[Solved] Nested loop join going single-threaded on large table updating all the rows

xhr489 Asks: Nested loop join going single-threaded on large table updating all the rows
I am looking at some code (reporting stored procedure) where some columns on a large table are updated based on a join. The correct indexes are there, but the execution plan shows a single-threaded nested loop join. My guess is that is chooses the nested loop join in order to avoid the sort after the join. But I don’t understand why is does not run in parallel.

I often see this, i.e. that nested loop join is only single-threaded. I cannot figure out why. The estimated cost of the query is larger than “cost threshold for parallelism” on the instance and as far as I have read none of the factors that cause a query to run single threaded are not present (“Parallelism-inhibiting operations”).

The code is like this:

FROM dbo.MyTable A
    INNER JOIN dbo.OtherTable B
         ON A.COL2 = B.COL2 AND A.COL3 = B.COL3;

Version of SQL Server is 2016.

Also I cannot get the actual execution plan since the query runs forever. I either have to drop the index on the A table or force it with a join hint to use a HASH MATCH JOIN. So I cannot see the “reason for not finding a valid parallel plan” in the root node of the execution plan.

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