint] NOT NULL,
    [creation_time] [datetime] NOT NULL,
    [last_execution_time] [datetime] NOT NULL,
    [DateAdded] [datetime] NOT NULL
) ON [PRIMARY]

CREATE UNIQUE CLUSTERED INDEX [IX_QueryStatsLast_sqlhandle_planhandle_statementstartoffset_U_C] ON [dbo].[QueryStatsLast] 
(
    [sql_handle] ASC,
    [plan_handle] ASC,
    [statement_start_offset] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
GO

CREATE TABLE [dbo].[QueryText](
    [sql_handle] [varbinary](64) NOT NULL,
    [QueryText] [nvarchar](max) NOT NULL,
    [DatabaseName] [nvarchar](128) NULL,
    [objtype] [nvarchar](20) NULL
) ON [PRIMARY]

CREATE UNIQUE CLUSTERED INDEX [IX_QueryText_sqlhandle_U_C] ON [dbo].[QueryText] 
(
    [sql_handle] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
GO

Now that the tables are created, here’s how I populate them.  Set this up to run once every 4 hours to keep the data small during typical running, and kick it up to once an hour or more if needed to pinpoint an issue if you need it.

DECLARE @interval_start smalldatetime
    , @interval_end smalldatetime

IF OBJECT_ID('tempdb..#QS') IS NOT NULL BEGIN
    DROP TABLE #QS 
END 

CREATE TABLE [dbo].[#QS](
    [sql_handle] [varbinary](64) NOT NULL,
    [plan_handle] [varbinary](64) NOT NULL,
    [statement_start_offset] [int] NOT NULL,
    [statement_end_offset] [int] NOT NULL,
    [objtype] [nvarchar](20) NOT NULL,
    [execution_count] [bigint] NOT NULL,
    [total_elapsed_time_ms] [bigint] NOT NULL,
    [min_elapsed_time_ms] [bigint] NOT NULL,
    [max_elapsed_time_ms] [bigint] NOT NULL,
    [total_worker_time_ms] [bigint] NOT NULL,
    [min_worker_time_ms] [bigint] NOT NULL,
    [max_worker_time_ms] [bigint] NOT NULL,
    [total_logical_reads] [bigint] NOT NULL,
    [min_logical_reads] [bigint] NOT NULL,
    [max_logical_reads] [bigint] NOT NULL,
    [total_physical_reads] [bigint] NOT NULL,
    [min_physical_reads] [bigint] NOT NULL,
    [max_physical_reads] [bigint] NOT NULL,
    [total_logical_writes] [bigint] NOT NULL,
    [min_logical_writes] [bigint] NOT NULL,
    [max_logical_writes] [bigint] NOT NULL,
    [creation_time] [datetime] NOT NULL,
    [last_execution_time] [datetime] NOT NULL,
    [DateAdded] [datetime] NOT NULL
) 
/*
--This sounded like a great idea, but it just slowed it down several seconds.
CREATE UNIQUE CLUSTERED INDEX TempQS_Cluster ON #QS  
(
    sql_handle
    , plan_handle
    , statement_start_offset
)
*/

--The Cached Plans Object Type is in here in case you want to treat ad-hoc or prepared statements differently
INSERT INTO #QS 
SELECT qs.sql_handle
    , qs.plan_handle
    , qs.statement_start_offset
    , qs.statement_end_offset
    , cp.objtype
    , qs.execution_count 
    , total_elapsed_time_ms = qs.total_elapsed_time/1000
    , min_elapsed_time_ms = qs.min_elapsed_time/1000
    , max_elapsed_time_ms = qs.max_elapsed_time/1000 
    , total_worker_time_ms = qs.total_worker_time/1000 
    , min_worker_time_ms = qs.min_worker_time/1000
    , max_worker_time_ms = qs.max_worker_time/1000
    , qs.total_logical_reads
    , qs.min_logical_reads
    , qs.max_logical_reads
    , qs.total_physical_reads
    , qs.min_physical_reads 
    , qs.max_physical_reads
    , qs.total_logical_writes 
    , qs.min_logical_writes 
    , qs.max_logical_writes
    , qs.creation_time
    , qs.last_execution_time
    , DateAdded = getDate()
FROM sys.dm_exec_query_stats AS qs
    INNER JOIN sys.dm_exec_cached_plans cp on qs.plan_handle = cp.plan_handle
--WHERE cp.objtype NOT IN ('Adhoc')

