Merge pull request ClickHouse#91457 from ClickHouse/backport/25.8/91396

Backport ClickHouse#91396 to 25.8: Remove Sparse columns from header in merge algorithms

Author: Avogar

src/Processors/Merges/Algorithms/MergedData.cpp

@@ -1,6 +1,7 @@
 #include <Columns/IColumn.h>
 #include <Core/Block.h>
 #include <Processors/Merges/Algorithms/MergedData.h>
+#include <Columns/ColumnSparse.h>
 
 namespace DB
 {
@@ -27,6 +28,8 @@ void MergedData::initialize(const Block & header, const IMergingAlgorithm::Input
 
     for (size_t i = 0; i != columns.size(); ++i)
     {
+        /// Sometimes header can contain Sparse columns, we don't support Sparse in merge algorithms.
+        columns[i] = recursiveRemoveSparse(std::move(columns[i]))->assumeMutable();
         if (columns[i]->hasDynamicStructure())
             columns[i]->takeDynamicStructureFromSourceColumns(source_columns[i]);
     }

tests/queries/0_stateless/03751_summing_coalescing_merge_tree_sparse_columns_in_header.reference

@@ -0,0 +1,12 @@
+drop table if exists src;
+create table src (x UInt64) engine=MergeTree order by tuple();
+insert into src select 0 from numbers(1000000);
+drop table if exists dst;
+create table dst (x UInt64) engine=CoalescingMergeTree order by tuple();
+insert into dst select * from src;
+drop table dst;
+create table dst (x UInt64) engine=SummingMergeTree order by tuple();
+insert into dst select * from src;
+drop table dst;
+drop table src;
+