Changelog category (leave one):

• New Feature

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

Added a setting enable_shared_storage_snapshot_in_query to enable sharing the same storage snapshot across all subqueries in a single query. This ensures consistent reads from the same table, even when the table is referenced multiple times within a query.

For example, the following query now produces correct and stable results when this setting is enabled:

SELECT count()
FROM events
WHERE _part_starting_offset + _part_offset IN (
    SELECT _part_starting_offset + _part_offset
    FROM events
    WHERE user_id = 42
)
SETTINGS enable_shared_storage_snapshot_in_query = 1

This helps #78429.

Additionally, introduced a test-only setting merge_tree_storage_snapshot_sleep_ms that allows injecting a delay during storage snapshot acquisition, to simulate concurrent modifications and verify snapshot consistency.

Note: This feature currently only applies to tables in the MergeTree family.

Caveat: Enabling this feature disables the merge_tree_enable_remove_parts_from_snapshot_optimization, which may lead to old data parts being retained longer during long-running queries. See #37913.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)