These extended statistics allow the query planner to know that the
prevalence of empty topics is very different in DMs, as opposed to
channel messages. The `ALTER STATISTICS` adjusts the size of the
"most common value" list -- it must be large enough to include the
empty-topic channel messages to be fully effective. Having this list
be over-large does not carry much risk, however.
The `ANALYZE zerver_message` step may be quite slow.
