fix

src/Processors/Transforms/MergeJoinTransform.cpp

@@ -9,7 +9,6 @@
 #include <base/defines.h>
 #include <base/types.h>
 
-#include <Common/logger_useful.h>
 #include <Columns/ColumnNullable.h>
 #include <Columns/ColumnsNumber.h>
 #include <Columns/IColumn.h>
@@ -19,6 +18,7 @@
 #include <Interpreters/FullSortingMergeJoin.h>
 #include <Interpreters/TableJoin.h>
 #include <Parsers/ASTTablesInSelectQuery.h>
+#include <Processors/Chunk.h>
 #include <Processors/Transforms/MergeJoinTransform.h>
 
 
@@ -260,6 +260,7 @@ void FullMergeJoinCursor::setChunk(Chunk && chunk)
         return;
     }
 
+    convertToFullIfSparse(chunk);
     current_chunk = std::move(chunk);
     cursor = SortCursorImpl(sample_block, current_chunk.getColumns(), desc);
 }

tests/queries/0_stateless/03174_merge_join_bug.reference

@@ -0,0 +1,10 @@
+0	0
+1	1
+2	2
+3	3
+4	4
+5	5
+6	6
+7	7
+8	8
+9	9

tests/queries/0_stateless/03174_merge_join_bug.sql

@@ -0,0 +1,10 @@
+-- Tags: no-random-settings
+
+-- https://github.com/ClickHouse/ClickHouse/issues/24395
+SET allow_experimental_analyzer=1, join_algorithm = 'full_sorting_merge';
+CREATE TABLE xxxx_yyy (key UInt32, key_b ALIAS key) ENGINE=MergeTree() ORDER BY key SETTINGS ratio_of_defaults_for_sparse_serialization=0.0;
+INSERT INTO xxxx_yyy SELECT number FROM numbers(10);
+
+SELECT *
+FROM xxxx_yyy AS a
+INNER JOIN xxxx_yyy AS b ON a.key = b.key_b;