ACCESS_METHODS_HOBT

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Description:

This latch class is when a thread is waiting for access to the in-memory representation of a HoBt (Heap-or-Btree) so that it can initialize/reinitialize it from metadata. The HoBt structure includes things like the list of columns, columns offsets, page and record counts, and allocation unit information (in-row data, LOB, row-overflow).

(Books Online description: “Used to synchronize access to the in-memory representation of a HoBt.”)

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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 HOBT map_value in sys.dm_xe_map_values.

The map_key value in sys.dm_xe_map_values is 9 in 2008 and 2008 R2, and 3 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. Refresh a HoBt structure (in this case, for a system table scan while looking up an object so a query can bind to it)
2. Refresh a HoBt structure (in this case, for a system table scan looking for Service Broker queues that support internal activation)

And many more similar stacks that end in a HoBt structure being initialized/reinitialized.

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

1. XeSqlPkg::latch_suspend_end::Publish+138
   LatchBase::Suspend+16b8
   LatchBase::AcquireInternal+415
   VisibleHoBt::Refresh+75
   HoBtFactory::GetHoBtAccess+a98
   GetRowsetProperties+177
   RowsetNewSS::Init+26e
   OpenRowsetSS::OpenRowset+10f
   OpenSystemTableRowset+2c7
   CMEDScanBase::Rowset+57d
   CMEDScan::StartSearch+4e7
   CMEDCatYukonObject::FLookupObjectFromId+25f
   CMEDCatalogObject::GetCachedObjectById+15c
   CMEDCatalogObject::GetCachedObjectByName+81
   CMEDProxySchema::GetObjectByName+82
   GetSystemObject+253
   IMetadataAccess::GetMultiNameObjectHelper+69c
   CMEDAccess::GetMultiNameObject+df
   CAlgTableMetadata::FPartialBind+796
   CAlgTableMetadata::Bind+31d
   CRelOp_Get::BindTree+429
   COptExpr::BindTree+85
   CRelOp_FromList::BindTree+37
   COptExpr::BindTree+85
2. XeSqlPkg::latch_suspend_end::Publish+138
   LatchBase::Suspend+16b8
   LatchBase::AcquireInternal+415
   VisibleHoBt::Refresh+75
   HoBtFactory::GetHoBtAccess+a98
   GetRowsetProperties+177
   RowsetNewSS::Init+26e
   OpenRowsetSS::OpenRowset+10f
   OpenSystemTableRowset+2c7
   CMEDScanBase::Rowset+57d
   CMEDScan::StartSearch+4e7
   ECatBitsYukon::GetNextSchemaObjectId+ac
   CMEDCatSB::GetNextServiceId+bf
   CMEDProxyDatabase::FGetNextServiceId+36
   CSbMappingMgr::NotifyBrokerInitialize+31d
   CActiveServiceBroker::BackgroundNotify+14b
   CActiveServiceBrokerMgr::ASBInitializer+19b
   CActiveServiceBrokerMgr::CSbASBInitializer::IdempotentInvoke+26
   CSbTaskManager::HandleReusableTask+2b8
   CSbTask::TaskStart+14b
   CSbTask::BeginTaskStart+17
   SOS_Task::Param::Execute+21e
   SOS_Scheduler::RunTask+ab