[YSQL] Add support for ALTER TABLE ALTER COLUMN TYPE

[fizaaluthra]

Jira Link: DB-663

This issue tracks support for ALTER TABLE ALTER COLUMN TYPE that require on-disk changes.

Note that we already support alter column type that do not require on-disk changes. See: [YSQL] Support ALTER COLUMN type that does not require on-disk changes #4424

[dobesv]

I guess the workaround currently would be to rename the column to a temporary name, create the new column with the correct type, run an UPDATE to copy/convert the old values to the new column, then DROP the old column.

[ajcaldera1]

Need this for Django support:

(.venv) $ python manage.py migrate
Operations to perform:
  Apply all migrations: admin, auth, contenttypes, sessions
Running migrations:
  Applying contenttypes.0001_initial... OK
  Applying auth.0001_initial... OK
  Applying admin.0001_initial... OK
  Applying admin.0002_logentry_remove_auto_add... OK
  Applying admin.0003_logentry_add_action_flag_choices... OK
  Applying contenttypes.0002_remove_content_type_name... OK
  Applying auth.0002_alter_permission_name_max_length...Traceback (most recent call last):
  File "/Users/acaldera/code/pyenv/.venv/lib/python3.8/site-packages/django/db/backends/utils.py", line 86, in _execute
    return self.cursor.execute(sql, params)
psycopg2.errors.FeatureNotSupported: ALTER TABLE ALTER column not supported yet
LINE 1: ALTER TABLE "auth_permission" ALTER COLUMN "name" TYPE varch...
                                      ^
HINT:  See https://github.com/YugaByte/yugabyte-db/issues/1124. Click '+' on the description to raise its priority

[kmuthukk]

Commit for issue #4424 addresses the most commonly used forms of this (such as the ones generated by Django framework) where the old and new column types are on-disk compatible.

[oneumyvakin]

Required for Laravel (Eloquent) support. Following statement

$table->string('field_name')->nullable()->change();

leads to

SQLSTATE[0A000]: Feature not supported: 7 ERROR:  This ALTER TABLE command is not yet supported. (SQL: ALTER TABLE table_name ALTER field_name TYPE VARCHAR(255))