## Code Analysis

### CASCADE is rejected during constraint-drop conversion
`server/ast/constraint_table_def.go:65-90`
```go
func nodeAlterTableDropConstraint(
	ctx *Context,
	node *tree.AlterTableDropConstraint,
	tableName vitess.TableName,
	ifExists bool) (*vitess.DDL, error) {

	if node.DropBehavior == tree.DropCascade {
		return nil, errors.Errorf("CASCADE is not yet supported for drop constraint")
	}

	return &vitess.DDL{
		Action:             "alter",
		Table:              tableName,
		IfExists:           ifExists,
		ConstraintAction:   "drop",
		ConstraintIfExists: node.IfExists,
		TableSpec: &vitess.TableSpec{
			Constraints: []*vitess.ConstraintDefinition{
				{Name: node.Constraint.String()},
			},
		},
	}, nil
}
```
The parser-side conversion explicitly returns an unsupported-feature error whenever the requested drop behavior is CASCADE. RESTRICT is not encoded in this conversion; it is emitted as a generic constraint-drop DDL.

### Unique constraints are delegated to index dropping
`server/analyzer/resolve_drop_constraint.go:75-110`
```go
if it, ok := table.(sql.IndexAddressable); ok {
	indexes, err := it.GetIndexes(ctx)
	if err != nil {
		return nil, transform.SameTree, err
	}
	for _, index := range indexes {
		if index.ID() != "PRIMARY" && index.IsUnique() && index.ID() == dropConstraint.Name {
			newNode, err := plan.NewAlterDropIndex(rt.Database(), rt, dropConstraint.IfExists, index.ID()).
				WithTargetSchema(rt.Schema(ctx))
			return newNode, transform.NewTree, err
		}
	}
}
```
The analyzer resolves a named non-primary unique constraint to an index-drop plan. This excerpt contains no dependent-foreign-key handling or translation to PostgreSQL's dependent-object SQLSTATE, which explains why the RESTRICT operation can surface the storage/index error instead of 2BP01.

### Result
The source path supports the reported outcomes: CASCADE is explicitly unsupported, while the RESTRICT path delegates the unique constraint to index handling without a visible dependency-error translation.

### Observed execution

#### qualified drop requests
```sql
ALTER TABLE ONLY auth.parent DROP CONSTRAINT parent_email_key RESTRICT;
ALTER TABLE ONLY auth.parent DROP CONSTRAINT parent_email_key CASCADE;
```

#### RESTRICT response
```text
ERROR:  cannot drop index: `parent_email_key` is used by foreign key `child_parent_email_fkey`
restrict_exit=1
```

#### state after RESTRICT
```text
 schema_name | table_name |         conname         | contype
-------------+------------+-------------------------+---------
 auth        | child      | child_parent_email_fkey | f
 auth        | child      | child_pkey              | p
 auth        | parent     | parent_email_key        | u
 auth        | parent     | parent_pkey             | p
(4 rows)
```

```text
 schemaname | tablename |        indexname
------------+-----------+-------------------------
 auth       | child     | child_parent_email_fkey
 auth       | child     | child_pkey
 auth       | parent    | parent_email_key
 auth       | parent    | parent_lower_email_idx
 auth       | parent    | parent_pkey
(5 rows)
```

The captured database evidence identifies the returned error as `XX000`, not the expected dependent-object code `2BP01`. The parent unique constraint, child foreign key, supporting indexes, and unrelated `parent_lower_email_idx` remained after the rejected operation.

#### CASCADE response
```text
ERROR:  CASCADE is not yet supported for drop constraint
cascade_exit=1
```

#### state after CASCADE
```text
 schema_name | table_name |         conname         | contype
-------------+------------+-------------------------+---------
 auth        | child      | child_parent_email_fkey | f
 auth        | child      | child_pkey              | p
 auth        | parent     | parent_email_key        | u
 auth        | parent     | parent_pkey             | p
(4 rows)
```

The index catalog also still contained `parent_lower_email_idx` and the supporting indexes after CASCADE returned its unsupported-feature error.

### Final result
REV-5 failed because RESTRICT rejected the dependent drop with `XX000` instead of `2BP01`, and CASCADE was unsupported; both operations preserved the existing constraints and indexes.