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ref/pgsql

ref/pgsql

Migrating a PostgreSQL Database to PostgreSQL

This command instructs pgloader to load data from a database connection. Automatic discovery of the schema is supported, including build of the indexes, primary and foreign keys constraints. A default set of casting rules are provided and might be overloaded and appended to by the command.

Here’s a short example of migrating a database from a PostgreSQL server to another:

load database
  from pgsql://localhost/pgloader
  into pgsql://localhost/copy

including only table names matching 'bits', ~/utilisateur/ in schema 'mysql'
including only table names matching ~/geolocations/ in schema 'public'
;

PostgreSQL Database Source Specification: FROM

Must be a connection URL pointing to a PostgreSQL database.

See the SOURCE CONNECTION STRING section above for details on how to write the connection string.

pgsql://[user[:password]@][netloc][:port][/dbname][?option=value&...]

PostgreSQL Database Migration Options: WITH

When loading from a PostgreSQL database, the following options are supported, and the default WITH clause is: no truncate, create schema, create tables, include drop, create indexes, reset sequences, foreign keys, downcase identifiers, uniquify index names, reindex.

  • include drop

    When this option is listed, pgloader drops all the tables in the target PostgreSQL database whose names appear in the MySQL database. This option allows for using the same command several times in a row until you figure out all the options, starting automatically from a clean environment. Please note that CASCADE is used to ensure that tables are dropped even if there are foreign keys pointing to them. This is precisely what include drop is intended to do: drop all target tables and recreate them.

    Great care needs to be taken when using include drop, as it will cascade to all objects referencing the target tables, possibly including other tables that are not being loaded from the source DB.

  • include no drop

    When this option is listed, pgloader will not include any DROP statement when loading the data.

  • truncate

    When this option is listed, pgloader issue the TRUNCATE command against each PostgreSQL table just before loading data into it.

  • no truncate

    When this option is listed, pgloader issues no TRUNCATE command.

  • disable triggers

    When this option is listed, pgloader issues an ALTER TABLE … DISABLE TRIGGER ALL command against the PostgreSQL target table before copying the data, then the command ALTER TABLE … ENABLE TRIGGER ALL once the COPY is done.

    This option allows loading data into a pre-existing table ignoring the foreign key constraints and user defined triggers and may result in invalid foreign key constraints once the data is loaded. Use with care.

  • create tables

    When this option is listed, pgloader creates the table using the meta data found in the MySQL file, which must contain a list of fields with their data type. A standard data type conversion from DBF to PostgreSQL is done.

  • create no tables

    When this option is listed, pgloader skips the creation of table before loading data, target tables must then already exist.

    Also, when using create no tables pgloader fetches the metadata from the current target database and checks type casting, then will remove constraints and indexes prior to loading the data and install them back again once the loading is done.

  • create indexes

    When this option is listed, pgloader gets the definitions of all the indexes found in the MySQL database and create the same set of index definitions against the PostgreSQL database.

  • create no indexes

    When this option is listed, pgloader skips the creating indexes.

  • drop indexes

    When this option is listed, pgloader drops the indexes in the target database before loading the data, and creates them again at the end of the data copy.

  • reindex

    When this option is used, pgloader does both drop indexes before loading the data and create indexes once data is loaded.

  • drop schema

    When this option is listed, pgloader drops the target schema in the target PostgreSQL database before creating it again and all the objects it contains. The default behavior doesn’t drop the target schemas.

  • foreign keys

    When this option is listed, pgloader gets the definitions of all the foreign keys found in the MySQL database and create the same set of foreign key definitions against the PostgreSQL database.

  • no foreign keys

    When this option is listed, pgloader skips creating foreign keys.

  • reset sequences

    When this option is listed, at the end of the data loading and after the indexes have all been created, pgloader resets all the PostgreSQL sequences created to the current maximum value of the column they are attached to.

    The options schema only and data only have no effects on this option.

  • reset no sequences

    When this option is listed, pgloader skips resetting sequences after the load.

    The options schema only and data only have no effects on this option.

  • downcase identifiers

    When this option is listed, pgloader converts all MySQL identifiers (table names, index names, column names) to downcase, except for PostgreSQL reserved keywords.

    The PostgreSQL reserved keywords are determined dynamically by using the system function pg_get_keywords().

  • quote identifiers

    When this option is listed, pgloader quotes all MySQL identifiers so that their case is respected. Note that you will then have to do the same thing in your application code queries.

  • schema only

    When this option is listed pgloader refrains from migrating the data over. Note that the schema in this context includes the indexes when the option create indexes has been listed.

