alter() on PostgreSQL cannot change an attribute: MODIFY is a syntax error, and a comment change is ignored #1625

Summary

On PostgreSQL, Table.alter() can add, drop and rename attributes, but it cannot change one:

• Changing an attribute's type, nullability or default raises a syntax error. _make_attribute_alter (declare.py) always emits MySQL's MODIFY <column definition>. PostgreSQL needs ALTER COLUMN ... TYPE ..., SET/DROP NOT NULL and SET/DROP DEFAULT.
• Changing only an attribute's comment does nothing, silently. On PostgreSQL the comment is not part of the column definition: it is applied with COMMENT ON COLUMN. So a comment-only change compares as unchanged, and alter() reports nothing to alter.

MySQL handles all five changes.

Reproduction

DataJoint master, PostgreSQL 15 and MySQL 8.0:

import datajoint as dj

schema = dj.Schema("scratch_alter_pg")


@schema
class Measurement(dj.Manual):
    definition = """
    measurement_id : int32
    ---
    value : float32   # the value
    """


Measurement.definition = """
measurement_id : int32
---
value : float64   # the value
"""
Measurement.alter(prompt=False)

Expected behavior

On PostgreSQL, alter() makes the same changes it makes on MySQL:

• a type change becomes ALTER COLUMN ... TYPE ..., with USING where the cast needs it, or a clear error when the backend cannot convert;
• a nullability change becomes SET/DROP NOT NULL;
• a default change becomes SET/DROP DEFAULT;
• a comment change becomes COMMENT ON COLUMN, with the core-type marker kept, so describe() still round-trips.

Repositioning a column has no PostgreSQL equivalent; it is already skipped there (supports_column_position).