INSERT INTO QueryText (sql_handle, QueryText, DatabaseName, objtype)
SELECT QS.sql_handle
    , QueryText = qt.text
    , DatabaseName = DB_NAME(max(qt.dbid))
    , max(QS.objtype)
FROM (SELECT #QS.sql_handle
        , #QS.objtype  
        FROM #QS
            LEFT JOIN QueryText QST ON #QS.sql_handle = QST.sql_handle
        WHERE QST.sql_handle IS NULL) QS
    CROSS APPLY sys.dm_exec_sql_text(QS.sql_handle) qt
GROUP BY QS.sql_handle
    , qt.text

--All the values are the same in each table
SELECT TOP 1 @interval_start = dateadded FROM QueryStatsLast
SELECT TOP 1 @interval_end = dateadded FROM #QS

IF @interval_start IS NULL BEGIN
    SELECT @interval_start = create_date
    FROM sys.databases
    WHERE name = 'tempdb'
END

INSERT INTO QueryStats (interval_start
    , interval_end
    , sql_handle
    , plan_handle
    , statement_start_offset
    , statement_end_offset
    , execution_count 
    , total_elapsed_time_ms
    , min_elapsed_time_ms 
    , max_elapsed_time_ms
    , total_worker_time_ms  
    , min_worker_time_ms 
    , max_worker_time_ms 
    , total_logical_reads
    , min_logical_reads
    , max_logical_reads
    , total_physical_reads
    , min_physical_reads 
    , max_physical_reads
    , total_logical_writes 
    , min_logical_writes 
    , max_logical_writes
    , creation_time
    , last_execution_time)
SELECT @interval_start
    , @interval_end
    , qs.sql_handle
    , qs.plan_handle
    , qs.statement_start_offset
    , qs.statement_end_offset
    , qs.execution_count - ISNULL(qsl.execution_count, 0)
    , qs.total_elapsed_time_ms - ISNULL(qsl.total_elapsed_time_ms, 0)
    , qs.min_elapsed_time_ms 
    , qs.max_elapsed_time_ms 
    , qs.total_worker_time_ms - ISNULL(qsl.total_worker_time_ms, 0)
    , qs.min_worker_time_ms
    , qs.max_worker_time_ms 
    , qs.total_logical_reads - ISNULL(qsl.total_logical_reads, 0)
    , qs.min_logical_reads 
    , qs.max_logical_reads 
    , qs.total_physical_reads - ISNULL(qsl.total_physical_reads, 0)
    , qs.min_physical_reads 
    , qs.max_physical_reads 
    , qs.total_logical_writes - ISNULL(qsl.total_logical_writes, 0)
    , qs.min_logical_writes 
    , qs.max_logical_writes 
    , qs.creation_time
    , qs.last_execution_time
FROM #QS qs
    LEFT OUTER JOIN QueryStatsLast qsl ON qs.sql_handle = qsl.sql_handle
        AND qs.plan_handle = qsl.plan_handle
        AND qs.statement_start_offset = qsl.statement_start_offset
        AND qs.creation_time = qsl.creation_time 
WHERE qs.execution_count - ISNULL(qsl.execution_count, 0) > 0 --Only keep records for when it was executed in that interval

TRUNCATE TABLE QueryStatsLast

INSERT INTO QueryStatsLast
SELECT sql_handle
    , plan_handle
    , statement_start_offset
    , statement_end_offset
    , objtype
    , execution_count 
    , total_elapsed_time_ms
    , min_elapsed_time_ms 
    , max_elapsed_time_ms
    , total_worker_time_ms  
    , min_worker_time_ms 
    , max_worker_time_ms 
    , total_logical_reads
    , min_logical_reads
    , max_logical_reads
    , total_physical_reads
    , min_physical_reads 
    , max_physical_reads
    , total_logical_writes 
    , min_logical_writes 
    , max_logical_writes
    , creation_time
    , last_execution_time
    , DateAdded
FROM #QS

Filed under: Monitoring, Scripts, SQL Server, System State Tagged: cache, PLE, sys.dm_exec_query_stats, Tuning

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