Trace Flag 834 And Columnstore Tables

Joe Obbish shows how trace flag 834 can solve a bottleneck when inserting into tables with clustered columnstore indexes:

In my experience, when we get into a situation with high memory waits caused by too much concurrent CCI activity all queries on the server that use a memory grant can be affected. For example, I’ve seen sp_whoisactive run for longer than 90 seconds.

It needs to be stated that not all CCIs will suffer from this scalability problem. I was able to achieve good scalability with some artificial tables, but all of the real target tables that I tested have excessive memory waits at high concurrency. Perhaps tables which require more CPU to compress naturally spread out their memory requests and the underlying OS is better able to keep up.

Read the whole thing, and also check out Lonny Niederstadt’s comment as it adds pertinent information about TF834.

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