Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

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

Fixed a LOGICAL_ERROR (Cannot create part with type Compact and storage type Full because table does not support polymorphic parts) raised when a MergeTree table loads a pre-existing Compact part while its current settings disable polymorphic parts (non-adaptive granularity). This could happen, for example, when a read-only table shares an on-disk path with another table and reloads its parts via SYSTEM RESTART DISK or on server startup. In debug and sanitizer builds the exception aborts the server.

Description

Found by the Stress test (amd_msan) check on an unrelated PR (STID 3286-0cd6), with the same error recurring across several unrelated PRs and check types (Stress msan/tsan/asan, Integration msan/tsan) over the last two months. No tracking issue existed.

Report: https://s3.amazonaws.com/clickhouse-test-reports/json.html?PR=109360&sha=3791a7836dd7b2f089113ba6666a14b6f9c26492&name_0=PR&name_1=Stress%20test%20%28amd_msan%29

Root cause. MergeTreeData::canUsePolymorphicParts() is equivalent to canUseAdaptiveGranularity(). When a table's current settings make it false (non-adaptive granularity), the part LOAD path rejected an already-existing Compact part in two places, both deriving the part's validity from the table's current create-time policy rather than the part's physical on-disk reality:

1. MergeTreeDataPartBuilder::build() threw the ... does not support polymorphic parts LOGICAL_ERROR directly.
2. Even without (1), the IMergeTreeDataPart constructor initializes index_granularity_info from the current policy, building MarkType(adaptive=false, Compact), which throws Non-Wide data part type with non-adaptive granularity.

But a Compact part is always adaptive by invariant (see the MarkType constructor), and an existing on-disk part's type is a fact, not a choice. The current policy only governs the format chosen for NEW parts, which MergeTreeData::choosePartFormat() already enforces (it returns {Wide, Full} when polymorphic parts are disabled).

Fix.

• MergeTreeIndexGranularityInfo(storage, settings, type): a Compact part type now always yields an adaptive mark type, regardless of the table's current policy. Wide keeps its policy-derived adaptivity (a Wide part may legitimately be non-adaptive). This covers both the load path and mutations that inherit a Compact source part's format.
• MergeTreeDataPartBuilder::build(): removed the redundant polymorphic-parts guard. New-part format selection is enforced by choosePartFormat; the type reaching build() otherwise belongs to an existing part (read from disk, or inherited by a mutation), where it is a physical fact.

New-part creation behavior is unchanged. The change only makes ClickHouse load pre-existing Compact parts it currently rejects.

Added regression test 04371_load_compact_part_non_adaptive_reader.sh (a non-adaptive reader shares a plain_rewritable path with an adaptive writer and loads the writer's Compact part via SYSTEM RESTART DISK). It aborts the server without the fix and passes with it.