We initially considered removing entries just before any change to
pg_dist_node. However, that ended-up being very complex and making
MX even more complex.
Instead, we're switching to a simpler solution, where we remove entries
when the counter gets to 0.
With certain workloads, this may have some performance penalty. But, two
notes on that:
- When counter == 0, it implies that the cluster is not busy
- With cached connections, that's not possible
With this commit, we're introducing a new infrastructure to throttle
connections to the worker nodes. This infrastructure is useful for
multi-shard queries, router queries are have not been affected by this.
The goal is to prevent establishing more than citus.max_shared_pool_size
number of connections per worker node in total, across sessions.
To do that, we've introduced a new connection flag OPTIONAL_CONNECTION.
The idea is that some connections are optional such as the second
(and further connections) for the adaptive executor. A single connection
is enough to finish the distributed execution, the others are useful to
execute the query faster. Thus, they can be consider as optional connections.
When an optional connection is not allowed to the adaptive executor, it
simply skips it and continues the execution with the already established
connections. However, it'll keep retrying to establish optional
connections, in case some slots are open again.
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