Postgres Database Functions Cursor Rules for AI

postgres-database-functions.mdc
---
description: This rule provides guidelines for writing PostgreSQL database functions.
globs: **/*.sql
alwaysApply: false
---

# Postgres Database Functions rules

You're a Postgres expert in writing database functions. Generate **high-quality PostgreSQL functions** that adhere to the following best practices:

## General Guidelines

1. **Default to `SECURITY INVOKER`:**

   - Functions should run with the permissions of the user invoking the function, ensuring safer access control.
   - Use `SECURITY DEFINER` only when explicitly required and explain the rationale.

2. **Set the `search_path` Configuration Parameter:**

   - Always set `search_path` to an empty string (`set search_path = '';`).
   - This avoids unexpected behavior and security risks caused by resolving object references in untrusted or unintended schemas.
   - Use fully qualified names (e.g., `schema_name.table_name`) for all database objects referenced within the function.

3. **Adhere to SQL Standards and Validation:**
   - Ensure all queries within the function are valid PostgreSQL SQL queries and compatible with the specified context (ie. Supabase).

## Best Practices

1. **Minimize Side Effects:**

   - Prefer functions that return results over those that modify data unless they serve a specific purpose (e.g., triggers).

2. **Use Explicit Typing:**

   - Clearly specify input and output types, avoiding ambiguous or loosely typed parameters.

3. **Default to Immutable or Stable Functions:**

   - Where possible, declare functions as `IMMUTABLE` or `STABLE` to allow better optimization by PostgreSQL. Use `VOLATILE` only if the function modifies data or has side effects.

4. **Triggers (if Applicable):**
   - If the function is used as a trigger, include a valid `CREATE TRIGGER` statement that attaches the function to the desired table and event (e.g., `BEFORE INSERT`).

## Example Templates

### Simple Function with `SECURITY INVOKER`

```sql
create or replace function my_schema.hello_world()
returns text
language plpgsql
security invoker
set search_path = ''
as $$
begin
  return 'hello world';
end;
$$;
```

### Function with Parameters and Fully Qualified Object Names

```sql
create or replace function public.calculate_total_price(order_id bigint)
returns numeric
language plpgsql
security invoker
set search_path = ''
as $$
declare
  total numeric;
begin
  select sum(price * quantity)
  into total
  from public.order_items
  where order_id = calculate_total_price.order_id;

  return total;
end;
$$;
```

### Function as a Trigger

```sql
create or replace function my_schema.update_updated_at()
returns trigger
language plpgsql
security invoker
set search_path = ''
as $$
begin
  -- Update the "updated_at" column on row modification
  new.updated_at := now();
  return new;
end;
$$;

create trigger update_updated_at_trigger
before update on my_schema.my_table
for each row
execute function my_schema.update_updated_at();
```

### Function with Error Handling

```sql
create or replace function my_schema.safe_divide(numerator numeric, denominator numeric)
returns numeric
language plpgsql
security invoker
set search_path = ''
as $$
begin
  if denominator = 0 then
    raise exception 'Division by zero is not allowed';
  end if;

  return numerator / denominator;
end;
$$;
```

### Immutable Function for Better Optimization

```sql
create or replace function my_schema.full_name(first_name text, last_name text)
returns text
language sql
security invoker
set search_path = ''
immutable
as $$
  select first_name || ' ' || last_name;
$$;
```
Cursor Rules for Postgres Database Functions

Cursor Rules for
Postgres Database Functions
