Implement 'drop index' migrations (#51)

Add support for **drop index** migrations. A drop index migration looks
like this:

```json
{
  "name": "11_drop_index",
  "operations": [
    {
      "drop_index": {
        "name": "_pgroll_idx_users_name"
      }
    }
  ]
}
```

* `Start` is a no-op.
* On `Complete` the index is removed from the underlying table.
* `Rollback` is a no-op.

examples/11_drop_index.json

@@ -0,0 +1,10 @@
+{
+  "name": "11_drop_index",
+  "operations": [
+    {
+      "drop_index": {
+        "name": "_pgroll_idx_fruits_name"
+      }
+    }
+  ]
+}

pkg/migrations/errors.go

@@ -43,3 +43,11 @@ type IndexAlreadyExistsError struct {
 func (e IndexAlreadyExistsError) Error() string {
 	return fmt.Sprintf("index %q already exists", e.Name)
 }
+
+type IndexDoesNotExistError struct {
+	Name string
+}
+
+func (e IndexDoesNotExistError) Error() string {
+	return fmt.Sprintf("index %q does not exist", e.Name)
+}

pkg/migrations/op_common.go

@@ -17,6 +17,7 @@ const (
 	OpNameAddColumn   OpName = "add_column"
 	OpNameDropColumn  OpName = "drop_column"
 	OpNameCreateIndex OpName = "create_index"
+	OpNameDropIndex   OpName = "drop_index"
 )
 
 func TemporaryName(name string) string {
@@ -90,6 +91,9 @@ func (v *Operations) UnmarshalJSON(data []byte) error {
 		case OpNameCreateIndex:
 			item = &OpCreateIndex{}
 
+		case OpNameDropIndex:
+			item = &OpDropIndex{}
+
 		default:
 			return fmt.Errorf("unknown migration type: %v", opName)
 		}
@@ -140,6 +144,9 @@ func (v Operations) MarshalJSON() ([]byte, error) {
 		case *OpCreateIndex:
 			opName = OpNameCreateIndex
 
+		case *OpDropIndex:
+			opName = OpNameDropIndex
+
 		default:
 			panic(fmt.Errorf("unknown operation for %T", op))
 		}

pkg/migrations/op_drop_index.go

@@ -0,0 +1,42 @@
+package migrations
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+
+	"pg-roll/pkg/schema"
+)
+
+type OpDropIndex struct {
+	Name string `json:"name"`
+}
+
+var _ Operation = (*OpDropIndex)(nil)
+
+func (o *OpDropIndex) Start(ctx context.Context, conn *sql.DB, schemaName string, stateSchema string, s *schema.Schema) error {
+	// no-op
+	return nil
+}
+
+func (o *OpDropIndex) Complete(ctx context.Context, conn *sql.DB) error {
+	// drop the index concurrently
+	_, err := conn.ExecContext(ctx, fmt.Sprintf("DROP INDEX CONCURRENTLY IF EXISTS %s", o.Name))
+
+	return err
+}
+
+func (o *OpDropIndex) Rollback(ctx context.Context, conn *sql.DB) error {
+	// no-op
+	return nil
+}
+
+func (o *OpDropIndex) Validate(ctx context.Context, s *schema.Schema) error {
+	for _, table := range s.Tables {
+		_, ok := table.Indexes[o.Name]
+		if ok {
+			return nil
+		}
+	}
+	return IndexDoesNotExistError{Name: o.Name}
+}

pkg/migrations/op_drop_index_test.go

@@ -0,0 +1,68 @@
+package migrations_test
+
+import (
+	"database/sql"
+	"testing"
+
+	"pg-roll/pkg/migrations"
+)
+
+func TestDropIndex(t *testing.T) {
+	t.Parallel()
+
+	idxName := migrations.GenerateIndexName("users", []string{"name"})
+
+	ExecuteTests(t, TestCases{{
+		name: "drop index",
+		migrations: []migrations.Migration{
+			{
+				Name: "01_add_table",
+				Operations: migrations.Operations{
+					&migrations.OpCreateTable{
+						Name: "users",
+						Columns: []migrations.Column{
+							{
+								Name:       "id",
+								Type:       "serial",
+								PrimaryKey: true,
+							},
+							{
+								Name:     "name",
+								Type:     "varchar(255)",
+								Nullable: false,
+							},
+						},
+					},
+				},
+			},
+			{
+				Name: "02_create_index",
+				Operations: migrations.Operations{
+					&migrations.OpCreateIndex{
+						Table:   "users",
+						Columns: []string{"name"},
+					},
+				},
+			},
+			{
+				Name: "03_drop_index",
+				Operations: migrations.Operations{
+					&migrations.OpDropIndex{
+						Name: idxName,
+					},
+				},
+			},
+		},
+		afterStart: func(t *testing.T, db *sql.DB) {
+			// The index has not yet been dropped.
+			IndexMustExist(t, db, "public", "users", idxName)
+		},
+		afterRollback: func(t *testing.T, db *sql.DB) {
+			// Rollback is a no-op.
+		},
+		afterComplete: func(t *testing.T, db *sql.DB) {
+			// The index has been removed from the underlying table.
+			IndexMustNotExist(t, db, "public", "users", idxName)
+		},
+	}})
+}