Fix Block structure mismatch with normal projections and parallel replicas by alexey-milovidov · Pull Request #99497 · ClickHouse/ClickHouse

alexey-milovidov · 2026-03-14T08:46:22Z

When optimizeUseNormalProjections replaces the read chain with a projection-based read where all parts come from the projection, the new chain may produce extra columns (e.g. from PREWHERE column restoration in splitAndFillPrewhereInfo). With parallel replicas, the local plan uses the projection while the remote plan does not, so the UnionStep combining them gets mismatched headers — the local side has extra columns the remote side doesn't.

The fix adds a converting step when the projection chain's output differs from the original chain's output. This mirrors the existing compatibility check already present for the mixed projection+parent-parts code path (line 459).

Closes #96131. Closes #95191.

CI report: https://s3.amazonaws.com/clickhouse-test-reports/json.html?REF=master&sha=73f45b0717e3a4763b7d1cde71950a076b5b00c7&name_0=MasterCI&name_1=Stress%20test%20%28azure%2C%20amd_msan%29

Changelog category (leave one):

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

Fix exception "Block structure mismatch in UnionStep stream" when using normal projections with parallel replicas.

Documentation entry for user-facing changes

Documentation is written (mandatory for new features)

… parallel replicas When `optimizeUseNormalProjections` replaces the read chain with a projection-based read, the new chain may output extra columns (e.g., `region` from PREWHERE restoration) that the original chain did not have. This causes a "Block structure mismatch in UnionStep stream" exception in debug/sanitizer builds when the local plan (with projection) is combined with the remote plan (without projection) in parallel replicas. The fix adds a converting step when the projection chain's output header differs from the expected output, ensuring headers stay consistent. This mirrors the existing header compatibility check already present for the mixed projection+parent-parts case (lines 459-460). https://s3.amazonaws.com/clickhouse-test-reports/json.html?REF=master&sha=73f45b0717e3a4763b7d1cde71950a076b5b00c7&name_0=MasterCI&name_1=Stress%20test%20%28azure%2C%20amd_msan%29 Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

clickhouse-gh · 2026-03-14T08:47:01Z

Workflow [PR], commit [eb9f32e]

Summary: ✅

AI Review

Summary

This PR adds a new stateless regression test for the UnionStep header mismatch around normal projections with parallel replicas. The test setup and data generation are clear, but the key assertion currently verifies projection usage (region_proj) rather than the specific header-conversion behavior this fix intends to protect, so it may not reliably catch regressions of that exact bug.

Findings

⚠️ Majors

[tests/queries/0_stateless/04039_normal_projection_parallel_replicas.sql:55-60] The regression check is not specific to the conversion path that prevents UnionStep header mismatch. It asserts that a projection is used, which can still be true even when conversion-specific behavior regresses.
- Suggested fix: add a deterministic assertion tied to the conversion path (for example via EXPLAIN PIPELINE/plan-shape evidence of the conversion stage or an assertion that fails on code without the conversion fix).

Tests

⚠️ Add a regression assertion that is specific to the header-alignment conversion, not only projection selection.

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: ⚠️ Request changes
Minimum required action: make the new regression check validate the conversion/header-alignment behavior directly, not only projection selection.

devcrafter · 2026-03-17T17:45:09Z

Here is the reason we can have unused columns after PREWHERE in header -

ClickHouse/src/Processors/QueryPlan/Optimizations/optimizePrewhere.cpp

Lines 73 to 98 in 35f2d31

    
           /// This is the leak of abstraction. 
        
           /// Split actions may have inputs which are needed only for PREWHERE. 
        
           /// This is fine for ActionsDAG to have such a split, but it breaks defaults calculation. 
        
           /// 
        
           /// See 00950_default_prewhere for example. 
        
