{"id":5104,"date":"2020-05-07T15:01:16","date_gmt":"2020-05-07T22:01:16","guid":{"rendered":"http:\/\/3.209.169.194\/blogs\/paul\/?p=5104"},"modified":"2020-05-07T15:01:16","modified_gmt":"2020-05-07T22:01:16","slug":"the-curious-case-of-extent-scan-fragmentation","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-extent-scan-fragmentation\/","title":{"rendered":"The Curious Case of\u2026 extent scan fragmentation"},"content":{"rendered":"<p style=\"text-align: justify;\"><em>(The Curious Case of\u2026<\/em>\u00a0used to be part of our bi-weekly\u00a0<a href=\"https:\/\/www.sqlskills.com\/join-the-sqlskills-insider-community\/\" target=\"_blank\" rel=\"noopener noreferrer\">newsletter<\/a>\u00a0but we decided to make it a\u00a0regular blog post instead so it can sometimes be more frequent. It covers something interesting one of us encountered when working with a client, doing some testing, or were asked in a random question from the community.)<\/p>\n<p style=\"text-align: justify;\">There was a Twitter #sqlhelp question this week about whether to be concerned about extent scan fragmentation or logical scan fragmentation when using <em>DBCC SHOWCONTIG<\/em>.<\/p>\n<p style=\"text-align: justify;\">The real answer is to stop using <em>DBCC SHOWCONTIG<\/em> as it was deprecated with the release of SQL Server 2005, and use <em>sys.dm_db_index_physical_stats<\/em>, the replacement DMV I designed, instead.<\/p>\n<p style=\"text-align: justify;\">The new DMV doesn&#8217;t report extent scan fragmentation because it doesn&#8217;t have to. If you think about it, extent scan fragmentation is a measure of how many times\u00a0a key-order scan of the\u00a0leaf level of the index moves from an extent to another extent which isn&#8217;t the next physically-contiguous extent. Logical fragmentation is a measure of how many times a key-order scan of the leaf level of the index moves from a page to another page which isn&#8217;t the next physically-contiguous page. So extent scan fragmentation is actually part of logical fragmentation, which is why the new DMV doesn&#8217;t report it.<\/p>\n<p style=\"text-align: justify;\">Back to the original question: use logical scan fragmentation if you&#8217;re still using the old DBCC command, but plan to move to the DMV.<\/p>\n<p style=\"text-align: justify;\">And there&#8217;s another reason: extent scan fragmentation in <em>DBCC SHOWCONTIG<\/em> is documented as not being accurate if the index is on a filegroup with multiple files. It&#8217;s been that way since it was originally written for SQL Server 7.0 and we decided not to fix it in SQL Server 2000 or later.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>(The Curious Case of\u2026\u00a0used to be part of our bi-weekly\u00a0newsletter\u00a0but we decided to make it a\u00a0regular blog post instead so it can sometimes be more frequent. It covers something interesting one of us encountered when working with a client, doing some testing, or were asked in a random question from the community.) There was a [&hellip;]<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[47,115],"tags":[],"class_list":["post-5104","post","type-post","status-publish","format-standard","hentry","category-indexes-from-every-angle","category-the-curious-case-of"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v26.5 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>The Curious Case of\u2026 extent scan fragmentation - Paul S. Randal<\/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\/paul\/the-curious-case-of-extent-scan-fragmentation\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The Curious Case of\u2026 extent scan fragmentation - Paul S. Randal\" \/>\n<meta property=\"og:description\" content=\"(The Curious Case of\u2026\u00a0used to be part of our bi-weekly\u00a0newsletter\u00a0but we decided to make it a\u00a0regular blog post instead so it can sometimes be more frequent. It covers something interesting one of us encountered when working with a client, doing some testing, or were asked in a random question from the community.) 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