DuckDB Postgres extension

The Postgres extension allows DuckDB to directly read and write data from a running Postgres database instance. The data can be queried directly from the underlying Postgres database. Data can be loaded from Postgres tables into DuckDB tables, or vice versa.

Reading Data from Postgres

To make a Postgres database accessible to DuckDB use the ATTACH command:

ATTACH 'dbname=postgresscanner' AS postgres_db (TYPE postgres);
USE postgres_db;

The ATTACH command takes as input a libpq connection string - which is a set of key=value pairs separated by spaces. Below are some example connection strings and commonly used parameters. A full list of available parameters can be found in the Postgres documentation.

dbname=postgresscanner
host=localhost port=5432 dbname=mydb connect_timeout=10

Name	Description	Default
host	Name of host to connect to	localhost
hostaddr	Host IP address	localhost
port	Port Number	5432
user	Postgres User Name	[OS user name]
password	Postgres Password
dbname	Database Name	[user]
passfile	Name of file passwords are stored in	~/.pgpass

The tables in the file can be read as if they were normal DuckDB tables, but the underlying data is read directly from Postgres at query time.

D SHOW TABLES;
┌───────────────────────────────────────┐
│                 name                  │
│                varchar                │
├───────────────────────────────────────┤
│ uuids                                 │
└───────────────────────────────────────┘
D SELECT * FROM uuids;
┌──────────────────────────────────────┐
│                  u                   │
│                 uuid                 │
├──────────────────────────────────────┤
│ 6d3d2541-710b-4bde-b3af-4711738636bf │
│ NULL                                 │
│ 00000000-0000-0000-0000-000000000001 │
│ ffffffff-ffff-ffff-ffff-ffffffffffff │
└──────────────────────────────────────┘

It might be desirable to create a copy of the Postgres databases in DuckDB to prevent the system from re-reading the tables from Postgres continuously, particularly for large tables.

Data can be copied over from Postgres to DuckDB using standard SQL, for example:

CREATE TABLE duckdb_table AS FROM postgres_db.postgres_tbl;

Writing Data to Postgres

In addition to reading data from Postgres, the extension allows you to create tables, ingest data into Postgres and make other modifications to a Postgres database using standard SQL queries.

This allows you to use DuckDB to, for example, export data that is stored in a Postgres database to Parquet, or read data from a Parquet file into Postgres.

Below is a brief example of how to create a new table in Postgres and load data into it.

ATTACH 'dbname=postgresscanner' AS postgres_db (TYPE postgres);
CREATE TABLE postgres_db.tbl(id INTEGER, name VARCHAR);
INSERT INTO postgres_db.tbl VALUES (42, 'DuckDB');

Many operations on Postgres tables are supported. All these operations directly modify the Postgres database, and the result of subsequent operations can then be read using Postgres. Note that if modifications are not desired, ATTACH can be run with the READ_ONLY property which prevents making modifications to the underlying database. For example:

ATTACH 'dbname=postgresscanner' AS postgres_db (TYPE postgres, READ_ONLY);

Below is a list of supported operations.

CREATE TABLE

CREATE TABLE postgres_db.tbl(id INTEGER, name VARCHAR);

INSERT INTO

INSERT INTO postgres_db.tbl VALUES (42, 'DuckDB');

SELECT

SELECT * FROM postgres_db.tbl;
┌───────┬─────────┐
│  id   │  name   │
│ int64 │ varchar │
├───────┼─────────┤
│    42 │ DuckDB  │
└───────┴─────────┘

COPY

COPY postgres_db.tbl TO 'data.parquet';
COPY postgres_db.tbl FROM 'data.parquet';

UPDATE

UPDATE postgres_db.tbl SET name='Woohoo' WHERE id=42;

DELETE

DELETE FROM postgres_db.tbl WHERE id=42;

ALTER TABLE

ALTER TABLE postgres_db.tbl ADD COLUMN k INTEGER;

DROP TABLE

DROP TABLE postgres_db.tbl;

CREATE VIEW

CREATE VIEW postgres_db.v1 AS SELECT 42;

CREATE SCHEMA/DROP SCHEMA

CREATE SCHEMA postgres_db.s1;
CREATE TABLE postgres_db.s1.integers(i int);
INSERT INTO postgres_db.s1.integers VALUES (42);
SELECT * FROM postgres_db.s1.integers;
┌───────┐
│   i   │
│ int32 │
├───────┤
│    42 │
└───────┘
DROP SCHEMA postgres_db.s1;

Transactions

CREATE TABLE postgres_db.tmp(i INTEGER);
BEGIN;
INSERT INTO postgres_db.tmp VALUES (42);
SELECT * FROM postgres_db.tmp;
┌───────┐
│   i   │
│ int64 │
├───────┤
│    42 │
└───────┘
ROLLBACK;
SELECT * FROM postgres_db.tmp;
┌────────┐
│   i    │
│ int64  │
├────────┤
│ 0 rows │
└────────┘

Settings

The extension exposes the following configuration parameters.

name	description	default
pg_debug_show_queries	DEBUG SETTING: print all queries sent to Postgres to stdout	false
pg_experimental_filter_pushdown	Whether or not to use filter pushdown (currently experimental)	false
pg_array_as_varchar	Read Postgres arrays as varchar - enables reading mixed dimensional arrays	false
pg_connection_limit	The maximum amount of concurrent Postgres connections	64
pg_pages_per_task	The amount of pages per task	1000
pg_use_binary_copy	Whether or not to use BINARY copy to read data	true

Building & Loading the Extension

To build, type

make

To run, run the bundled duckdb shell:

 ./build/release/duckdb -unsigned  # allow unsigned extensions

Then, load the Postgres extension like so:

LOAD 'build/release/extension/postgres_scanner/postgres_scanner.duckdb_extension';