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PostgreSQL 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.See the official announcement for implementation details and background.

Installing and Loading

To install the postgres extension, run:

INSTALL postgres;

The extension is loaded automatically upon first use. If you prefer to load it manually, run:

LOAD postgres;

Connecting

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

-- connect to the "public" schema of the postgres instance running on localhost
ATTACH '' AS postgres_db (TYPE postgres);
-- connect to the Postgres instance with the given parameters
ATTACH 'dbname=postgres user=postgres host=127.0.0.1' AS db (TYPE postgres);

The ATTACH command takes as input either a libpq connection string or a PostgreSQL URI.

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
dbname Database name [user]
host Name of host to connect to localhost
hostaddr Host IP address localhost
passfile Name of file passwords are stored in ~/.pgpass
password Postgres password (empty)
port Port number 5432
user Postgres user name current user

An example URI is postgresql://username@hostname/dbname.

Postgres connection information can also be specified with environment variables. This can be useful in a production environment where the connection information is managed externally and passed in to the environment.

export PGPASSWORD="secret"
export PGHOST=localhost
export PGUSER=owner
export PGDATABASE=mydatabase
duckdb
ATTACH '' AS p (TYPE postgres);

Usage

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

SHOW TABLES;
┌───────────────────────────────────────┐
│                 name                  │
│                varchar                │
├───────────────────────────────────────┤
│ uuids                                 │
└───────────────────────────────────────┘
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 │
└───────┘
```sql
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 │
└────────┘

Running SQL Queries in Postgres with postgres_query

The postgres_query function allows you to run arbitrary SQL within an attached database. postgres_query takes the name of the attached Postgres database to execute the query in, as well as the SQL query to execute. The result of the query is returned. Single-quote strings are escaped by repeating the single quote twice.

postgres_query(attached_database::VARCHAR, query::VARCHAR)

Example

ATTACH 'dbname=postgresscanner' AS s (TYPE POSTGRES);
SELECT * FROM postgres_query('s', 'SELECT * FROM cars LIMIT 3');
┌──────────────┬───────────┬─────────┐
│    brand     │   model   │  color  │
│   varchar    │  varchar  │ varchar │
├──────────────┼───────────┼─────────┤
│ ferari       │ testarosa │ red     │
│ aston martin │ db2       │ blue    │
│ bentley      │ mulsanne  │ gray    │
└──────────────┴───────────┴─────────┘

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_connection_cache Whether or not to use the connection cache true
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

Schema Cache

To avoid having to continuously fetch schema data from Postgres, DuckDB keeps schema information - such as the names of tables, their columns, etc. - cached. If changes are made to the schema through a different connection to the Postgres instance, such as new columns being added to a table, the cached schema information might be outdated. In this case, the function pg_clear_cache can be executed to clear the internal caches.

CALL pg_clear_cache();

Deprecated The old postgres_attach function is deprecated. It is recommended to switch over to the new ATTACH syntax.