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NESTING_TRANSACTION_READONLY

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Description:

This latch class, along with NESTING_TRANSACTION_FULL, is used to control access to the transaction description structure (called an XDES) for the parent transaction of a parallel nested transaction. A query that involves a parallel operator must start a sub-transaction for each parallel thread that is used, so these transactions are sub-transactions (called parallel nested transactions) of the main transaction. The _READONLY is for a transaction that’s read-only, i.e. it hasn’t changed the database.

This latch class replaced the NESTED_TRANSACTION_READONOY latch class from SQL Server 2008 onward.

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

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Added in SQL Server version:

2008

Removed in SQL Server version:

N/A

Extended Events latch_class value:

Maps to the TRAN_NESTING_RO map_value in sys.dm_xe_map_values.

The map_key value in sys.dm_xe_map_values is 112 in 2008 and 2008 R2, and 109 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:

These latches indicate that parallelism is happening, so for contention issues, follow the general steps for troubleshooting parallelism problems (see the CXPACKET wait type). I’ve heard anecdotal evidence that these latches can be problematic under certain circumstances when RCSI (read-committed snapshot isolation) is enabled, and there is a known bug (fixed) in SQL Server 2012 and 2014 that is described in KB 2806979.

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. Creating a rowset (a way of transferring rows from the Storage Engine to the Query Processor) (in this case, for a parallel table scan)
  2. Creating a rowset (in this case, as part of processing a sort for a top-N select)

And many, many more similar stacks.

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

  1. XeSqlPkg::latch_suspend_end::Publish+138
    LatchBase::Suspend+16b8
    LatchBase::AcquireInternal+415
    ParNestedXact::GetXdesInternal+f8
    GetTableCursor+e1
    GetQETableCursor+21a
    CQScanRowsetNew::ParallelGetRowset+6cf
    CQScanTableScanNew::CQScanTableScanNew+12b
    CXteTableScan::QScanGet+af
    CBpXte::PqsInitializeBatchProcessingTree+43
    CQScanXProducerNew::CQScanXProducerNew+251
    FnProducerThread+78f
    SubprocEntrypoint+a7f
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+a8
  2. XeSqlPkg::latch_suspend_end::Publish+138
    LatchBase::Suspend+16b8
    LatchBase::AcquireInternal+415
    ParNestedXact::GetXdesInternal+f8
    OpenRowsetSS::OpenWorkRowset+e6
    GetTableCursor+81
    CQPWorkTable::GetIRowsetAndWakeup+2c1
    CHashWorktablePartitionGroup::CreateWorktable+36f
    CHashWorktablePartitionGroup::CHashWorktablePartitionGroup+fa
    CHashWorktablePartitionGroupCompile::PhpgCreatePartitionGroup+67
    CQScanHash::CQScanHash+3ed
    CXteHash::QScanGet+82
    CQScanSortNew::CQScanSortNew+cf
    CQScanTopSortNew::CQScanTopSortNew+28
    CXteTopSort::QScanGet+a5
    CBpXte::PqsInitializeBatchProcessingTree+43
    CQScanXProducerNew::CQScanXProducerNew+251
    FnProducerThread+78f
    SubprocEntrypoint+a7f
    SOS_Task::Param::Execute+21e
    SOS_Scheduler::RunTask+a8