If you have really wide table with many thousands of columns and you want to drop / modify column.
For wide parts it working fine, but for compact clickhouse would try to allocate a lot of memory and going to be killed via OOM killer.



**Does it reproduce on recent release?**

Yes, 21.9

**How to reproduce**
```
clickhouse-client -mn --query="SELECT 'CREATE TABLE xxx_really_wide( ' || arrayStringConcat(groupArray('column_'|| toString(number) || ' Nullable(UInt32)'), ',') || ') ENGINE=MergeTree ORDER BY assumeNotNull(column_0)' FROM numbers(6000) FORMAT TSVRaw" | clickhouse-client -mn

clickhouse-client -mn --query="SELECT arrayStringConcat(replicate('1', range(6000)), ',') FROM numbers(300) FORMAT TSVRaw" | clickhouse-client -mn --query "INSERT INTO xxx_really_wide FORMAT CSV " --send_logs_level='trace'
```

```
SELECT *
FROM system.merges
FORMAT Vertical

Query id: d9751847-a689-49f0-aab3-f868d9c08e34

Row 1:
──────
database:                    default
table:                       xxx_really_wide
elapsed:                     84.8952157
progress:                    0
num_parts:                   1
source_part_names:           ['all_1_1_0']
result_part_name:            all_1_1_0_3
source_part_paths:           ['/var/lib/clickhouse/data/default/xxx_really_wide/all_1_1_0/']
result_part_path:            /var/lib/clickhouse/data/default/xxx_really_wide/all_1_1_0_3/
partition_id:                all
is_mutation:                 1
total_size_bytes_compressed: 456027
total_size_marks:            2
bytes_read_uncompressed:     0
rows_read:                   0
bytes_written_uncompressed:  0
rows_written:                0
columns_written:             0
memory_usage:                16836533272
thread_id:                   16171
merge_type:
merge_algorithm:

Row 2:
──────
database:                    default
table:                       xxx_really_wide
elapsed:                     84.8947852
progress:                    0
num_parts:                   1
source_part_names:           ['all_2_2_0']
result_part_name:            all_2_2_0_3
source_part_paths:           ['/var/lib/clickhouse/data/default/xxx_really_wide/all_2_2_0/']
result_part_path:            /var/lib/clickhouse/data/default/xxx_really_wide/all_2_2_0_3/
partition_id:                all
is_mutation:                 1
total_size_bytes_compressed: 468027
total_size_marks:            2
bytes_read_uncompressed:     0
rows_read:                   0
bytes_written_uncompressed:  0
rows_written:                0
columns_written:             0
memory_usage:                16706294896
thread_id:                   16174
merge_type:
merge_algorithm:
```

for (const auto & kv : prepared_stages[i].column_to_updated)
            prepared_stages[i].output_columns.insert(kv.first);
    }
    
    
    for (size_t i = prepared_stages.size() - 1; i > 0; --i)
    {
        auto & stage = prepared_stages[i];
        ASTPtr all_asts = std::make_shared<ASTExpressionList>();
        for (const auto & ast : stage.filters)
            all_asts->children.push_back(ast);
        for (const auto & kv : stage.column_to_updated)
            all_asts->children.push_back(kv.second);
        
        for (const String & column : stage.output_columns)
            all_asts->children.push_back(std::make_shared<ASTIdentifier>(column));
        
        
        bool execute_scalar_subqueries = !dry_run;
        auto syntax_result = TreeRewriter(context).analyze(
            all_asts, all_columns, storage, storage_snapshot,
            false, true, execute_scalar_subqueries);
        if (execute_scalar_subqueries && context->hasQueryContext())
            for (const auto & it : syntax_result->getScalars())
                context->getQueryContext()->addScalar(it.first, it.second);
        stage.analyzer = std::make_unique<ExpressionAnalyzer>(all_asts, syntax_result, context);
        ExpressionActionsChain & actions_chain = stage.expressions_chain;
        for (const auto & ast : stage.filters)