Support ALTER TABLE ... EXPORT PARTITION for non-replicated MT tables

[svb-alt]

Summary

Add support for ALTER TABLE ... EXPORT PARTITION on plain (non-replicated) MergeTree tables.

Today, partition export functionality exists for ReplicatedMergeTree tables. However, equivalent functionality is not available for standalone MergeTree tables, even though ALTER TABLE ... EXPORT PART is already supported for individual parts. In order to export an entire partition, users operating non-replicated MergeTree tables currently have to

1. Enumerate all parts belonging to the partition.
2. Export each part individually.
3. Reconstruct partition-level semantics externally.

This feature request proposes extending partition export to non-replicated MergeTree engines.

References

• ALTER TABLE ... EXPORT PARTITION implementation for replicated tables:
  • Export partition to apache iceberg #1618
• ALTER TABLE ... EXPORT PART support:
  • Export merge tree part docs #1147
• Original export-related discussion:
  • ALTER TABLE EXPORT to external table #595