Optimize default value handling during type changes

[Copilot]

The default value handling logic unnecessarily dropped and re-added defaults for all type changes, even when PostgreSQL could implicitly convert types (e.g., integer → bigint).

Changes

• Modified internal/diff/column.go to evaluate needsUsingClause() once and use it to determine when defaults must be dropped/re-added
• DROP/SET DEFAULT now only executes when a USING clause is required (e.g., text → enum)
• Implicit conversions (integer → bigint) now preserve defaults automatically

Example

For a column changing from integer DEFAULT 1 to bigint DEFAULT 1:

Before:

ALTER TABLE t ALTER COLUMN id DROP DEFAULT;
ALTER TABLE t ALTER COLUMN id TYPE bigint;
ALTER TABLE t ALTER COLUMN id SET DEFAULT 1;

After:

ALTER TABLE t ALTER COLUMN id TYPE bigint;

For text DEFAULT 'pending' to action_type DEFAULT 'pending', the DROP/SET DEFAULT sequence is still correctly generated since USING is required.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[tianzhou]

LGTM