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Triggers are database functions that execute automatically when specific events occur on a table, such as INSERT, UPDATE, DELETE, or TRUNCATE operations. They’re powerful tools for maintaining data integrity, enforcing business rules, and automating workflows.

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How Triggers Work

A trigger consists of two parts:
  1. Trigger Function: A function that contains the code to execute
  2. Trigger Definition: Specifies when and how to call the function
-- Step 1: Create the trigger function
create or replace function trigger_function_name()
returns trigger
language plpgsql
as $$
begin
  -- Your trigger logic here
  return new;  -- or return old, or return null
end;
$$;

-- Step 2: Create the trigger
create trigger trigger_name
  after insert on table_name
  for each row
  execute function trigger_function_name();

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Trigger Timing

Triggers can fire at different times relative to the triggering event.

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BEFORE Triggers

Execute before the operation. Can modify the data being inserted/updated. 
-- Validate and transform data before insert
create or replace function validate_email()
returns trigger
language plpgsql
as $$
begin
  -- Convert email to lowercase
  new.email := lower(trim(new.email));
  
  -- Validate email format
  if new.email !~ '^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$' then
    raise exception 'Invalid email format: %', new.email;
  end if;
  
  return new;
end;
$$;

create trigger validate_user_email
  before insert or update on users
  for each row
  execute function validate_email();

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AFTER Triggers

Execute after the operation completes. Cannot modify the data. 
-- Log changes after they occur
create or replace function log_price_change()
returns trigger
language plpgsql
as $$
begin
  insert into price_history (product_id, old_price, new_price, changed_at)
  values (new.id, old.price, new.price, now());
  
  return new;
end;
$$;

create trigger track_price_changes
  after update on products
  for each row
  when (old.price is distinct from new.price)
  execute function log_price_change();

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Trigger Events

Triggers can respond to different types of operations.

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INSERT Trigger

-- Set default values or generate IDs
create or replace function set_user_defaults()
returns trigger
language plpgsql
as $$
begin
  -- Generate username if not provided
  if new.username is null then
    new.username := 'user_' || new.id;
  end if;
  
  -- Set default avatar
  if new.avatar_url is null then
    new.avatar_url := 'https://example.com/default-avatar.png';
  end if;
  
  return new;
end;
$$;

create trigger set_defaults
  before insert on users
  for each row
  execute function set_user_defaults();

​
UPDATE Trigger

-- Automatically update timestamps
create or replace function update_timestamp()
returns trigger
language plpgsql
as $$
begin
  new.updated_at := now();
  return new;
end;
$$;

create trigger update_products_timestamp
  before update on products
  for each row
  execute function update_timestamp();

​
DELETE Trigger

-- Soft delete implementation
create or replace function soft_delete()
returns trigger
language plpgsql
as $$
begin
  -- Instead of deleting, update deleted_at
  update users
  set deleted_at = now()
  where id = old.id;
  
  -- Prevent the actual delete
  return null;
end;
$$;

create trigger soft_delete_users
  before delete on users
  for each row
  execute function soft_delete();

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Multiple Events

create or replace function audit_changes()
returns trigger
language plpgsql
as $$
begin
  if TG_OP = 'INSERT' then
    insert into audit_log (table_name, operation, new_data)
    values (TG_TABLE_NAME, 'INSERT', row_to_json(new));
    return new;
  elsif TG_OP = 'UPDATE' then
    insert into audit_log (table_name, operation, old_data, new_data)
    values (TG_TABLE_NAME, 'UPDATE', row_to_json(old), row_to_json(new));
    return new;
  elsif TG_OP = 'DELETE' then
    insert into audit_log (table_name, operation, old_data)
    values (TG_TABLE_NAME, 'DELETE', row_to_json(old));
    return old;
  end if;
end;
$$;

create trigger audit_products
  after insert or update or delete on products
  for each row
  execute function audit_changes();

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Trigger Variables

Trigger functions have access to special variables:
  • NEW: The new row data (INSERT/UPDATE)
  • OLD: The old row data (UPDATE/DELETE)
  • TG_OP: The operation type (‘INSERT’, ‘UPDATE’, ‘DELETE’, ‘TRUNCATE’)
  • TG_TABLE_NAME: The name of the table that fired the trigger
  • TG_TABLE_SCHEMA: The schema of the table
  • TG_WHEN: ‘BEFORE’ or ‘AFTER’
  • TG_LEVEL: ‘ROW’ or ‘STATEMENT’
create or replace function trigger_info()
returns trigger
language plpgsql
as $$
begin
  raise notice 'Trigger: % on %.%', TG_OP, TG_TABLE_SCHEMA, TG_TABLE_NAME;
  raise notice 'When: %, Level: %', TG_WHEN, TG_LEVEL;
  
  if TG_OP = 'UPDATE' then
    raise notice 'Old: %, New: %', old.id, new.id;
  end if;
  
  return new;
end;
$$;

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Real Example: Salary History Tracking

Here’s a complete example of tracking employee salary changes: 
-- Create the salary history table
create table salary_log (
  id bigint generated always as identity primary key,
  employee_id bigint not null,
  old_salary numeric(10,2),
  new_salary numeric(10,2),
  changed_at timestamptz default now(),
  changed_by text
);

-- Create the trigger function
create or replace function update_salary_log()
returns trigger
language plpgsql
security definer
set search_path = public
as $$
begin
  -- Only log if salary actually changed
  if old.salary is distinct from new.salary then
    insert into salary_log (
      employee_id,
      old_salary,
      new_salary,
      changed_by
    ) values (
      new.id,
      old.salary,
      new.salary,
      current_user
    );
  end if;
  
  return new;
end;
$$;

