{"id":2437,"date":"2020-12-15T07:00:00","date_gmt":"2020-12-15T04:00:00","guid":{"rendered":"https:\/\/www.dataplatform.gr\/?p=2437"},"modified":"2026-03-30T16:28:29","modified_gmt":"2026-03-30T13:28:29","slug":"pos-vriskoyme-ti-kathysterei-ta-queries-ston-sql-se","status":"publish","type":"post","link":"https:\/\/www.dataplatform.gr\/en\/pos-vriskoyme-ti-kathysterei-ta-queries-ston-sql-se\/","title":{"rendered":"How do we find what&#039;s slowing down queries in SQL Server?"},"content":{"rendered":"<p class=\"wp-block-paragraph\">Many times we will see that a query is delayed and we would like to know what is delaying it. To do this we need to know the <strong>wait types<\/strong> and what each of them means.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Specifically <strong>types<\/strong> they can show what delays a query during execution. Similarly, high waits in time and number can be seen aggregated and at the instance level.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">There is a detailed list of all wait types <a href=\"https:\/\/docs.microsoft.com\/en-us\/sql\/relational-databases\/system-dynamic-management-views\/sys-dm-os-wait-stats-transact-sql?view=sql-server-ver15\" target=\"_blank\" rel=\"noreferrer noopener\">here<\/a>.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">The most common wait types<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>SOS_SCHEDULER_YIELD<\/strong>: indicates high CPU usage. It may be due to the use of intensive CPU tasks such as recompiling footage.<\/li>\n\n\n\n<li><strong>CXPACKET<\/strong>: this wait is due to waiting for all parallel threads to complete. It is created as each thread receives a non-proportional weight for completing each other&#039;s work. The solution to the problem may be to increase it <em>cost threshold of parallelism<\/em>.<\/li>\n\n\n\n<li><strong>WRITELOG<\/strong>: shows transaction log I\/O latency. Moving the transaction logs to different fast disks can reduce this.<\/li>\n\n\n\n<li><strong>LCK_M_IX<\/strong>: happens when there is a lock on the table or its page from an insert or update, it may be due to the lack of an index or the fact that it is fragmented.<\/li>\n\n\n\n<li><strong>LCK_M_X<\/strong>: indicates that there is a delay due to row locks. It may be due to the use of a limiter <a href=\"https:\/\/www.dataplatform.gr\/en\/ti-einai-ta-isolation-levels-kai-pos-mporoyn-na-epire\/\">isolation level<\/a>.<\/li>\n\n\n\n<li><strong>ASYNC_NETWORK_IO<\/strong>: indicates that there is a delay from SQL Server waiting for the client to finish processing the data.<\/li>\n\n\n\n<li><strong>PAGEIOLATCH_SH<\/strong>: shows the delay in loading pages from disk into memory. It may be due to reduced RAM which leads to not being able to hold a large number of pages, but it may also be due to low I\/O on the physical disks.<\/li>\n\n\n\n<li><strong>PAGELATCH_EX<\/strong>: usually due to multiple consecutive inserts with identity in the primary key. It may also indicate that they don&#039;t have enough database files in tempdb.<\/li>\n\n\n\n<li>\u200b<strong>RESOURCE_SEMAPHORE<\/strong>: occurs when a query is waiting to allocate the necessary memory (memory grant) to start its execution. This wait type is particularly critical, as it indicates that the instance has run out of available memory to execute new tasks. It is usually due to very large queries that require huge memory grants (e.g. due to large sorts or hashes) or to incorrect estimates of the optimizer (cardinality estimation) that lead to excessive resource allocations. The solution may include optimizing the queries to consume less memory, creating appropriate indexes, or even increasing the physical RAM of the server.<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">How do we find what is delaying a specific query?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">When a query is executed we can through the dynamic view <em>sys.dm_exec_requests<\/em> let&#039;s find it <strong>last_wait_type<\/strong> so we can see what seems to be delaying the query at the moment.