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ACCESS_METHODS_DATASET_PARENT

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Description:

This latch class is when a thread in a parallel scan needs access to the parent dataset, generally to get the next range of rows to scan.

(Books Online description: “Used to synchronize child dataset access to the parent dataset during parallel operations.”)

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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 DATASET_PARENT map_value in sys.dm_xe_map_values.

The map_key value in sys.dm_xe_map_values is 7 in 2008 and 2008 R2, and 1 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:

Although this latch can be a contention point, it’s only applicable to parallel scans, and so the parallelism itself is what should be investigated. See the CXPACKET wait type for information on investigating and addressing unwanted or skewed parallelism.

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):

  1. Getting the next range to scan from the parent dataset (in this case, while scanning a heap as part of a nested-loop join)
  2. Setting up the scan for a child thread (in this case, while scanning an index as part of a sort operation)

And many, many more similar stacks around parallel scans.

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

  1. XeSqlPkg::latch_suspend_end::Publish+138
    LatchBase::Suspend+16b8
    LatchBase::AcquireInternal+415
    HeapDataSetSession::GetNextRangeForChildScan+70
    HeapDataSetSession::SetupNextSubScan+285
    HeapDataSetSession::StartScan+192
    RowsetNewSS::EnsureStartOfScanLong+b1
    RowsetNewSS::FetchNextRow+36
    CQScanTableScanNew::GetRow+f2
    CQScanNLJoinTrivialNew::GetRow+e5
    CQScanNLJoinTrivialNew::GetRow+e5
    CQScanNLJoinTrivialNew::GetRow+e5
    CQScanNLJoinTrivialNew::GetRow+e5
    CQScanUpdateNew::GetRow+76
    CQScanXProducerNew::GetRowHelper+53
    CQScanXProducerNew::GetRow+15
    FnProducerOpen+57
    FnProducerThread+8c3
    SubprocEntrypoint+a7f
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+a8
  2. XeSqlPkg::latch_suspend_end::Publish+138
    LatchBase::Suspend+16b8
    LatchBase::AcquireInternal+415
    IndexDataSetSession::SetupNextChildSubScan+bc
    IndexDataSetSession::GetNextRowValuesInternal+17a2
    RowsetNewSS::FetchNextRow+197
    CQScanTableScanNew::GetRow+f2
    CQScanSortNew::BuildSortTable+246
    CQScanSortNew::OpenHelper+c0
    CQScanNew::OpenHelper+41
    CQScanXProducerNew::Open+c8
    FnProducerOpen+44
    FnProducerThread+8c3
    SubprocEntrypoint+a7f