PRQL in PostgreSQL!

PL/PRQL is a PostgreSQL extension that lets you write functions with PRQL. The extension supports PostgreSQL v12-16 on Linux and macOS.

What is PRQL?

PRQL (Pipelined Relational Query Language) is an open source query language for data manipulation and analysis that compiles to SQL. PRQL introduces a pipeline concept (similar to Unix pipes) that transforms data line-by-line. The sequential series of transformations reduces the complexity often encountered with nested SQL queries and makes your data manipulation logic easier to read and write.

Key features

• Write functions with PRQL - Useful for large analytical queries
• PRQL compiler - Useful for development and debugging
• Inline execution - Useful for prototyping and custom queries in ORMs

Functions

PRQL shines when your SQL queries becomes very long and complex. You can manage this complexity by porting your most impressive SQL incantations to PRQL functions, which can then be used in dashboards, business logic or other database code. For example:

create function match_stats(int) returns table(player text, kd_ratio float) as $$
  from matches
  filter match_id == $1
  group player (
    aggregate {
      total_kills = sum kills,
      total_deaths = sum deaths
    }
  )
  filter total_deaths > 0
  derive kd_ratio = total_kills / total_deaths
  select { player, kd_ratio }
$$ language plprql;

select * from match_stats(1001)
    
 player  | kd_ratio 
---------+----------
 Player1 |    0.625
 Player2 |      1.6
(2 rows)

PRQL Compiler

You can use prql_to_sql() to see the SQL statements that PostgreSQL executes under the hood. This function invokes the PRQL compiler and shows you the resulting SQL code. Using the example above:

select prql_to_sql('...'); -- statements above omitted for brevity

 prql_to_sql 
-------------
WITH table_0 AS (
  SELECT player, COALESCE(SUM(kills), 0) AS _expr_0, COALESCE(SUM(deaths), 0) AS _expr_1
  FROM matches
  WHERE match_id = $1
  GROUP BY player
)
SELECT player, _expr_0 / _expr_1 AS kd_ratio
FROM table_0
WHERE _expr_1 > 0
-- Generated by PRQL compiler version:0.11.1 (https://prql-lang.org)
(1 row)

Inline Execution

You can run PRQL code directly with the prql function. This is useful for e.g. custom queries in ORMs:

select prql('from matches | filter player == ''Player1''') 
as (id int, match_id int, round int, player text, kills int, deaths int) 
limit 2;

 id | match_id | round | player  | kills | deaths 
----+----------+-------+---------+-------+--------
  1 |     1001 |     1 | Player1 |     4 |      1
  3 |     1001 |     2 | Player1 |     1 |      7
(2 rows)
 
-- Same as above, but returns cursor
select prql('from matches | filter player == ''Player1''', 'player1_cursor');
fetch 2 from player1_cursor;

For more information on the design of the extension, see the design document.

For more information on PRQL, visit the PRQL website, playground or repository.

Note

PRQL supports select statements only. insert, update, and delete statements, and your other database code, will continue to live in vanilla SQL, ORMs, or other database frameworks.

Getting Started

You can install the PL/PRQL extension in four ways:

• Install Deb File: Download .deb file from releases page.
• Install From Source: Clone the repository and build the extension on your own machine.
• Run Dockerfile: Build a docker image with PostgreSQL and the extension.
• Run Shell Script: Download and run a shell script builds the extension on your own machine for you.

The instruction assume you use Ubuntu or Debian.

Install Deb File

Follow these steps to install PL/PRQL from one of the released deb files:

1. Download the deb file that matches your operating system from the Releases page.

2. Open a terminal and change to the directory where the .deb file was downloaded. Install the package with dpkg, e.g.:

   sudo dpkg -i plprql-0.1.0-postgresql-16-debian-bookworm-amd64.deb

3. If dpkg reports missing dependencies, run the following command to fix them:

   sudo apt-get install -f

This only requires that you have PostgreSQL installed on beforehand. Replace the major version of PostgreSQL in the deb's filename if needed. Supported versions are 12, 13, 14, 15, and 16.

