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This wait type is when parallel threads for a columnstore index build operation are waiting for the first segment of the index to be built single-threaded.

Questions/comments on this wait type? Click here to send Paul an email, especially if you have any information to add to this topic.

Added in SQL Server version:


Removed in SQL Server version:


Extended Events wait_type value:

The map_key value in sys.dm_xe_map_values is 1008 in 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:

When a columnstore index is being built, the memory grant estimate is based on a segment size of one million rows. The first segment of the index is built using a single thread so the real, required per-thread memory grant is found. Then the memory grants are given per thread and the other segments are built multi-threaded. Although all the threads for the operation are allocated at the start of the build, only one thread is used for the first segment and all the others incur a COLUMNSTORE_BUILD_THROTTLE wait.

If the build of the first segment is taking a long time, the cumulative time for this wait type can make it become one of the most prevalent waits on your server. Check that you haven’t made a mistake in the definition of the index being built (a good example of that is in Kendra Little’s post here).

Known occurrences in SQL Server (list number matches call stack list):

  1. TBD

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

  1. TBD