           /// Table has structure `APIKey UInt8, SessionType UInt8` and default `OperatingSystem = SessionType+1` 
        
           /// For a query with `SELECT OperatingSystem WHERE APIKey = 42 AND SessionType = 42` we push everything to PREWHERE 
        
           /// and columns APIKey, SessionType are removed from inputs (cause only OperatingSystem is needed). 
        
           /// However, column OperatingSystem is calculated after PREWHERE stage, based on SessionType value. 
        
           /// If column SessionType is removed by PREWHERE actions, we use zero as default, and get a wrong result. 
        
           /// 
        
           /// So, here we restore removed inputs for PREWHERE actions 
        
           { 
        
               std::unordered_set<const ActionsDAG::Node *> first_outputs( 
        
                   split_result.first.getOutputs().begin(), split_result.first.getOutputs().end()); 
        
               for (const auto * input : split_result.first.getInputs()) 
        
               { 
        
                   if (!first_outputs.contains(input)) 
        
                   { 
        
                       split_result.first.getOutputs().push_back(input); 
        
                       /// Add column to second actions as input. 
        
                       /// Do not add it to result, so it would be removed. 
        
                       split_result.second.addInput(input->result_name, input->result_type); 
        
                   } 
        
               } 
        
           }

It fixes another issue. So, it seems it's a job for query_plan_remove_unused_columns optimization, but currently it's not happening

devcrafter

It's more symptom fix. Proper fix will take some effort - some thoughts here

The previous test had too few rows (2) to trigger `optimizePrewhere`, which is needed to produce the extra PREWHERE restoration columns that cause the Block structure mismatch. With 10000 rows, column sizes are meaningful and the `MergeTreeWhereOptimizer` moves the filter to PREWHERE. The PREWHERE column restoration then adds `region` as an extra output that persists through `ActionsDAG::mergeInplace` into the projection query DAG, causing the header mismatch with the remote replica plan. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

The bugfix validation was failing because commit 0f305e6 (already in master) moved the debug assertion in `UnionStep::updatePipeline` after the header conversion code. This means the pipeline-level safety net silently handles mismatched headers even in debug builds, so the test could never reproduce the runtime error against the master binary. Add an EXPLAIN PLAN check that detects the plan-level fix: the converting `ExpressionStep` added by `optimizeUseNormalProjection` creates two consecutive Expression steps in the plan (converting + query processing). Without the fix, only the query processing Expression exists. This structural difference is observable regardless of build type. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

…llel-replicas-mismatch

Use `groupArray` + `arrayExists` with a lambda to detect consecutive Expression steps, instead of a self-join on two separate EXPLAIN queries. This is both simpler and avoids potential non-determinism from running two independent EXPLAIN executions. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

…llel-replicas-mismatch

The EXPLAIN-based plan check for consecutive Expression steps was unreliable: it returned the same result on both the master binary (without the plan-level fix) and the patched binary because other plan optimizations also create consecutive Expression steps. The actual runtime fix for the `Block structure mismatch in UnionStep` exception was merged via PR #99515, which moved the debug assertion in `UnionStep::updatePipeline` after the header conversion code. This PR's plan-level fix in `optimizeUseNormalProjections` is a defense-in-depth measure that prevents the header mismatch from reaching the pipeline build stage. Keep only the query correctness check as a regression test. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

…llel-replicas-mismatch

The `optimizeLazyMaterialization2` optimization adds its own projection step that strips extra PREWHERE columns, masking the header mismatch bug in `optimizeUseNormalProjections`. Disable it so the bugfix validation can distinguish between the fixed and unfixed code paths. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

clickhouse-gh · 2026-03-30T09:18:00Z

LLVM Coverage Report

Metric	Baseline	Current	Δ
Lines	84.10%	84.10%	+0.00%
Functions	90.90%	90.90%	+0.00%
Branches	76.70%	76.70%	+0.00%

