sql/postgres: allow creating/modifying pk using existing index

sql/postgres/migrate.go

@@ -460,13 +460,21 @@ func (s *state) modifyTable(modify *schema.ModifyTable) error {
 
 type (
 	// AddUniqueConstraint to the table using the given index. Note, if the index
-	// name does not match the constraint name, PostgreSQL implicitly renames it to
-	// the constraint name.
+	// name does not match the unique constraint name, PostgreSQL implicitly renames
+	// it to the constraint name.
 	AddUniqueConstraint struct {
 		schema.Change
 		Name  string        // Name of the constraint.
 		Using *schema.Index // Index to use for the constraint.
 	}
+	// AddPKConstraint to the table using the given index. Note, if the index
+	// name does not match the primary-key constraint name, PostgreSQL implicitly
+	// renames it to the constraint name.
+	AddPKConstraint struct {
+		schema.Change
+		Name  string        // Name of the constraint.
+		Using *schema.Index // Index to use for the constraint.
+	}
 )
 
 // alterTable modifies the given table by executing on it a list of changes in one SQL statement.
@@ -514,6 +522,16 @@ func (s *state) alterTable(t *schema.Table, changes []schema.Change) error {
 				// Translated to the DROP CONSTRAINT below,
 				// which drops the index as well.
 				reverse = append(reverse, &schema.DropIndex{I: drop})
+			case *AddPKConstraint:
+				b.P("ADD CONSTRAINT").Ident(change.Name).P("PRIMARY KEY USING INDEX").Ident(change.Using.Name)
+				drop := change.Using
+				if drop.Name != change.Name {
+					drop = sqlx.P(*change.Using)
+					drop.Name = change.Name
+				}
+				// Translated to the DROP CONSTRAINT below,
+				// which drops the index as well.
+				reverse = append(reverse, &schema.DropIndex{I: drop})
 			case *schema.AddIndex:
 				b.P("ADD")
 				if err := s.unique(b, change.I); err != nil {

sql/postgres/migrate_test.go

@@ -486,6 +486,38 @@ func TestPlanChanges(t *testing.T) {
 				},
 			},
 		},
+		// Add a primary key using index.
+		{
+			changes: []schema.Change{
+				func() schema.Change {
+					users := schema.NewTable("users").
+						SetSchema(schema.New("test")).
+						AddColumns(
+							schema.NewIntColumn("id", "bigint"),
+						)
+					users.SetPrimaryKey(schema.NewPrimaryKey(users.Columns...))
+					return &schema.ModifyTable{
+						T: users,
+						Changes: []schema.Change{
+							&AddPKConstraint{
+								Name:  "users_pkey",
+								Using: schema.NewUniqueIndex("users_pkey"),
+							},
+						},
+					}
+				}(),
+			},
+			wantPlan: &migrate.Plan{
+				Reversible:    true,
+				Transactional: true,
+				Changes: []*migrate.Change{
+					{
+						Cmd:     `ALTER TABLE "test"."users" ADD CONSTRAINT "users_pkey" PRIMARY KEY USING INDEX "users_pkey"`,
+						Reverse: `ALTER TABLE "test"."users" DROP CONSTRAINT "users_pkey"`,
+					},
+				},
+			},
+		},
 		// Modify a primary key.
 		{
 			changes: []schema.Change{