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DBCC_CHECK_TABLE_INIT

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Description:

This latch class is when a thread is waiting for access to the DBCC MultiObjectScanner data structure that DBCC consistency checks use to drive scanning through the database. In this case, the latch controls the access necessary to load the MultiObjectScanner with the allocation units from one or more objects (multiple objects can be checked simultaneously). The various scanning threads inside DBCC then acquire the DBCC_MULTIOBJECT_SCANNER latch to request the next page (or range of pages) to scan for consistency problems. You can read more about this architecture in this blog post.

(Books Online description: “Internal use only.”)

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Added in SQL Server version:

pre-2005/2005

Removed in SQL Server version:

N/A

Extended Events latch_class value:

Maps to the DBCC_CHECK_TABLES_INIT map_value in sys.dm_xe_map_values.

The map_key value in sys.dm_xe_map_values is 44 in 2008 and 2008 R2, and 38 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.

Other information:

I have not seen this latch be a noticeable contention point.

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):

  1. Adding more allocation units into the DBCC MultiObjectScanner

Abbreviated call stacks (list number matches known occurrences list):

  1. XeSqlPkg::latch_suspend_end::Publish+138
    LatchBase::Suspend+16b8
    LatchBase::AcquireInternal+415
    CheckTables::ProcessNextData+185
    CheckAggregateSingleInstance::GetNextFact+28e
    CTRowsetInstance::FGetNextRow+3c
    CQScanRmtScanNew::GetRowHelper+3b5
    CQScanXProducerNew::GetRowHelper+53
    CQScanXProducerNew::GetRow+15
    FnProducerOpen+57
    FnProducerThread+8c3