dialect/sql/schema: initial interface for postgres API

Summary: Pull Request resolved: https://github.com/facebookincubator/ent/pull/101

Reviewed By: alexsn

Differential Revision: D17977541

fbshipit-source-id: 118f935e11449661fd91f87b94dde2fb62c9b912

dialect/sql/schema/postgres.go

@@ -0,0 +1,65 @@
+// Copyright 2019-present Facebook Inc. All rights reserved.
+// This source code is licensed under the Apache 2.0 license found
+// in the LICENSE file in the root directory of this source tree.
+
+package schema
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/facebookincubator/ent/dialect"
+	"github.com/facebookincubator/ent/dialect/sql"
+)
+
+// Postgres is a postgres migration driver.
+type Postgres struct {
+	dialect.Driver
+	version string
+}
+
+// init loads the Postgres version from the database for later use in the migration process.
+// It returns an error if the server version is lower than v10.
+func (d *Postgres) init(ctx context.Context, tx dialect.Tx) error {
+	rows := &sql.Rows{}
+	if err := tx.Query(ctx, "SHOW server_version_num", []interface{}{}, rows); err != nil {
+		return fmt.Errorf("postgres: querying server version %v", err)
+	}
+	defer rows.Close()
+	if !rows.Next() {
+		return fmt.Errorf("postgres: server_version_num variable was not found")
+	}
+	var version string
+	if err := rows.Scan(&version); err != nil {
+		return fmt.Errorf("postgres: scanning version: %v", err)
+	}
+	if len(version) < 6 {
+		return fmt.Errorf("postgres: malformed version: %s", version)
+	}
+	d.version = fmt.Sprintf("%s.%s.%s", version[:2], version[2:4], version[4:])
+	if compareVersions(d.version, "10.0.0") == -1 {
+		return fmt.Errorf("postgres: unsupported version: %s", d.version)
+	}
+	return nil
+}
+
+// tableExist checks if a table exists in the database and current schema.
+func (d *Postgres) tableExist(ctx context.Context, tx dialect.Tx, name string) (bool, error) {
+	query, args := sql.Dialect(dialect.Postgres).
+		Select(sql.Count("*")).From(sql.Table("INFORMATION_SCHEMA.TABLES").Unquote()).
+		Where(sql.EQ("table_schema", sql.Raw("CURRENT_SCHEMA()")).And().EQ("table_name", name)).Query()
+	return exist(ctx, tx, query, args...)
+}
+
+// tableExist checks if a foreign-key exists in the current schema.
+func (d *Postgres) fkExist(ctx context.Context, tx dialect.Tx, name string) (bool, error) {
+	query, args := sql.Dialect(dialect.Postgres).
+		Select(sql.Count("*")).From(sql.Table("INFORMATION_SCHEMA.TABLE_CONSTRAINTS").Unquote()).
+		Where(sql.EQ("table_schema", sql.Raw("CURRENT_SCHEMA()")).And().EQ("constraint_type", "FOREIGN KEY").And().EQ("constraint_name", name)).Query()
+	return exist(ctx, tx, query, args...)
+}
+
+// setRange sets restart the identity column to the given offset. Used by the universal-id option.
+func (d *Postgres) setRange(ctx context.Context, tx dialect.Tx, name string, value int) error {
+	return tx.Exec(ctx, fmt.Sprintf("ALTER TABLE %s ALTER COLUMN id RESTART WITH %d", name, value), []interface{}{}, new(sql.Result))
+}