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This latch class is when a thread is waiting for access to the ExpressionEvaluator object that the Query Processor provides to DBCC consistency checks so that the thread can verify the stored value of a computed column is correct.
(Books Online description: “Internal use only.”)
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Added in SQL Server version:
Removed in SQL Server version:
Extended Events latch_class value:
The map_key value in sys.dm_xe_map_values is 49 in 2008 and 2008 R2, and 43 in 2012 and 2014 RTM. After 2014 RTM, you must check the DMV to get the latest value as some map_key values have changed in later builds.
This latch is a major bottleneck for DBCC consistency checks when indexes on computed columns exist, as the ExpressionEvaluator latch must be acquired for each row of an index on a computed column, by any thread checking one of these indexes, and can only be acquired in EX mode. (Just to be clear, although I was responsible for DBCC for SQL Server 2005, the ExpressionEvaluator functionality and it’s EX-latch limitation wasn’t written by me 🙂 The only solution is to disable those indexes before running the consistency checks, and as far as I know, this bottleneck will not be addressed by Microsoft. You can read more about this issue in this blog post.
Also see the LATCH_EX wait type for more information about latches in general and how to troubleshoot them.
Known occurrences in SQL Server (list number matches call stack list):
- Verifying the value of a persisted computed column from a data page
- Calculating the value of a computed column as part of the nonclustered index cross-checks (see the blog post link above for details)
Abbreviated call stacks (list number matches known occurrences list):