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KochetovNicolai · KochetovNicolai · commit 12f81b320fbd · 2026-01-13T15:24:56.000Z
diff --git a/tests/integration/test_projection_rebuild_with_required_columns/test.py b/tests/integration/test_projection_rebuild_with_required_columns/test.py
@@ -19,14 +19,21 @@ def started_cluster():
 
 
 def test_projection_rebuild_uses_only_required_columns(started_cluster):
+    # Here we check that projection rebuild does not create too many temporary parts.
+    # The size of temporary projection part is limited by min_insert_block_size_bytes/min_insert_block_size_rows, so they are changed in the config.
+
     node1.query("drop table if exists tab")
     node1.query("create table tab (x UInt64, y UInt64, data String codec(NONE), v UInt8, projection p (select _part_offset order by y)) engine = ReplacingMergeTree(v) order by x settings allow_part_offset_column_in_projections=1, deduplicate_merge_projection_mode='rebuild';")
+    # Here we expect 3 parts to be inserted, contrilled by max_block_size=min_insert_block_size_rows=10000
     node1.query("insert into tab select number, number, rightPad('', 100, 'a'), 0 from numbers(30000) settings max_block_size=10000;")
+    # Here we merge parts, and projections should be rebuild
+    # Initially we kept `data` column in projection squash, ~10 temporary parts were created by min_insert_block_size_bytes limit
     node1.query("optimize table tab final settings mutations_sync=2, alter_sync=2;")
     node1.query("system flush logs;")
 
     uuid = node1.query("select uuid from system.tables where table = 'tab';").strip()
     cnt = node1.query("select count() from system.text_log where query_id like '{}::all_%_2' and message like '%Reading%from part p_%from the beginning of the part%'".format(uuid))
+    # One projection part per source part
     assert (cnt == '3\n')
-
+    # Here we check that _parent_part_offset is calculated properly. It was fixed in https://github.com/ClickHouse/ClickHouse/pull/93827
     assert(node1.query("select min(_parent_part_offset), max(_parent_part_offset) from mergeTreeProjection(default, tab, 'p')") == '0\t29999\n')
