GitHub user michaeljmarshall added a comment to the discussion: Zero receiver 
queue size for multi topic consumers

The documentation indicating that it cannot be done for a partitioned topic is 
going to provide the same justification: 
https://github.com/apache/pulsar/blame/master/site2/website/versioned_docs/version-2.10.x/cookbooks-message-queue.md#L25.

For a single topic, the zero queue consumer sends a request for a message and 
waits indefinitely until a message is sent by the broker. In the case of a 
multi-topic consumer, like the regex consumer, the routing would be overly 
complex. The consumer would need to try to consume from individual topics, but 
not at the same time, otherwise the consumer could have more than zero messages 
in the queue.

The docs could be made clearer though to indicate that any multi-topic consumer 
(e.g. a partitioned topic consumer, a regex consumer, or a topics list 
consumer) is not able to use a zero queue consumer.

GitHub link: 
https://github.com/apache/pulsar/discussions/18944#discussioncomment-4417051

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