bug: should use `executor_id` to count mview creation progress

[BugenZhao]

a fragment contains more than one chain operator and we need to take care

Yes. I believe there will some problems with the current MV on MV creation reporting since we're using the actor id. 😢

Originally posted by @BugenZhao in https://github.com/risingwavelabs/risingwave/issues/6233#issuecomment-1305547119

• 2773

[BugenZhao]

https://github.com/risingwavelabs/risingwave/issues/6270 will be a prerequisite.

[github-actions[bot]]

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[BugenZhao]

This is not a problem anymore, as there's no chance that two Backfill executors reside in the same actor. Otherwise,

• If they are for the same table (like self-join), they should be shared.
• If they are for different tables, it's impossible because different upstream tables have different distribution in the optimizer and won't be backfilled in the same fragment.

[xxchan]

In theory, we can have multiple singleton executors in the same fragment (e.g., count(*) mv, or StreamValues). In our current code, it seems this won't happen, since JOIN/UNION will force a hash shard to inputs, and doesn't have the specialization for 2 singleton inputs.