Migrating a MS SQL Database to PostgreSQL

This command instructs pgloader to load data from a MS SQL database. Automatic discovery of the schema is supported, including build of the indexes, primary and foreign keys constraints.

Here’s an example:

load database
     from mssql://user@host/dbname
     into postgresql:///dbname

including only table names like 'GlobalAccount' in schema 'dbo'

set work_mem to '16MB', maintenance_work_mem to '512 MB'

before load do $$ drop schema if exists dbo cascade; $$;

The mssql command accepts the following clauses and options.

MS SQL Database Source Specification: FROM

Connection string to an existing MS SQL database server that listens and welcome external TCP/IP connection. As pgloader currently piggybacks on the FreeTDS driver, to change the port of the server please export the TDSPORT environment variable.

MS SQL Database Migration Options: WITH

When loading from a MS SQL database, the same options as when loading a MS SQL database are supported. Please refer to the MS SQL section. The following options are added:

  • create schemas

    When this option is listed, pgloader creates the same schemas as found on the MS SQL instance. This is the default.

  • create no schemas

    When this option is listed, pgloader refrains from creating any schemas at all, you must then ensure that the target schema do exist.

MS SQL Database Casting Rules

CAST

The cast clause allows to specify custom casting rules, either to overload the default casting rules or to amend them with special cases.

Please refer to the MS SQL CAST clause for details.

MS SQL Views Support

MS SQL views support allows pgloader to migrate view as if they were base tables. This feature then allows for on-the-fly transformation from MS SQL to PostgreSQL, as the view definition is used rather than the base data.

MATERIALIZE VIEWS

This clause allows you to implement custom data processing at the data source by providing a view definition against which pgloader will query the data. It’s not possible to just allow for plain SQL because we want to know a lot about the exact data types of each column involved in the query output.

This clause expect a comma separated list of view definitions, each one being either the name of an existing view in your database or the following expression:

*name* `AS` `$$` *sql query* `$$`

The name and the sql query will be used in a CREATE VIEW statement at the beginning of the data loading, and the resulting view will then be dropped at the end of the data loading.

MATERIALIZE ALL VIEWS

Same behaviour as MATERIALIZE VIEWS using the dynamic list of views as returned by MS SQL rather than asking the user to specify the list.

MS SQL Partial Migration

INCLUDING ONLY TABLE NAMES LIKE

Introduce a comma separated list of table name patterns used to limit the tables to migrate to a sublist. More than one such clause may be used, they will be accumulated together.

Example:

including only table names like 'GlobalAccount' in schema 'dbo'

EXCLUDING TABLE NAMES LIKE

Introduce a comma separated list of table name patterns used to exclude table names from the migration. This filter only applies to the result of the INCLUDING filter.

excluding table names matching 'LocalAccount' in schema 'dbo'

MS SQL Schema Transformations

ALTER SCHEMA ‘…’ RENAME TO ‘…’

Allows to rename a schema on the flight, so that for instance the tables found in the schema ‘dbo’ in your source database will get migrated into the schema ‘public’ in the target database with this command:

alter schema 'dbo' rename to 'public'

ALTER TABLE NAMES MATCHING … IN SCHEMA ‘…’

Introduce a comma separated list of table names or regular expressions that you want to target in the pgloader ALTER TABLE command. Available actions are SET SCHEMA, RENAME TO, and SET:

ALTER TABLE NAMES MATCHING ~/_list$/, 'sales_by_store', ~/sales_by/
  IN SCHEMA 'dbo'
 SET SCHEMA 'mv'

ALTER TABLE NAMES MATCHING 'film' IN SCHEMA 'dbo' RENAME TO 'films'

ALTER TABLE NAMES MATCHING ~/./ IN SCHEMA 'dbo' SET (fillfactor='40')

ALTER TABLE NAMES MATCHING ~/./ IN SCHEMA 'dbo' SET TABLESPACE 'tlbspc'

You can use as many such rules as you need. The list of tables to be migrated is searched in pgloader memory against the ALTER TABLE matching rules, and for each command pgloader stops at the first matching criteria (regexp or string).

No ALTER TABLE command is sent to PostgreSQL, the modification happens at the level of the pgloader in-memory representation of your source database schema. In case of a name change, the mapping is kept and reused in the foreign key and index support.

The SET () action takes effect as a WITH clause for the CREATE TABLE command that pgloader will run when it has to create a table.

The SET TABLESPACE action takes effect as a TABLESPACE clause for the CREATE TABLE command that pgloader will run when it has to create a table.

The matching is done in pgloader itself, with a Common Lisp regular expression lib, so doesn’t depend on the LIKE implementation of MS SQL, nor on the lack of support for regular expressions in the engine.

MS SQL Driver setup and encoding

pgloader is using the FreeTDS driver, and internally expects the data to be sent in utf-8. To achieve that, you can configure the FreeTDS driver with those defaults, in the file ~/.freetds.conf:

[global]
    tds version = 7.4
    client charset = UTF-8

Default MS SQL Casting Rules

When migrating from MS SQL the following Casting Rules are provided:

Numbers:

type tinyint to smallint

type float to float   using float-to-string
type real to real     using float-to-string
type double to double precision     using float-to-string
type numeric to numeric     using float-to-string
type decimal to numeric     using float-to-string
type money to numeric     using float-to-string
type smallmoney to numeric     using float-to-string

Texts:

type char      to text drop typemod
type nchat     to text drop typemod
type varchar   to text drop typemod
type nvarchar  to text drop typemod
type xml       to text drop typemod

Binary:

type binary    to bytea using byte-vector-to-bytea
type varbinary to bytea using byte-vector-to-bytea

Date:

type datetime    to timestamptz
type datetime2   to timestamptz

Others:

type bit to boolean
type hierarchyid to bytea
type geography to bytea
type uniqueidentifier to uuid using sql-server-uniqueidentifier-to-uuid