Reading over the archive on the subject of threads has left me mostly still 
confused about how h2 handles concurrency, so I've been doing some tests, 
instead.

I started with largish queries that all have a "where ... in" clause with 
500 values (like "where x in [v0, v1, v2, .... v499]"), which ran with 
average time 120 seconds.

I then re-ran the same queries as sets of 500 queries (each with a single 
value, like "where x in [v0]"), splitting the queries across different 
threads. These sets of 500 queries ran much faster, average 30 seconds. It 
also made much more use of the disk: io stats where much higher, which is 
consistent with the queries running concurrently.

The docs seem to indicate that a query in one thread blocks queries in 
other threads, so I'm not sure how to interpret these numbers. Maybe I 
misunderstood the docs, and queries do run concurrently? I'm not using 
MULTI_THREADED.

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