Parallel replicas feature is Beta

[alexey-milovidov]

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• Improvement

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The "parallel replicas" feature is no longer experimental and it is now in beta. The setting allow_experimental_parallel_replicas is renamed to use_parallel_replicas.

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[robot-ch-test-poll3]

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[alexey-milovidov]

Currently, there are four modes for parallel replicas:

1. Sample key;
2. Task-based;
3. Custom key by hash;
4. Custom key by range;

Confusingly, the "sample key" mode is available even without use_parallel_replicas,
and the choice between "custom key" and "task-based" modes is determined by setting the value of "parallel_replicas_custom_key".

We should introduce a new setting "parallel_replicas_mode" which will decide between these modes.

[alesapin]

Let's have at least one test with old name for compatibility check

[devcrafter]

LGTM

[nikitamikhaylov]

@alexey-milovidov Parallel replicas modes 1, 3, and 4 are currently working on top of distributed table. While task-based parallel replicas work on top of MergeTree. We should unify this as well.

[nikitamikhaylov]

Let's make the parallel replicas with custom key work on top of MergeTree first and introduce the setting parallel_replicas_mode: #63521

[devcrafter]

Let's make the parallel replicas with custom key work on top of MergeTree first and introduce the setting parallel_replicas_mode: #63521

Unifying settings and implementing custom key on top of MergeTree are 2 different things and can be done independently