# Introduction

pgloader loads data from various sources into PostgreSQL. It can transform the data it reads on the fly and submit raw SQL before and after the loading. It uses the COPY PostgreSQL protocol to stream the data into the server, and manages errors by filling a pair of reject.dat and reject.log files.

pgloader knows how to read data from different kind of sources:

> - Files

>   - CSV

>   - Fixed Format

>   - DBF

> - Databases

>   - SQLite

>   - MySQL

>   - MS SQL Server

>   - PostgreSQL

>   - Redshift

pgloader knows how to target different products using the PostgresQL Protocol:

> - PostgreSQL

> - Citus

> - Redshift

The level of automation provided by pgloader depends on the data source type. In the case of CSV and Fixed Format files, a full description of the expected input properties must be given to pgloader. In the case of a database, pgloader connects to the live service and knows how to fetch the metadata it needs directly from it.

## Features Matrix

Here’s a comparison of the features supported depending on the source database engine. Some features that are not supported can be added to pgloader, it’s just that nobody had the need to do so yet. Those features are marked with ✗. Empty cells are used when the feature doesn’t make sense for the selected source database.

Feature | SQLite | MySQL | MS SQL | PostgreSQL | Redshift
--- | --- | --- | --- | --- | ---
One-command migration | ✓ | ✓ | ✓ | ✓ | ✓
Continuous Migration | ✓ | ✓ | ✓ | ✓ | ✓
Schema discovery | ✓ | ✓ | ✓ | ✓ | ✓
Partial Migrations | ✓ | ✓ | ✓ | ✓ | ✓
Schema only | ✓ | ✓ | ✓ | ✓ | ✓
Data only | ✓ | ✓ | ✓ | ✓ | ✓
Repeatable (DROP+CREATE) | ✓ | ✓ | ✓ | ✓ | ✓
User defined casting rules | ✓ | ✓ | ✓ | ✓ | ✓
Encoding Overrides |  | ✓ |  |  |
On error stop | ✓ | ✓ | ✓ | ✓ | ✓
On error resume next | ✓ | ✓ | ✓ | ✓ | ✓
Pre/Post SQL commands | ✓ | ✓ | ✓ | ✓ | ✓
Post-Schema SQL commands | ✗ | ✓ | ✓ | ✓ | ✓
Primary key support | ✓ | ✓ | ✓ | ✓ | ✓
Foreign key support | ✓ | ✓ | ✓ | ✓ |
Online ALTER schema | ✓ | ✓ | ✓ | ✓ | ✓
Materialized views | ✗ | ✓ | ✓ | ✓ | ✓
Distribute to Citus | ✗ | ✓ | ✓ | ✓ | ✓

For more details about what the features are about, see the specific reference pages for your database source.

For some of the features, missing support only means that the feature is not needed for the other sources, such as the capability to override MySQL encoding metadata about a table or a column. Only MySQL in this list is left completely unable to guarantee text encoding. Or Redshift not having foreign keys.

## Commands

pgloader implements its own Command Language, a DSL that allows to specify every aspect of the data load and migration to implement. Some of the features provided in the language are only available for a specific source type.

## Command Line

The pgloader command line accepts those two variants:

    pgloader [<options>] [<command-file>]...
    pgloader [<options>] SOURCE TARGET

Either you have a command-file containing migration specifications in the pgloader Command Language, or you can give a Source for the data and a PostgreSQL database connection Target where to load the data into.