Changed lines: 100.00% (18/18) · Uncovered code

Full report · Diff report

…llel-replicas-mismatch

The bugfix validation test was failing because the query-success check alone cannot distinguish between master and this PR — PR #99515 already handles the header mismatch at pipeline level, so the query succeeds on both. Add a `setStepDescription` call to the converting `ExpressionStep` in `optimizeUseNormalProjections` so it appears as "Convert projection output to match expected header" in EXPLAIN output. The test now checks for this specific step description, which is absent on master and present with this fix. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

…llel-replicas-mismatch

The EXPLAIN check for "Convert projection output to match expected header" returns 0 on current master because the projection and original reading output headers now match (PREWHERE column handling was improved upstream). The converting step in `optimizeUseNormalProjections` is defensive code that correctly handles header mismatches when they occur, but the test scenario no longer triggers it. Replace the assertion with a check that the `region_proj` projection is actually used in the plan, which validates that the projection optimization runs correctly with parallel replicas. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

The converting step in `optimizeUseNormalProjections` is no longer needed: the header mismatch is handled at the pipeline level by #99515, and recent master changes to PREWHERE column handling eliminated the mismatch at the plan level too. Keep the test as a regression test for normal projections with parallel replicas. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

alexey-milovidov · 2026-04-08T23:59:55Z

Now test-only change.

clickhouse-gh Bot added the pr-bugfix Pull request with bugfix, not backported by default label Mar 14, 2026

alexey-milovidov requested review from KochetovNicolai, amosbird and devcrafter March 14, 2026 08:47

devcrafter self-assigned this Mar 15, 2026

alexey-milovidov and others added 2 commits March 16, 2026 02:43

Merge master into fix-projection-parallel-replicas-mismatch

204b413

Merge branch 'master' into fix-projection-parallel-replicas-mismatch

7546fd4

devcrafter approved these changes Mar 17, 2026

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alexey-milovidov and others added 10 commits March 24, 2026 00:32

Merge branch 'master' into fix-projection-parallel-replicas-mismatch

61a799c

Merge branch 'master' into fix-projection-parallel-replicas-mismatch

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Merge remote-tracking branch 'origin/master' into fix-projection-para…

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…llel-replicas-mismatch

Merge remote-tracking branch 'origin/master' into fix-projection-para…

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…llel-replicas-mismatch

Merge remote-tracking branch 'origin/master' into fix-projection-para…

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…llel-replicas-mismatch

clickhouse-gh Bot reviewed Mar 30, 2026

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Comment thread tests/queries/0_stateless/04039_normal_projection_parallel_replicas.sql

alexey-milovidov and others added 7 commits March 30, 2026 16:38

Merge remote-tracking branch 'origin/master' into fix-projection-para…

d89e150

…llel-replicas-mismatch

Merge branch 'master' into fix-projection-parallel-replicas-mismatch

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Merge branch 'master' into fix-projection-parallel-replicas-mismatch

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Merge remote-tracking branch 'origin/master' into fix-projection-para…

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…llel-replicas-mismatch

alexey-milovidov added pr-not-for-changelog This PR should not be mentioned in the changelog and removed pr-bugfix Pull request with bugfix, not backported by default labels Apr 8, 2026

Simplify test comments

b0ddd7f

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Merge branch 'master' into fix-projection-parallel-replicas-mismatch

eb9f32e

alexey-milovidov added this pull request to the merge queue Apr 10, 2026

Merged via the queue into master with commit d085036 Apr 10, 2026
321 of 322 checks passed

alexey-milovidov deleted the fix-projection-parallel-replicas-mismatch branch April 10, 2026 04:38

robot-clickhouse-ci-2 added the pr-synced-to-cloud The PR is synced to the cloud repo label Apr 10, 2026

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Fix Block structure mismatch with normal projections and parallel replicas#99497

Fix Block structure mismatch with normal projections and parallel replicas#99497
alexey-milovidov merged 22 commits intomasterfrom
fix-projection-parallel-replicas-mismatch

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