{"id":5153,"date":"2020-09-07T13:40:21","date_gmt":"2020-09-07T20:40:21","guid":{"rendered":"http:\/\/3.209.169.194\/blogs\/paul\/?p=5153"},"modified":"2020-09-07T13:40:21","modified_gmt":"2020-09-07T20:40:21","slug":"the-curious-case-of-almost-online-index-rebuilds","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/","title":{"rendered":"The Curious Case of&#8230; almost online index rebuilds"},"content":{"rendered":"<p><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>I had a comment on my blog a few weeks ago from someone who hadn&#8217;t known that online index operations acquire blocking table locks &#8211; yes they do!<\/p>\n<p>When I&#8217;m teaching the index fragmentation module of our IEPTO1 class, I like to joke that the marketing team back in 2005 called them &#8216;online index operations&#8217; because &#8216;almost online index operations&#8217; wasn&#8217;t a good marketing term :-)<\/p>\n<p>Using an online index rebuild as an example, here are the basic operations that happen:<\/p>\n<ul>\n<li>Acquire a table S (Share) lock. This makes sure that there are no open transactions making changes to the table. This is necessary because the next step is to&#8230;<\/li>\n<li>Create the empty new index and start a versioning scan of the old index. It doesn&#8217;t enable snapshot isolation, but it uses the same underlying mechanism to preserve a point-in-time view (for want of a better word) of the old index.<\/li>\n<li>Cause all query plans that affect the old index to recompile, so changes are made to both the old and the new index.<\/li>\n<li>Downgrade the table S lock to an IS (Intent-Share) lock, which remains in place for the duration of the operation. This allows read and writes to the table.<\/li>\n<li>Populate the new index. Any changes to the old index are also made to the new index, with logic in place to cope with pathological problems.<\/li>\n<li>Acquire a table Sch-M (Schema Modification) lock, which is not compatible with any other lock, so no readers or writers can be active on the table.<\/li>\n<li>Swap the new index for the old one, queue the old index for <a href=\"https:\/\/sqlperformance.com\/2013\/05\/sql-performance\/drop-truncate-log-myth\">deferred drop<\/a>, recompile all query plans.<\/li>\n<li>Drop the table lock<\/li>\n<\/ul>\n<p>So you can see there are two points at which an online operation must acquire a table lock that could cause blocking. This can be mitigated somewhat from SQL Server 2014 onward using the <a href=\"https:\/\/www.sqlskills.com\/blogs\/paul\/low-priority-locking-wait-types\/\" target=\"_blank\" rel=\"noopener noreferrer\"><em>WAIT_AT_LOW_PRIORITY<\/em> option<\/a>, but it&#8217;s still a necessity.<\/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.) I had 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-5153","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... almost online index rebuilds - 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-almost-online-index-rebuilds\/\" \/>\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... almost online index rebuilds - 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.) I had a [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/\" \/>\n<meta property=\"og:site_name\" content=\"Paul S. Randal\" \/>\n<meta property=\"article:published_time\" content=\"2020-09-07T20:40:21+00:00\" \/>\n<meta name=\"author\" content=\"Paul Randal\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Paul Randal\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"2 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":\"WebPage\",\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/\",\"url\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/\",\"name\":\"The Curious Case of... almost online index rebuilds - Paul S. Randal\",\"isPartOf\":{\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/#website\"},\"datePublished\":\"2020-09-07T20:40:21+00:00\",\"author\":{\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/#\/schema\/person\/ffcec826c18782e1e0adf173826a7fce\"},\"breadcrumb\":{\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/#breadcrumb\"},\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"ReadAction\",\"target\":[\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/\"]}]},{\"@type\":\"BreadcrumbList\",\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/#breadcrumb\",\"itemListElement\":[{\"@type\":\"ListItem\",\"position\":1,\"name\":\"Home\",\"item\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/\"},{\"@type\":\"ListItem\",\"position\":2,\"name\":\"The Curious Case of&#8230; almost online index rebuilds\"}]},{\"@type\":\"WebSite\",\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/#website\",\"url\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/\",\"name\":\"Paul S. Randal\",\"description\":\"In Recovery...