docs: Add documentation on PostgreSQL extension (#4124)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: eitsupi <50911393+eitsupi@users.noreply.github.com>

web/book/src/SUMMARY.md

@@ -96,6 +96,7 @@
   - [VS Code](./project/integrations/vscode.md)
   - [Rill](./project/integrations/rill.md)
   - [Syntax highlighting](./project/integrations/syntax-highlighting.md)
+  - [PostgreSQL](./project/integrations/postgresql.md)
 
 - [Contributing to PRQL](./project/contributing/README.md)
 

web/book/src/project/integrations/README.md

@@ -8,5 +8,6 @@ PRQL is building integrations with lots of external tools, including:
 - [Prefect](./prefect.md)
 - [VS Code](./vscode.md)
 - [Rill](./rill.md)
+- [PostgreSQL](./postgresql.md)
 
 We also have a CLI, [`prqlc`](./prqlc-cli.md)

web/book/src/project/integrations/postgresql.md

@@ -0,0 +1,49 @@
+# PostgreSQL
+
+PL/PRQL is a PostgreSQL extension that lets you write functions with PRQL.
+
+PL/PRQL functions serve as intermediaries, compiling the user's PRQL code into SQL statements that PostgreSQL executes. The extension is based on the [pgrx](https://github.com/pgcentralfoundation/pgrx) for developing PostgreSQL extensions in Rust. This framework manages the interaction with PostgreSQL's internal APIs, type conversions, and other function hooks necessary to integrate PRQL with PostgreSQL.
+
+
+## Examples
+PL/PRQL functions are defined using the `plprql` language specifier:
+```sql
+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)
+```
+
+You can also run PRQL directly with the `prql` function which is useful for custom SQL in ORMs:
+
+```sql
+select prql('from matches | filter player == ''Player1''', 'player1_cursor');
+
+fetch 2 from player1_cursor;
+
+ id | match_id | round | player  | kills | deaths
+----+----------+-------+---------+-------+--------
+  1 |     1001 |     1 | Player1 |     4 |      1
+  3 |     1001 |     2 | Player1 |     1 |      7
+(2 rows)
+```
+
+## Getting Started
+For installation instructions and more information on the extension, see the [PL/PRQL repository](https://github.com/kaspermarstal/plprql).

web/website/content/_index.md

@@ -194,6 +194,10 @@ integrations_section:
       link: https://github.com/ywelsch/duckdb-prql
       text: A DuckDB extension to execute PRQL
 
+    - label: "PostgreSQL"
+      link: https://github.com/kaspermarstal/plprql
+      text: Write PRQL functions in PostgreSQL
+
 bindings_section:
   enable: true
   title: "Bindings"