Identify the (Top 20) most expensive queries across your SQL Server using Query Store

I’m a big fan of using queries based on the dynamic management view sys.dm_exec_query_stats to identify the most resource hungry queries across a SQL instance. There are lots of versions of this sort of query around the place if you google for “Top 20 queries”.

That approach has some limitations though. First, it is cleared out every time an instance restarts, and second it only keeps figures for currently cached plans, so when a query recompiles, data is lost.

The DMVs provided by SQL Server Query Store solve these issues, as data is persisted in the database over time, so nothing is lost on restarts etc. And one extra thing you gain by using the Query Store DMVs is that you can slice by time interval, for instance if you want to look at the before and after states relating to a change that has been made – or you want to look at an interval where performance degradation has been reported.

Some time ago I wrote a query store version of the “Top 20 queries” query that will produce a ranked list of your most expensive queries – and I’ve ended up using this a lot.

The only downside of using the DMVs for Query Store is that they are per database whereas dm_exec_query_stats is a view across the whole instance. So I had to use a cursor and a temp table, populating the temp table for each database in turn.

Here’s the query:

--Gather and report on most resource hungry queries
DECLARE @Reportinginterval int;
DECLARE @Database sysname;
DECLARE @StartDateText varchar(30);
DECLARE @TotalExecutions decimal(20,3);
DECLARE @TotalDuration decimal(20,3);
DECLARE @TotalCPU decimal(20,3);
DECLARE @TotalLogicalReads decimal(20,3);
DECLARE @SQL varchar(MAX);

--Set Reporting interval in days
SET @Reportinginterval = 1;

SET @StartDateText = CAST(DATEADD(DAY, -@Reportinginterval, GETUTCDATE()) AS varchar(30));

--Cursor to step through the databases
DECLARE curDatabases CURSOR FAST_FORWARD FOR
SELECT [name]
FROM sys.databases 
WHERE is_query_store_on = 1;

--Temp table to store the results
DROP TABLE IF EXISTS #Stats;
CREATE TABLE #Stats (
   DatabaseName sysname,
   SchemaName sysname NULL,
   ObjectName sysname NULL,
   QueryText varchar(1000),
   TotalExecutions bigint,
   TotalDuration decimal(20,3),
   TotalCPU decimal(20,3),
   TotalLogicalReads bigint
);

OPEN curDatabases;
FETCH NEXT FROM curDatabases INTO @Database;

--Loop through the datbases and gather the stats
WHILE @@FETCH_STATUS = 0
BEGIN
    
    SET @SQL = '
	   USE [' + @Database + ']
	   INSERT intO #Stats
	   SELECT 
		  DB_NAME(),
		  s.name AS SchemaName,
		  o.name AS ObjectName,
		  SUBSTRING(t.query_sql_text,1,1000) AS QueryText,
		  SUM(rs.count_executions) AS TotalExecutions,
		  SUM(rs.avg_duration * rs.count_executions) AS TotalDuration,
		  SUM(rs.avg_cpu_time * rs.count_executions) AS TotalCPU,
		  SUM(rs.avg_logical_io_reads * rs.count_executions) AS TotalLogicalReads
	   FROM sys.query_store_query q
	   INNER JOIN sys.query_store_query_text t
		  ON q.query_text_id = t.query_text_id
	   INNER JOIN sys.query_store_plan p
		  ON q.query_id = p.query_id
	   INNER JOIN sys.query_store_runtime_stats rs
		  ON p.plan_id = rs.plan_id
	   INNER JOIN sys.query_store_runtime_stats_interval rsi
		  ON rs.runtime_stats_interval_id = rsi.runtime_stats_interval_id
	   LEFT JOIN sys.objects o
		  ON q.OBJECT_ID = o.OBJECT_ID
	   LEFT JOIN sys.schemas s
		  ON o.schema_id = s.schema_id     
	   WHERE rsi.start_time > ''' + @StartDateText + '''
	   GROUP BY s.name, o.name, SUBSTRING(t.query_sql_text,1,1000)
	   OPTION(RECOMPILE);';

    EXEC (@SQL);

    FETCH NEXT FROM curDatabases INTO @Database;
END;

CLOSE curDatabases;
DEALLOCATE curDatabases;

--Aggregate some totals
SELECT 
    @TotalExecutions = SUM(TotalExecutions),
    @TotalDuration = SUM (TotalDuration),
    @TotalCPU  = SUM(TotalCPU),
    @TotalLogicalReads = SUM(TotalLogicalReads)
FROM #Stats

--Produce output
SELECT TOP 20
    DatabaseName,
    SchemaName,
    ObjectName,
    QueryText,
    TotalExecutions,
    CAST((TotalExecutions/@TotalExecutions)*100 AS decimal(5,2)) AS [TotalExecutions %],
    CAST(TotalDuration/1000000 AS decimal(19,2)) AS [TotalDuration(s)],
    CAST((TotalDuration/@TotalDuration)*100 AS decimal(5,2)) AS [TotalDuration %],
    CAST((TotalDuration/TotalExecutions)/1000 AS decimal(19,2)) AS [AverageDuration(ms)],
    CAST(TotalCPU/1000000  AS decimal(19,2)) [TotalCPU(s)],
    CAST((TotalCPU/@TotalCPU)*100 AS decimal(5,2)) AS [TotalCPU %],
    CAST((TotalCPU/TotalExecutions)/1000 AS decimal(19,2)) AS [AverageCPU(ms)],   
    TotalLogicalReads,
    CAST((TotalLogicalReads/@TotalLogicalReads)*100 AS decimal(5,2)) AS [TotalLogicalReads %],
  CAST((TotalLogicalReads/TotalExecutions) AS decimal(19,2)) AS [AverageLogicalReads]   
FROM #Stats
--Order by the resource you're most interested in

--ORDER BY TotalExecutions DESC
--ORDER BY TotalDuration DESC
ORDER BY TotalCPU DESC
--ORDER BY TotalLogicalReads DESC

DROP TABLE #Stats;

The script limits itself to looking at databases where query store is enabled.

If you want to bring back more results you can just change the TOP statement, and if you want to look at the results ordered by a different resource (e.g. Reads) then just make sure the relevant ORDER BY clause is uncommented. With other small modifications I find this script useful in a myriad of scenarios. I hope you find it useful too.

Related:

How does Query Store capture cross database queries?

Introduction to SQL Server Query Store

3 thoughts on “Identify the (Top 20) most expensive queries across your SQL Server using Query Store

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s