Run tests on an isolated, temporary PostgreSQL database

bitbucket/overview | download 2.2


Get an interactive shell to a new database with a predefined schema

psql $uri -f schema.sql 
psql $uri


Tagwatch: using pg_tmp to facilitate application development

WEBM (31MB, vp8-vorbis) MP4 (27MB, h264-aac)

Theory and Operation

pg_tmp is a compact shell script designed to make unit testing, integration testing with PostgreSQL easy in any language.

Sometimes SQLite is used as drop-in replacement for PostgreSQL when running tests in order to make tests self-contained and therefore free of side-effects. This technique seems to suffice for simple interactions, but it does so by reducing the set of features that an application can use to the functionality common to both platforms. PostgreSQL is not a heavy weight key-value store it is more like a specialized programming environment.

The engineering genius of unit tests is mainly the ability they have to shape the design of the internal interfaces used to build an application. To ensure that we are testing the module and not the environment we mock APIs to external resources, but never the runtime or the language itself. An application built on Postgres is an application where where individual functions span more more than process—and more than one runtime. Testing concretely requires verification of methods as complete “units”.

To facilitate the creation of tests that run autonomously and concurrently two libraries have been developed to start up and tear down a database in the test code itself: Test-postgresql (Perl), and testing.postgresql (Python). Both of these are well-maintained and highly configurable, but they have some limitations:

  1. Each is tied tightly to it's implementation language and supporting libraries, making them cumbersome to integrate when the main project is not built with Python or Perl
  2. Using automatic setup and teardown within the library itself creates deadlocks if the programmer is not careful to use objects in the correct scope
  3. The instance of PostgreSQL that is created is not optimized for limited memory and low latency by default

pg_tmp is less configurable from the command line, is impervious to deadlocks, requires no external libraries, and is already tuned for the singular task of reducing the startup time to less than a second. This makes a full-featured PostgreSQL database useful for unit testing and other tests that require rapid feedback.

Performance Techniques

Total Wait Time Speedup Strategy
7.2 Naive in-process creation and teardown
6.4 0.8 Disabling runtime fsync ( pg_ctl -F )
6.0 0.4 Spinning to make the first connection (instead of pg_ctl -w )
2.2 3.8 Background database initialization ( initdb --nosync )
1 second 1.3 Shut down database asynchronously


While most PostgreSQL libraries support the URI syntax, some frameworks do not. The following are links to culpability shims that allow you to configure your application to accept a URI-formatted connection string.

Ruby (pg adapter) ruby-pg-url
Django dj-database-url

News & Discussion

April 19, 2016
Overcoming First Principles:
A guide for accessing the features of PostgreSQL in test-driven development
» video
» WEBM (640x360, 99MB) MP4 (640x360, 125M) MP4 (1280x768, 437MB)

November 17, 2015
Database testing in Python

July 29, 2015
Minimal postgres instance for testing in Java

Last updated on February 10, 2017
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