Install From Source

PL/PRQL is built on top of the pgrx framework for writing PostgreSQL extensions in Rust. This framework comes with development tools that you need to install. Follow these steps to set up your development environment:

1. Install cargo.

   curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y

2. Install cargo-pgrx.

   cargo install --locked --version=0.11.3 cargo-pgrx

   The version of cargo-pgrx must match the version of pgrx in plprql/Cargo.toml.

3. Initialize pgrx for your system.

   cargo pgrx init --pg16 <PG16>

   where <PG16> is the path to your system installation's pg_config tool (typically /usr/bin/pg_config). Supported versions are PostgreSQL v12-16. You can also run cargo pgrx init and have pgrx download, install, and compile PostgreSQL v12-16. These installations are managed by pgrx and used for development and testing. Individual pgrx-managed installations can be installed using e.g. cargo pgrx init --pg16 download.

4. Clone this repository.

   git clone https://github.com/kaspermarstal/plprql

5. cd into root directory and install the extension to the PostgreSQL specified by the pg_config currently on your $PATH.

   cd plprql/plprql
   cargo pgrx install --release

   You can target a specific PostgreSQL installation by providing the path of another pg_config using the -c flag.

6. Fire up your system PostgreSQL installation and start writing functions right away! You can also try out PL/PRQL in an installation managed by pgrx:

   $ cargo pgrx run pg16
   psql> create extension plprql;
   psql> create function match_stats(int) 
         returns table(total_kills real, total_deaths real) as $$
           from rounds
           filter match_id == $1
           aggregate {
             total_kills = sum kills,
             total_deaths = sum deaths
           }
         $$ language plprql
   psql> select match_stats(1);

Run Dockerfile

The docker/plprql.Dockerfile builds the postgres:16-bookworm docker image with the extension installed. You run this Dockerfile on your own machine with the following commands:

curl --proto '=https' --tlsv1.2 -sSf https://raw.githubusercontent.com/kaspermarstal/plprql/main/docker/plprql.Dockerfile > plprql.Dockerfile
docker build --tag 'plprql' . -f plprql.Dockerfile

The dockerfile downloads a .deb file from the releases page and installs it into the official postgres:16-bookworm image.

You can quickly test that the extension is installed and works as expected:

CONTAINER_ID=$(docker run -d -e POSTGRES_HOST_AUTH_METHOD=trust plprql)
docker exec $CONTAINER_ID psql -U postgres -c "create extension plprql;"
docker exec $CONTAINER_ID psql -U postgres -c "select prql_to_sql1('from table')"

Run Shell Script

Run the following command to download and execute the shell script in scripts/install.sh:

curl --proto '=https' --tlsv1.2 -sSf https://raw.githubusercontent.com/kaspermarstal/plprql/main/scripts/install.sh | bash

This will install the tip of the main branch using pg_config on your path.

You can customize the PostgreSQL installation and/or the PL/PRQL version using the --pg-config and --revision flags:

curl --proto '=https' --tlsv1.2 -sSf https://raw.githubusercontent.com/kaspermarstal/plprql/main/scripts/install.sh > install.sh
chmod +x ./install.sh
./install.sh --pg-version /usr/bin/pg_config --revision 186faea

You need the following packages for the shell script to run:

• A C compiler
• PostgreSQL and header files
• Rust, Cargo, and pgrx
• Utilities for the shell script (curl, wget, gnupg, lsb-release, git, jq)

You can install these dependencies with the following commands:

sudo apt-get update && apt-get upgrade
sudo apt-get install -y curl wget gnupg lsb-release git build-essential
sh -c 'echo "deb https://apt.postgresql.org/pub/repos/apt $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list'
wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | apt-key add -
sudo apt-get update
sudo apt-get install -y postgresql-16 postgresql-server-dev-16
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
source ~/.bashrc
cargo install --locked --version=0.11.3 cargo-pgrx
cargo pgrx init --pg16 $(which pg_config)

Running Tests

You can run tests using cargo pgrx test pg16. Unit tests are in the main plprql crate while integration tests are in the plprql-tests crate. From the root source directory:

cd plprql && echo "\q" | cargo pgrx run pg16 && cargo test --no-default-features --features pg16
cd ../plprql-tests && echo "\q" | cargo pgrx run pg16 && cargo test --no-default-features --features pg16

Supported PostgreSQL versions are pg12, pg13, pg14, pg15, and pg16.

License

Apache 2.0 License