{"id":468,"date":"2012-10-24T12:06:02","date_gmt":"2012-10-24T12:06:02","guid":{"rendered":"\/blogs\/jonathan\/post\/How-useful-are-query_hash-and-query_plan_hash-for-troubleshooting.aspx"},"modified":"2017-04-13T14:40:49","modified_gmt":"2017-04-13T18:40:49","slug":"how-useful-are-query_hash-and-query_plan_hash-for-troubleshooting","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/jonathan\/how-useful-are-query_hash-and-query_plan_hash-for-troubleshooting\/","title":{"rendered":"How useful are query_hash and query_plan_hash for troubleshooting?"},"content":{"rendered":"<p>After my last post, I was contacted by email about the usefulness of query_hash in actual troubleshooting scenarios since the query_hash value is based on the statement and not the database in a system.&#160; It just happened that the example template from SQL Server 2012 that I showed had the query_hash included in it&#8217;s definition.&#160; The premise for the question was that Software as a Service (Saas) solutions have multiple databases, sometimes hundreds to even thousands on the same server, that have the exact same code base, so using query_hash to aggregate data is not as useful in these scenarios since the data is not guaranteed to be evenly distributed.&#160; My first feedback on this was that my blog post didn&#8217;t actually relate any information to the query_hash action in Extended Events directly, but there happens to be more to this situation.<\/p>\n<p>The query_hash and query_plan_hash provide the <a href=\"https:\/\/blogs.msdn.microsoft.com\/bartd\/2008\/09\/03\/query-fingerprints-and-plan-fingerprints-the-best-sql-2008-feature-that-youve-never-heard-of\/\" target=\"_blank\">query finger prints in SQL Server 2008+,<\/a> and simplify the effort required for analyzing adhoc\/prepared workloads as well as workloads that use stored procedures in multiple databases.&#160; To look at how to leverage this information in SaaS scenarios to identify the database that caused the most executions of the query_hash being reviewed we&#8217;ll make use of the following workload to show the accumulated effects of a specific query_hash per database. <\/p>\n<blockquote>\n<p>CREATE DATABASE Test;     <br \/>GO      <br \/>USE Test;      <br \/>GO      <br \/>CREATE PROCEDURE dbo.TestProc      <br \/>AS      <br \/>BEGIN      <br \/>&#160;&#160;&#160; SELECT TOP 10 *      <br \/>&#160;&#160;&#160; FROM master..spt_values      <br \/>&#160;&#160;&#160; WHERE type = N&#8217;P&#8217;;      <br \/>END      <br \/>GO      <br \/>CREATE DATABASE Test1      <br \/>GO      <br \/>USE Test1      <br \/>GO      <br \/>CREATE PROCEDURE dbo.TestProc      <br \/>AS      <br \/>BEGIN      <br \/>&#160;&#160;&#160; SELECT TOP 10 *      <br \/>&#160;&#160;&#160; FROM master..spt_values      <br \/>&#160;&#160;&#160; WHERE type = N&#8217;P&#8217;;      <br \/>END      <br \/>GO      <br \/>CREATE DATABASE Test2      <br \/>GO      <br \/>USE Test2      <br \/>GO      <br \/>CREATE PROCEDURE dbo.TestProc      <br \/>AS      <br \/>BEGIN      <br \/>&#160;&#160;&#160; SELECT TOP 10 *      <br \/>&#160;&#160;&#160; FROM master..spt_values      <br \/>&#160;&#160;&#160; WHERE type = N&#8217;P&#8217;;      <br \/>END      <br \/>GO      <br \/>DBCC FREEPROCCACHE;      <br \/>GO<\/p>\n<\/blockquote>\n<p>After creating the databases and objects, we&#8217;ll setup an Extended Events session to capture the statement level completed events for the session_id running the test queries and capture the query_hash to show how to use it to look at execution information per database from the query statistics in SQL Server. Since I am using SQL Server 2012, I am not using a target with the event session, but will instead capture the information using the Live Data Viewer to make it easier to show in this post.<\/p>\n<blockquote>\n<p>CREATE EVENT SESSION [SQLskills_Query_Hash] ON SERVER      <br \/>ADD EVENT sqlserver.sp_statement_completed(      <br \/>&#160;&#160;&#160; ACTION(sqlserver.query_hash)      <br \/>&#160;&#160;&#160; WHERE ([sqlserver].[session_id]=(66))),      <br \/>ADD EVENT sqlserver.sql_statement_completed(      <br \/>&#160;&#160;&#160; ACTION(sqlserver.query_hash)      <br \/>&#160;&#160;&#160; WHERE ([sqlserver].