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PostgreSQL driver for Elixir.



Add Postgrex as a dependency in your mix.exs file.

def deps do
  [{:postgrex, "~> 0.8"} ]

After you are done, run mix deps.get in your shell to fetch and compile Postgrex. Start an interactive Elixir shell with iex -S mix.

iex> {:ok, pid} = Postgrex.Connection.start_link(hostname: "localhost", username: "postgres", password: "postgres", database: "postgres")
{:ok, #PID<0.69.0>}
iex> Postgrex.Connection.query!(pid, "SELECT user_id, text FROM comments", [])
%Postgrex.Result{command: :select, empty?: false, columns: ["user_id", "text"], rows: [[3,"hey"],[4,"there"]], size: 2}}
iex> Postgrex.Connection.query!(pid, "INSERT INTO comments (user_id, text) VALUES (10, 'heya')", [])
%Postgrex.Result{command: :insert, columns: nil, rows: nil, num_rows: 1}}


Data representation

PostgreSQL      Elixir
----------      ------
NULL            nil
bool            true | false
char            "é"
int             42
float           42.0
text            "eric"
bytea           <<42>>
numeric         #Decimal<42.0> *
date            %Postgrex.Date{year: 2013, month: 10, day: 12}
time            %Postgrex.Time{hour: 0, min: 37, sec: 14, msec: 0}
timestamp(tz)   %Postgrex.Timestamp{year: 2013 month: 10, day: 12, hour: 0, min: 37, sec: 14, msec: 0}
interval        %Postgrex.Interval{months: 14, days: 40, secs: 10920}
array           [1, 2, 3]
composite type  {42, "title", "content"}
range           %Postgrex.Range{lower: 1, upper: 5}
uuid            <<160,238,188,153,156,11,78,248,187,109,107,185,189,56,10,17>>
hstore          %{"foo" => "bar"}
oid types       42

* Decimal


Extensions are used to extend Postgrex’ built-in type encoding/decoding.

Here is a JSON extension that supports encoding/decoding Elixir maps to the Postgres’ JSON type.

To use the extension pass it to the connection as seen below:

Postgrex.Connection.start_link(extensions: [{Postgrex.Extensions.JSON, library: Poison}], ...)

OID type encoding

PostgreSQL’s wire protocol supports encoding types either as text or as binary. Unlike most client libraries Postgrex uses the binary protocol, not the text protocol. This allows for efficient encoding of types (e.g. 4-byte integers are encoded as 4 bytes, not as a string of digits) and automatic support for arrays and composite types.

Unfortunately the PostgreSQL binary protocol transports OID types as integers while the text protocol transports them as string of their name, if one exists, and otherwise as integer.

This means you either need to supply oid types as integers or perform an explicit cast (which would be automatic when using the text protocol) in the query.

# Fails since $1 is regclass not text.
query("select nextval($1)", ["some_sequence"])

# Perform an explicit cast, this would happen automatically when using a
# client library that uses the text protocol.
query("select nextval($1::text::regclass)", ["some_sequence"])

# Determine the oid once and store it for later usage. This is the most
# efficient way, since PostgreSQL only has to perform the lookup once. Client
# libraries using the text protocol do not support this.
%{rows: [{sequence_oid}]} = query("select $1::text::regclass", ["some_sequence"])
query("select nextval($1)", [sequence_oid])


To contribute you need to compile Postgrex from source and test it:

$ git clone
$ cd postgrex
$ mix test

The tests requires some modifications to your hba file. The path to it can be found by running $ psql -U postgres -c "SHOW hba_file" in your shell. Put the following above all other configurations (so that they override):

host    all             postgrex_md5_pw    md5
host    all             postgrex_cleartext_pw    password

The server needs to be restarted for the changes to take effect. Additionally you need to setup a Postgres user with the same username as the local user and give it trust or ident in your hba file. Or you can export $PGUSER and $PGPASSWORD before running tests.

Testing hstore on 9.0

Postgres versions 9.0 does not have the CREATE EXTENSION commands. This means we have to locate the postgres installation and run the hstore.sql in contrib to install hstore. Below is an example command to test 9.0 on OS X with homebrew installed postgres:

$ PGVERSION=9.0 PGPATH=/usr/local/share/postgresql9/ mix test


Copyright 2013 Eric Meadows-Jönsson

Licensed under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.