\",\"potentialAction\":[{\"@type\":\"SearchAction\",\"target\":{\"@type\":\"EntryPoint\",\"urlTemplate\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/?s={search_term_string}\"},\"query-input\":{\"@type\":\"PropertyValueSpecification\",\"valueRequired\":true,\"valueName\":\"search_term_string\"}}],\"inLanguage\":\"en-US\"},{\"@type\":\"Person\",\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/#\/schema\/person\/ffcec826c18782e1e0adf173826a7fce\",\"name\":\"Paul Randal\",\"image\":{\"@type\":\"ImageObject\",\"inLanguage\":\"en-US\",\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/#\/schema\/person\/image\/\",\"url\":\"https:\/\/secure.gravatar.com\/avatar\/0b6a266bba2f088f2551ef529293001bd73bf026bc1908b9866728c062beeeb6?s=96&d=mm&r=g\",\"contentUrl\":\"https:\/\/secure.gravatar.com\/avatar\/0b6a266bba2f088f2551ef529293001bd73bf026bc1908b9866728c062beeeb6?s=96&d=mm&r=g\",\"caption\":\"Paul Randal\"},\"sameAs\":[\"http:\/\/3.209.169.194\/blogs\/paul\"],\"url\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/author\/paul\/\"}]}<\/script>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"The Curious Case of... almost online index rebuilds - Paul S. Randal","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/","og_locale":"en_US","og_type":"article","og_title":"The Curious Case of... almost online index rebuilds - Paul S. Randal","og_description":"(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.) I had a [&hellip;]","og_url":"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/","og_site_name":"Paul S. Randal","article_published_time":"2020-09-07T20:40:21+00:00","author":"Paul Randal","twitter_misc":{"Written by":"Paul Randal","Est. reading time":"2 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"WebPage","@id":"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/","url":"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/","name":"The Curious Case of... almost online index rebuilds - Paul S. Randal","isPartOf":{"@id":"https:\/\/www.sqlskills.com\/blogs\/paul\/#website"},"datePublished":"2020-09-07T20:40:21+00:00","author":{"@id":"https:\/\/www.sqlskills.com\/blogs\/paul\/#\/schema\/person\/ffcec826c18782e1e0adf173826a7fce"},"breadcrumb":{"@id":"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/#breadcrumb"},"inLanguage":"en-US","potentialAction":[{"@type":"ReadAction","target":["https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/"]}]},{"@type":"BreadcrumbList","@id":"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-almost-online-index-rebuilds\/#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/www.sqlskills.com\/blogs\/paul\/"},{"@type":"ListItem","position":2,"name":"The Curious Case of&#8230; almost online index rebuilds"}]},{"@type":"WebSite","@id":"https:\/\/www.sqlskills.com\/blogs\/paul\/#website","url":"https:\/\/www.sqlskills.com\/blogs\/paul\/","name":"Paul S. Randal","description":"In Recovery...","potentialAction":[{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https:\/\/www.sqlskills.com\/blogs\/paul\/?s={search_term_string}"},"query-input":{"@type":"PropertyValueSpecification","valueRequired":true,"valueName":"search_term_string"}}],"inLanguage":"en-US"},{"@type":"Person","@id":"https:\/\/www.sqlskills.com\/blogs\/paul\/#\/schema\/person\/ffcec826c18782e1e0adf173826a7fce","name":"Paul Randal","image":{"@type":"ImageObject","inLanguage":"en-US","@id":"https:\/\/www.sqlskills.com\/blogs\/paul\/#\/schema\/person\/image\/","url":"https:\/\/secure.gravatar.com\/avatar\/0b6a266bba2f088f2551ef529293001bd73bf026bc1908b9866728c062beeeb6?s=96&d=mm&r=g","contentUrl":"https:\/\/secure.gravatar.com\/avatar\/0b6a266bba2f088f2551ef529293001bd73bf026bc1908b9866728c062beeeb6?s=96&d=mm&r=g","caption":"Paul Randal"},"sameAs":["http:\/\/3.209.169.194\/blogs\/paul"],"url":"https:\/\/www.sqlskills.com\/blogs\/paul\/author\/paul\/"}]}},"_links":{"self":[{"href":"https:\/\/www.sqlskills.com\/blogs\/paul\/wp-json\/wp\/v2\/posts\/5153","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.sqlskills.com\/blogs\/paul\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.sqlskills.com\/blogs\/paul\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.sqlskills.com\/blogs\/paul\/wp-json\/wp\/v2\/users\/5"}],"replies":[{"embeddable":true,"href":"https:\/\/www.sqlskills.com\/blogs\/paul\/wp-json\/wp\/v2\/comments?post=5153"}],"version-history":[{"count":0,"href":"https:\/\/www.sqlskills.com\/blogs\/paul\/wp-json\/wp\/v2\/posts\/5153\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.sqlskills.com\/blogs\/paul\/wp-json\/wp\/v2\/media?parent=5153"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.sqlskills.com\/blogs\/paul\/wp-json\/wp\/v2\/categories?post=5153"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.sqlskills.com\/blogs\/paul\/wp-json\/wp\/v2\/tags?post=5153"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}