Waits With Outsized Importance

Brent Ozar has a few wait types whose appearance in your “uh-oh” list is disproportionate to the wait type’s relative percentage:

RESOURCE_SEMAPHORE_QUERY_COMPILE – this means a query came in, and SQL Server didn’t have an execution plan cached for it. In order to build an execution plan, SQL Server needs a little memory – not a lot, just a little – but that memory wasn’t available. SQL Server had to wait for memory to become available before it could even build an execution plan. For more details and a reproduction script, check out my Bad Idea Jeans: Dynamically Generating Ugly Queries post. In this scenario, cached query plans (and small ones) may be able to proceed just fine (depending on how much pressure the server is under), but the ugly ones will feel frozen.

Read on for a couple more examples of poison wait types.

Related Posts

New Diagnostics For Synchronous Statistics Updates

Joe Sack announces a new wait type and request command: Consider the following query execution scenario: You execute a SELECT query that triggers an automatic synchronous statistics update. The synchronous statistics update begins execution and your query waits (is essentially blocked) until the fresh statistics are generated. The query compilation and execution does not resume […]

Read More

Taking Action With Wait Stats

Aaron Bertrand lays out a course of action (or inaction) when dealing with the most common wait types in SQL Server: I started going a little further than this, mapping out some of the more common wait types, and noting some of the properties they shared. Translated into questions a tuner might have about a […]

Read More

Categories

August 2017
MTWTFSS
« Jul Sep »
 123456
78910111213
14151617181920
21222324252627
28293031