  • data only

    When this option is listed pgloader only issues the COPY statements, without doing any other processing.

  • rows per range

    How many rows are fetched per SELECT query when using multiple readers per thread, see above for details.

PostgreSQL Database Casting Rules

The command CAST introduces user-defined casting rules.

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

A casting rule is expected to follow one of the forms:

type <type-name> [ <guard> ... ] to <pgsql-type-name> [ <option> ... ]
column <table-name>.<column-name> [ <guards> ] to ...

It’s possible for a casting rule to either match against a PostgreSQL data type or against a given column name in a given table name. So it’s possible to migrate a table from a PostgreSQL database while changing and int column to a bigint one, automatically.

The casting rules are applied in order, the first match prevents following rules to be applied, and user defined rules are evaluated first.

The supported guards are:

  • when default ‘value’

    The casting rule is only applied against MySQL columns of the source type that have given value, which must be a single-quoted or a double-quoted string.

  • when typemod expression

    The casting rule is only applied against MySQL columns of the source type that have a typemod value matching the given typemod expression. The typemod is separated into its precision and scale components.

    Example of a cast rule using a typemod guard:

    type char when (= precision 1) to char keep typemod

    This expression casts MySQL char(1) column to a PostgreSQL column of type char(1) while allowing for the general case char(N) will be converted by the default cast rule into a PostgreSQL type varchar(N).

  • with extra auto_increment

    The casting rule is only applied against PostgreSQL attached to a sequence. This can be the result of doing that manually, using a serial or a bigserial data type, or an identity column.

The supported casting options are:

  • drop default, keep default

    When the option drop default is listed, pgloader drops any existing default expression in the MySQL database for columns of the source type from the CREATE TABLE statement it generates.

    The spelling keep default explicitly prevents that behaviour and can be used to overload the default casting rules.

  • drop not null, keep not null, set not null

    When the option drop not null is listed, pgloader drops any existing NOT NULL constraint associated with the given source MySQL datatype when it creates the tables in the PostgreSQL database.

    The spelling keep not null explicitly prevents that behaviour and can be used to overload the default casting rules.

    When the option set not null is listed, pgloader sets a NOT NULL constraint on the target column regardless whether it has been set in the source MySQL column.

  • drop typemod, keep typemod

    When the option drop typemod is listed, pgloader drops any existing typemod definition (e.g. precision and scale) from the datatype definition found in the MySQL columns of the source type when it created the tables in the PostgreSQL database.

    The spelling keep typemod explicitly prevents that behaviour and can be used to overload the default casting rules.

  • using

    This option takes as its single argument the name of a function to be found in the pgloader.transforms Common Lisp package. See above for details.

    It’s possible to augment a default cast rule (such as one that applies against ENUM data type for example) with a transformation function by omitting entirely the type parts of the casting rule, as in the following example:

    column enumerate.foo using empty-string-to-null

PostgreSQL Views Support

PostgreSQL views support allows pgloader to migrate view as if they were base tables. This feature then allows for on-the-fly transformation of the source schema, 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 PostgreSQL rather than asking the user to specify the list.

PostgreSQL Partial Migration

INCLUDING ONLY TABLE NAMES MATCHING

Introduce a comma separated list of table names or regular expression used to limit the tables to migrate to a sublist.

Example:

including only table names matching ~/film/, 'actor' in schema 'public'

EXCLUDING TABLE NAMES MATCHING

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

excluding table names matching ~<ory> in schema 'public'

PostgreSQL Schema Transformations

ALTER TABLE NAMES MATCHING

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 'public'
 SET SCHEMA 'mv'

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

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

ALTER TABLE NAMES MATCHING ~/./ IN SCHEMA 'public' SET TABLESPACE 'pg_default'

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.

PostgreSQL Migration: limitations

The only PostgreSQL objects supported at this time in pgloader are extensions, schema, tables, indexes and constraints. Anything else is ignored.

  • Views are not migrated,

    Supporting views might require implementing a full SQL parser for the MySQL dialect with a porting engine to rewrite the SQL against PostgreSQL, including renaming functions and changing some constructs.

    While it’s not theoretically impossible, don’t hold your breath.

  • Triggers are not migrated

    The difficulty of doing so is not yet assessed.

  • Stored Procedures and Functions are not migrated.

Default PostgreSQL Casting Rules

When migrating from PostgreSQL the following Casting Rules are provided:

type int with extra auto_increment to serial
type bigint with extra auto_increment to bigserial
type "character varying" to text drop typemod