Disable pure parallel replicas if trivial count optimization is possible

[Algunenano]

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• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

• Disable pure parallel replicas if trivial count optimization is possible

Documentation entry for user-facing changes

XXX

Using parallel replicas to do count() operations when trivial count is possible is extremely disadvantageous (in all cases I'd say) as it means both initializing parallel replicas, organizing the work and then reading the smallest column (vs checking just the part metadata). So in those cases we disable parallel replicas and do all the work in the initiator.

I've only attempted to implement this for pure parallel replicas (both in the old interpreter and the new analyzer) as custom key replicas isn't working right now and dealing with cluster() vs normal storage was more complex (but mainly the first reason).

[robot-clickhouse-ci-2]

This is an automated comment for commit 29e886f with description of existing statuses. It's updated for the latest CI running
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Check name	Description	Status
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[Algunenano]

• Integration tests (asan) [6/6] -> test_ssl_cert_authentication/test.py::test_https_non_ssl_auth. Flaky. Flaky test test_ssl_cert_authentication #49400
• Stateless tests (msan) [3/6]. Timeouts in 00534_functions_bad_arguments which seem flaky. Reported at 00534_functions_bad_arguments tests are flaky #50619
• Stateless tests (ubsan) [2/2] -> 01676_clickhouse_client_autocomplete -> 01676_clickhouse_client_autocomplete is flaky #50626

[nikitamikhaylov]

Will queries which read from Distributed tables break or return possibly wrong result?

[Algunenano]

AFAICS no for 2 reasons:

• The checks is kept in the original place (InterpreterSelectQuery::executeFetchColumns) before calling getTrivialCount during execution. If processing_stage != QueryProcessingStage::FetchColumns then it won't use trivial count, as before.

• StorageDistributed can't use trivial count, which means that getTrivialCount during analysis will return an empty value, so it won't change any setting.

I might be missing other scenarios though.