Add regression test for UnionStep block structure mismatch with PREWHERE and projections

alexey-milovidov · claude · alexey-milovidov · commit 21e09b2df950 · 2026-03-21T10:47:35.000Z
This test exercises the PREWHERE + projection + `query_plan_remove_unused_columns = 0`
path that caused a debug assertion failure in `UnionStep::updatePipeline`.

Co-Authored-By: Claude Opus 4.6 (1M context) &lt;noreply@anthropic.com&gt;
diff --git a/tests/queries/0_stateless/04040_union_block_structure_prewhere_projection.reference b/tests/queries/0_stateless/04040_union_block_structure_prewhere_projection.reference
@@ -0,0 +1 @@
+https://example.com/page1
diff --git a/tests/queries/0_stateless/04040_union_block_structure_prewhere_projection.sql b/tests/queries/0_stateless/04040_union_block_structure_prewhere_projection.sql
@@ -0,0 +1,40 @@
+-- Regression test for Block structure mismatch in UnionStep with PREWHERE and projections.
+-- The debug assertion in UnionStep::updatePipeline fired before the header conversion code
+-- could fix the mismatch, causing an exception in debug/sanitizer builds.
+-- Triggered by PREWHERE optimization adding extra pass-through columns to ReadFromMergeTree
+-- output that are not consumed by the expression DAG above.
+-- https://github.com/ClickHouse/ClickHouse/issues/96131
+
+SET enable_analyzer = 1;
+SET parallel_replicas_local_plan = 1;
+SET optimize_aggregation_in_order = 0;
+SET query_plan_remove_unused_columns = 0;
+SET min_table_rows_to_use_projection_index = 1000000;
+
+DROP TABLE IF EXISTS test_union_prewhere_proj;
+CREATE TABLE test_union_prewhere_proj
+(
+    id UInt64,
+    event_date Date,
+    user_id UInt32,
+    url String,
+    region String,
+    PROJECTION region_url_proj
+    (
+        SELECT _part_offset ORDER BY region, url
+    )
+)
+ENGINE = MergeTree
+ORDER BY (event_date, id)
+SETTINGS
+    index_granularity = 1, min_bytes_for_wide_part = 0,
+    min_bytes_for_full_part_storage = 0, enable_vertical_merge_algorithm = 0;
+
+INSERT INTO test_union_prewhere_proj VALUES (1, '2023-01-01', 101, 'https://example.com/page1', 'europe');
+INSERT INTO test_union_prewhere_proj VALUES (2, '2023-01-01', 102, 'https://example.com/page2', 'us_west');
+
+OPTIMIZE TABLE test_union_prewhere_proj FINAL;
+
+SELECT url FROM test_union_prewhere_proj WHERE region = 'europe' ORDER BY ALL;
+
+DROP TABLE test_union_prewhere_proj;

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	`+https://example.com/page1`