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 & stage : stages)
                {
                    stages_copy.emplace_back(context);
                    stages_copy.back().column_to_updated = stage.column_to_updated;
                    stages_copy.back().output_columns = stage.output_columns;
                    stages_copy.back().filters = stage.filters;
                }
                const ASTPtr select_query = prepareInterpreterSelectQuery(stages_copy,  true);
                InterpreterSelectQuery interpreter{
                    select_query, context, storage, metadata_snapshot,
                    SelectQueryOptions().analyze( false).ignoreLimits().ignoreProjections()};
                auto first_stage_header = interpreter.getSampleBlock();
                QueryPlan plan;
                auto source = std::make_shared<NullSource>(first_stage_header);
                plan.addStep(std::make_unique<ReadFromPreparedSource>(Pipe(std::move(source))));
                auto pipeline = addStreamsForLaterStages(stages_copy, plan);
                updated_header = std::make_unique<Block>(pipeline.getHeader());
            }
            
            stages.emplace_back(context);
            for (const auto & column : unchanged_columns)
                stages.back().column_to_updated.emplace(
                    column, std::make_shared<ASTIdentifier>(column));
        }
    }
    is_prepared = true;
    return prepareInterpreterSelectQuery(stages, dry_run);
}
ASTPtr MutationsInterpreter::prepareInterpreterSelectQuery(std::vector<Stage> & prepared_stages, bool dry_run)