-- Create the trigger
create trigger salary_update_trigger
  after update on employees
  for each row
  execute function update_salary_log();

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Real Example: Auto-updating Timestamps

Real example from Supabase source code: 
-- Create a reusable timestamp update function
create or replace function utils.update_timestamp()
returns trigger
set search_path = ''
language plpgsql
as $$
begin
  new.updated_at = now();
  return new;
end;
$$;

-- Grant execution permissions
grant execute on function utils.update_timestamp() to anon;
grant execute on function utils.update_timestamp() to authenticated;

-- Apply to a table
create trigger sync_updated_at_content_service
  before update on content.service
  for each row
  execute function utils.update_timestamp();

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Real Example: Soft Delete with Rule

Another pattern from Supabase source: 
create or replace rule soft_delete_content_service as
  on delete to content.service
  do instead (
    update content.service
    set deleted_at = now()
    where id = old.id
  );

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Row Level vs Statement Level

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FOR EACH ROW

Executes once per affected row (most common): 
create trigger row_trigger
  after insert on orders
  for each row  -- Fires once per inserted row
  execute function process_order();

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FOR EACH STATEMENT

Executes once per SQL statement, regardless of rows affected: 
create trigger statement_trigger
  after insert on orders
  for each statement  -- Fires once per INSERT statement
  execute function log_bulk_insert();

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Conditional Triggers

Use WHEN clause to conditionally execute triggers: 
-- Only trigger when specific columns change
create trigger notify_price_change
  after update on products
  for each row
  when (old.price is distinct from new.price)
  execute function notify_price_update();

-- Only trigger for high-value orders
create trigger verify_large_order
  before insert on orders
  for each row
  when (new.total_amount > 10000)
  execute function verify_order();

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Realtime Integration

Supabase triggers can publish real-time events. Here’s an example from the Slack clone: 
-- Enable realtime for specific tables
alter publication supabase_realtime add table public.messages;
alter publication supabase_realtime add table public.channels;

-- Set replica identity for change data
alter table public.messages replica identity full;
Subscribe to changes in your app: 
const channel = supabase
  .channel('messages-changes')
  .on(
    'postgres_changes',
    {
      event: 'INSERT',
      schema: 'public',
      table: 'messages'
    },
    (payload) => {
      console.log('New message:', payload.new)
    }
  )
  .subscribe()

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Advanced: Auth Hook Trigger

Real example from Slack clone - automatically create user profile: 
-- Create user profile when auth user is created
create or replace function public.handle_new_user()
returns trigger
language plpgsql
security definer
set search_path = auth, public
as $$
declare
  is_admin boolean;
begin
  -- Insert into public users table
  insert into public.users (id, username)
  values (new.id, new.email);
  
  -- Check if this is the first user
  select count(*) = 1 from auth.users into is_admin;
  
  -- Assign roles based on email
  if position('+supaadmin@' in new.email) > 0 then
    insert into public.user_roles (user_id, role)
    values (new.id, 'admin');
  elsif position('+supamod@' in new.email) > 0 then
    insert into public.user_roles (user_id, role)
    values (new.id, 'moderator');
  end if;
  
  return new;
end;
$$;

-- Trigger on auth.users insert
create trigger on_auth_user_created
  after insert on auth.users
  for each row
  execute function public.handle_new_user();

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Managing Triggers

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Disable/Enable Trigger

-- Disable a specific trigger
alter table products disable trigger update_timestamp_trigger;

-- Enable it again
alter table products enable trigger update_timestamp_trigger;

-- Disable all triggers on a table
alter table products disable trigger all;

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Drop Trigger

-- Drop the trigger
drop trigger if exists update_timestamp_trigger on products;

-- Drop the function (cascade drops all dependent triggers)
drop function if exists update_timestamp() cascade;

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List Triggers

-- View all triggers in a schema
select
  trigger_name,
  event_manipulation,
  event_object_table,
  action_timing
from information_schema.triggers
where trigger_schema = 'public'
order by event_object_table, trigger_name;

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Common Use Cases

Track all changes to sensitive tables for compliance and debugging.
Enforce complex business rules that can’t be expressed with simple constraints.
Automatically update calculated fields or summary tables.
Update related records when a parent record changes.
Send alerts or trigger workflows when important events occur.

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Best Practices

Triggers can impact performance. Keep them fast and simple.
Complex logic should be in the application or database functions, not triggers.
Don’t create triggers that fire other triggers (can lead to infinite loops).
Filter trigger execution to only when necessary.
For bulk operations, statement-level triggers are more efficient.
Always set search_path on security definer trigger functions.

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Troubleshooting

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Infinite Trigger Loops

-- Bad: Can cause infinite loop
create trigger bad_trigger
  after update on products
  for each row
  execute function update_products();  -- Updates products table!

-- Good: Prevent loop with conditional
create or replace function update_products_safe()
returns trigger as $$
begin
  if new.updated_at = old.updated_at then
    new.updated_at := now();
  end if;
  return new;
end;
$$ language plpgsql;

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Debugging Triggers

create or replace function debug_trigger()
returns trigger as $$
begin
  raise notice 'Trigger fired: %', TG_OP;
  raise notice 'Old: %', row_to_json(old);
  raise notice 'New: %', row_to_json(new);
  return new;
end;
$$ language plpgsql;

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Next Steps

Functions

Learn more about database functions

Tables

Understand table structures and relationships

Extensions

Explore PostgreSQL extensions

Realtime

Subscribe to database changes in real-time

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