Fix flattened Dynamic type serialization with binary encoded data types

[Avogar]

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):

Fix flattened Dynamic type serialization with binary encoded data types. Closes #101911

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

[clickhouse-gh]

Workflow [PR], commit [9a89987]

Summary: ❌

job_name	test_name	status	info	comment
Unit tests (asan_ubsan)		failure
	FunctionsStress.stress	failure	cidb	IGNORED
	AllTests	failure	cidb	IGNORED

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AI Review

Summary

This PR fixes flattened Dynamic serialization when output_format_native_encode_types_in_binary_format=1 by switching to the stream-writing encodeDataType overload in SerializationDynamic, and adds a focused stateless roundtrip test. I did not find correctness, safety, performance, or compatibility issues in the diff. High-level verdict: approve from code-review perspective.

Missing context

• ⚠️ Full CI is still in progress (only partial statuses are currently available), so final merge readiness still depends on CI completion.

ClickHouse Rules

Item	Status	Notes
Deletion logging	➖
Serialization versioning	✅
Core-area scrutiny	✅
No test removal	✅
Experimental gate	➖
No magic constants	✅
Backward compatibility	✅
SettingsChangesHistory.cpp	➖
PR metadata quality	✅
Safe rollout	✅
Compilation time	✅
No large/binary files	✅

Final Verdict

• Status: ✅ Approve

[clickhouse-gh]

LLVM Coverage Report

Metric	Baseline	Current	Δ
Lines	84.00%	84.10%	+0.10%
Functions	90.90%	90.90%	+0.00%
Branches	76.60%	76.60%	+0.00%

Changed lines: 100.00% (4/4) · Uncovered code

Full report · Diff report

[Avogar]

Unit tests (asan_ubsan) - fixed in master

[clickgapai]

Hi — this PR may need backporting to 26.3 (LTS), 26.2, 26.1, 25.8 (LTS), but no backport label was found.

Affected code: SerializationDynamic::serializeBinaryBulkStatePrefix — FLATTENED path binary type encoding at line 172 — introduced in 25.8.

Why: The FLATTENED serialization path was introduced in PR #80499 (merged 2025-06-04), before the 25.8 release (2025-08-29). The buggy encodeDataType(type) call was present since that commit. The encode_types_in_binary_format setting interaction was introduced even earlier in PR #75832 (2025-02-14). All supported branches contain the affected code.

If this should be backported, consider adding pr-must-backport or a version-specific label (e.g. v26.3-must-backport). Ignore this if backporting is not applicable.