[session_id]=(66)));<\/p>\n<\/blockquote>\n<p>Once the session is created and started, we can run the following tests and review the output:<\/p>\n<blockquote>\n<p>USE Test     <br \/>GO<\/p>\n<p>EXECUTE dbo.TestProc     <br \/>GO 2<\/p>\n<p>SELECT TOP 10 *     <br \/>FROM master..spt_values      <br \/>WHERE type = N&#8217;P&#8217;;      <br \/>GO 5<\/p>\n<p>USE Test1     <br \/>GO<\/p>\n<p>EXECUTE dbo.TestProc     <br \/>GO 4<\/p>\n<p>SELECT TOP 10 *     <br \/>FROM master..spt_values      <br \/>WHERE type = N&#8217;P&#8217;;      <br \/>GO 7<\/p>\n<p>USE Test2     <br \/>GO<\/p>\n<p>EXECUTE dbo.TestProc     <br \/>GO 8<\/p>\n<p>SELECT TOP 10 *     <br \/>FROM master..spt_values      <br \/>WHERE type = N&#8217;P&#8217;;      <br \/>GO 3<\/p>\n<\/blockquote>\n<p>After collecting the data, I disconnected the UI from the live stream and then grouped the events by the query_hash and summed the values for logical_reads, duration, cpu_time, and writes<\/p>\n<p><a href=\"https:\/\/www.SQLskills.com\/blogs\/jonathan\/wp-content\/uploads\/windows-live-writer\/how-useful-are-query_hash-and-query_plan\/215bc0d7\/image.png\"><img fetchpriority=\"high\" decoding=\"async\" style=\"background-image: none; border-bottom: 0px; border-left: 0px; padding-left: 0px; padding-right: 0px; display: inline; border-top: 0px; border-right: 0px; padding-top: 0px\" title=\"image\" border=\"0\" alt=\"image\" src=\"https:\/\/www.SQLskills.com\/blogs\/jonathan\/wp-content\/uploads\/windows-live-writer\/how-useful-are-query_hash-and-query_plan\/72960834\/image_thumb.png\" width=\"644\" height=\"92\" \/><\/a><\/p>\n<p>Here we can see that all of the statements that executed had the same query_hash value, even though they were a mix of adhoc executions and procedures from different databases.&#160; If we want to break this down to determine each databases executions we can do that by taking the query_hash value, converting it into a binary(8) data type from the bigint value produced by Extended Events, and then query sys.dm_exec_query_stats.<\/p>\n<blockquote>\n<p>SELECT      <br \/>&#160;&#160;&#160; sql_handle,       <br \/>&#160;&#160;&#160; plan_handle,       <br \/>&#160;&#160;&#160; execution_count,       <br \/>&#160;&#160;&#160; total_logical_reads,       <br \/>&#160;&#160;&#160; total_elapsed_time,       <br \/>&#160;&#160;&#160; dbid,       <br \/>&#160;&#160;&#160; objectid       <br \/>FROM sys.dm_exec_query_stats AS qs      <br \/>CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) AS qp      <br \/>WHERE query_hash = CONVERT(BINARY(8), CONVERT(BIGINT, 1640387627010439277));      <\/p>\n<\/blockquote>\n<p><a href=\"https:\/\/www.SQLskills.com\/blogs\/jonathan\/wp-content\/uploads\/windows-live-writer\/how-useful-are-query_hash-and-query_plan\/18f7eb80\/image.png\"><img decoding=\"async\" style=\"background-image: none; border-bottom: 0px; border-left: 0px; padding-left: 0px; padding-right: 0px; display: inline; border-top: 0px; border-right: 0px; padding-top: 0px\" title=\"image\" border=\"0\" alt=\"image\" src=\"https:\/\/www.SQLskills.com\/blogs\/jonathan\/wp-content\/uploads\/windows-live-writer\/how-useful-are-query_hash-and-query_plan\/46790b43\/image_thumb.png\" width=\"644\" height=\"104\" \/><\/a><\/p>\n<p>Using this method we can further breakdown the information to see where our biggest impacts are.&#160; Using the query_hash at the server level in Extended Events makes it possible to identify statements in a workload that might not have long executions but execute frequently leading to a &quot;death by a thousand cuts&quot;.&#160; If you need to dig into the database level information, you could add the sqlserver.database_id action to the statement level events, or just query the information out of the query statistics maintained inside of SQL Server.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>After my last post, I was contacted by email about the usefulness of query_hash in actual troubleshooting scenarios since the query_hash value is based on the statement and not the database in a system.&#160; It just happened that the example template from SQL Server 2012 that I showed had the query_hash included in it&#8217;s definition.&#160; [&hellip;]<\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[19,23,39],"tags":[],"class_list":["post-468","post","type-post","status-publish","format-standard","hentry","category-database-administration","category-extended-events","category-sql-server-2012"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>How useful are query_hash and query_plan_hash for troubleshooting? - Jonathan Kehayias<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.sqlskills.com\/blogs\/jonathan\/how-useful-are-query_hash-and-query_plan_hash-for-troubleshooting\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" 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