Use multiple threads while reading the metadata of tables to restore

[vitlibar]

Changelog category:

• Improvement

Changelog entry:

Use multiple threads while reading the metadata of tables from a backup while executing the RESTORE command.

Before this PR the metadata of tables was always read from a backup in a single thread. That was slow if the number of tables in a backup was high.

[robot-ch-test-poll3]

This is an automated comment for commit 0df2a64 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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ClickBench	Runs [ClickBench](https://github.com/ClickHouse/ClickBench/) with instant-attach table	✅ success
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Performance Comparison	Measure changes in query performance. The performance test report is described in detail here. In square brackets are the optional part/total tests	✅ success
SQLTest	There's no description for the check yet, please add it to tests/ci/ci_config.py:CHECK_DESCRIPTIONS	✅ success
SQLancer	Fuzzing tests that detect logical bugs with SQLancer tool	✅ success
Sqllogic	Run clickhouse on the sqllogic test set against sqlite and checks that all statements are passed	✅ success
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Check name	Description	Status
AST fuzzer	Runs randomly generated queries to catch program errors. The build type is optionally given in parenthesis. If it fails, ask a maintainer for help	❌ failure
CI running	A meta-check that indicates the running CI. Normally, it's in success or pending state. The failed status indicates some problems with the PR	⏳ pending
Integration tests	The integration tests report. In parenthesis the package type is given, and in square brackets are the optional part/total tests	❌ failure
Stateless tests	Runs stateless functional tests for ClickHouse binaries built in various configurations -- release, debug, with sanitizers, etc	❌ failure

[vitlibar]

@SmitaRKulkarni Please take a look

[SmitaRKulkarni]

@vitlibar
Do you think it makes sense to wrap this under a flag ?

[SmitaRKulkarni]

Rest all LGTM

[vitlibar]

@vitlibar Do you think it makes sense to wrap this under a flag ?

I don't think we should provide user a choice here, because using multiple threads seems to be faster and as correct as it was.

[vitlibar]

Test failures:

• 02122_parallel_formatting_Template - known, see issue 58829
• 02962_system_sync_replica_lightweight_from_modifier - see issue 60480
• test_failed_mutations/test.py::test_exponential_backoff_with_merge_tree[node0-True] - known, see PR 58036