**Use case**

PostgreSQL is using `ALTER TABLE t ALTER COLUMN c TYPE type` instead of `ALTER TABLE t MODIFY COLUMN c type`.
There is a risk that some people already get used to PostgreSQL syntax.

Let's implement it in ClickHouse.

```
tutorial=# ALTER TABLE hits_100m_obfuscated MODIFY COLUMN UserAgentMinor TEXT
tutorial-# ;
ERROR:  syntax error at or near "MODIFY"
LINE 1: ALTER TABLE hits_100m_obfuscated MODIFY COLUMN UserAgentMino...
                                         ^
tutorial=# ALTER TABLE hits_100m_obfuscated ALTER COLUMN UserAgentMinor TYPE TEXT
;
ALTER TABLE
tutorial=#
```

**Proposed implementation**

Implement it in parser level.
if (s_in_partition.ignore(pos, expected))
                {
                    if (!parser_partition.parse(pos, command->partition, expected))
                        return false;
                }
                if (!s_where.ignore(pos, expected))
                    return false;
                if (!parser_exp_elem.parse(pos, command->predicate, expected))
                    return false;
                command->type = ASTAlterCommand::DELETE;
            }
            else if (s_update.ignore(pos, expected))
            {
                if (!parser_assignment_list.parse(pos, command->update_assignments, expected))
                    return false;
                if (s_in_partition.ignore(pos, expected))
                {
                    if (!parser_partition.parse(pos, command->partition, expected))
                        return false;
                }
                if (!s_where.ignore(pos, expected))
                    return false;
                if (!parser_exp_elem.parse(pos, command->predicate, expected))
                    return false;
                command->type = ASTAlterCommand::UPDATE;
            }
            else if (s_comment_column.ignore(pos, expected))
            {
                if (s_if_exists.ignore(pos, expected))
                    command->if_exists = true;