disclaimer: i haven't followed all turns and details on this task so sorry if my comments are outdated (or generally uninteresting).

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i think it will be well invested time to add infrastructure to be able to restrict when parallel applying will be performed.

typically i think it would be nice to

- only allow parallel transaction in same engine

- decrease parallelism when performing DDL

...(more of these can probably be added)

restrictions that will make it easier to reason about correctness and that will not affect performance

for 99% of the cases (i think :-)

if adding such restrictions was an option, the resolving review comments on weird corner cases would be much easier.

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from what I can tell, there are currently no such restrictions,

and no infrastructure to add them either.

i realize that it might not be trivial to introduce such restrictions,

but personally i think it will be worth the effort to get a solid solution in place.

<quick thoughts on implementation>

for row-based replication this seems quite "easy".

for statement-based replication i image that you would have to add hooks into the "real" code

after parsing has been performed, but before the actual execution is started (and yes, i know that there is sometimes a blurry line here)

</thoughts>

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/Jonas