From experimenting in production we found a "!=" caused the engine
to use a sequential scan, whereas explicitly listing all the types
ensured an index scan was used.
We also found that querying for many (over 100K) items leads to
the task stalling - no logs, but no evidence of it running either -
so we also add a limit to the query.
Since the query now only returns a subset of notifications, we need
to ensure the subsequent "update" query operates on the same batch.
Also, as a temporary measure, we have a loop in the task code to
ensure it operates on the total set of notifications to "time out",
which we assume is less than 500K for the time being.
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