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There are several modes of parallel replicas and tons of settings. In order to make it usable we need to do the following:
All the modes should work on top the MergeTree tables without additional Distributed on top.
Introduce a setting parallel_replicas_mode with Enum type with values read_tasks, key_hash, key_range and sample_offset. Later on we will introduce another mode auto.
This is kind of backward-incompatible change, because for sample-offset mode you will need to set: use_parallel_replicas=true, parallel_replicas_mode='sample_offset', max_parallel_replicas=X instead of just max_parallel_replicas=X. I think this is Ok, because this mode is not popular among our users + we can document it well.
Same for other modes, but all other ones are considered experimental.
Use case
There are several modes of parallel replicas and tons of settings. In order to make it usable we need to do the following:
parallel_replicas_mode
with Enum type with valuesread_tasks
,key_hash
,key_range
andsample_offset
. Later on we will introduce another modeauto
.use_parallel_replicas=true, parallel_replicas_mode='sample_offset', max_parallel_replicas=X
instead of justmax_parallel_replicas=X
. I think this is Ok, because this mode is not popular among our users + we can document it well.Additional context
Take a look at comment in this PR: #63151
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