<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"sql\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\" data-no-translation=\"\" data-no-auto-translation=\"\">SELECT d.name,p.command,p.session_id,t.text,p.start_time,p.status,p.total_elapsed_time\/1000 as elapsed_time_secs,wait_time\/1000 as wait_time,last_wait_type,p.blocking_session_id,m.granted_memory_kb,m.grant_time,p.plan_handle,ph.query_plan,p.sql_handle,dr.host_name,dr.program_name,dr.login_name\nFROM sys.dm_exec_requests p\ninner join sys.databases d on d.database_id = p.database_id\nouter apply sys.dm_exec_sql_text(p.sql_handle) t\nouter apply sys.dm_exec_query_plan(p.plan_handle) as ph\ninner join sys.dm_exec_sessions dr on dr.session_id = p.session_id\nleft join sys.dm_exec_query_memory_grants m on m.session_id = p.session_id\nwhere 1=1\nand text is not null\n--and p.session_id in (select spid from sysprocesses where blocked&lt;>0) --find blocked only\norder by  p.start_time\n<\/pre>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"839\" height=\"98\" src=\"https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/10\/ws-01.png\" alt=\"\" class=\"wp-image-2438\" srcset=\"https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/10\/ws-01.png 839w, https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/10\/ws-01-300x35.png 300w, https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/10\/ws-01-768x90.png 768w\" sizes=\"auto, (max-width: 839px) 100vw, 839px\" \/><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">Wait statistics on the entire instance<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">However, we can also see the total top 10 waits that the SQL Server instance had since the last time it was restarted or <em>DBCC SQLPERF(&#039;sys.dm_os_wait_stats&#039;, CLEAR)<\/em>:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"sql\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\" data-no-translation=\"\" data-no-auto-translation=\"\">select top 10 * from sys.dm_os_wait_stats \norder by  wait_time_ms desc<\/pre>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"652\" height=\"253\" src=\"https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/10\/ws-03.png\" alt=\"\" class=\"wp-image-2441\" srcset=\"https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/10\/ws-03.png 652w, https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/10\/ws-03-300x116.png 300w\" sizes=\"auto, (max-width: 652px) 100vw, 652px\" \/><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">Sources:<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><a href=\"https:\/\/docs.microsoft.com\/en-us\/sql\/relational-databases\/system-dynamic-management-views\/sys-dm-os-wait-stats-transact-sql?view=sql-server-ver15\" target=\"_blank\" rel=\"noreferrer noopener\">sys.dm_os_wait_stats (Transact-SQL)<\/a><\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><\/p>","protected":false},"excerpt":{"rendered":"<p>Many times we will see that a query is delayed and we would like to know what is delaying it. To do this we need to know the wait types and what each of them means. Specific types can show what delays a query during execution. Similarly, high waits in time [...]<\/p>","protected":false},"author":1,"featured_media":702,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[11,15],"tags":[23,48,6,131],"class_list":["post-2437","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-databases","category-ms-sqlserver","tag-microsoft","tag-performance_tuning","tag-sqlserver","tag-wait-statistics"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>\u03a0\u03ce\u03c2 \u03b2\u03c1\u03af\u03c3\u03ba\u03bf\u03c5\u03bc\u03b5 \u03c4\u03b9 \u03ba\u03b1\u03b8\u03c5\u03c3\u03c4\u03b5\u03c1\u03b5\u03af \u03c4\u03b1 queries \u03c3\u03c4\u03bf\u03bd SQL Server - DataPlatform.gr<\/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.dataplatform.gr\/en\/pos-vriskoyme-ti-kathysterei-ta-queries-ston-sql-se\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"\u03a0\u03ce\u03c2 \u03b2\u03c1\u03af\u03c3\u03ba\u03bf\u03c5\u03bc\u03b5 \u03c4\u03b9 \u03ba\u03b1\u03b8\u03c5\u03c3\u03c4\u03b5\u03c1\u03b5\u03af \u03c4\u03b1 queries \u03c3\u03c4\u03bf\u03bd SQL Server - 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