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:
```

{
    
    
    
    
    auto select = std::make_shared<ASTSelectQuery>();
    select->setExpression(ASTSelectQuery::Expression::SELECT, std::make_shared<ASTExpressionList>());
    auto count_func = std::make_shared<ASTFunction>();
    count_func->name = "count";
    count_func->arguments = std::make_shared<ASTExpressionList>();
    select->select()->children.push_back(count_func);
    ASTs conditions;
    for (const MutationCommand & command : commands)
    {
        if (ASTPtr condition = getPartitionAndPredicateExpressionForMutationCommand(command, storage, context))
            conditions.push_back(std::move(condition));
    }
    if (conditions.size() > 1)
    {
        auto coalesced_predicates = makeASTFunction("or");
        coalesced_predicates->arguments->children = std::move(conditions);
        select->setExpression(ASTSelectQuery::Expression::WHERE, std::move(coalesced_predicates));
    }
    else if (conditions.size() == 1)
    {
        select->setExpression(ASTSelectQuery::Expression::WHERE, std::move(conditions.front()));
    }
    return select;
}
ColumnDependencies getAllColumnDependencies(const StorageMetadataPtr & metadata_snapshot, const NameSet & updated_columns)