Debugging performance issues

explain() is a method that provides the Postgres EXPLAIN execution plan of a query. It is a powerful tool for debugging slow queries and understanding how Postgres will execute a given query. This feature is applicable to any query, including those made through rpc() or write operations.

Enabling `explain()`

explain() is disabled by default to protect sensitive information about your database structure and operations. We recommend using explain() in a non-production environment. Run the following SQL to enable explain():

-- enable explain
alter role authenticator
set pgrst.db_plan_enabled to 'true';

-- reload the config
notify pgrst, 'reload config';

Using `explain()`

To get the execution plan of a query, you can chain the explain() method to a Supabase query:

const { data, error } = await supabase
  .from('instruments')
  .select()
  .explain()

Example data

To illustrate, consider the following setup of a instruments table:

create table instruments (
  id int8 primary key,
  name text
);

insert into instruments
  (id, name)
values
  (1, 'violin'),
  (2, 'viola'),
  (3, 'cello');

Expected response

The response would typically look like this:

Aggregate  (cost=33.34..33.36 rows=1 width=112)
  ->  Limit  (cost=0.00..18.33 rows=1000 width=40)
        ->  Seq Scan on instruments  (cost=0.00..22.00 rows=1200 width=40)

By default, the execution plan is returned in TEXT format. However, you can also retrieve it as JSON by specifying the format parameter.

Production use with pre-request protection

If you need to enable explain() in a production environment, ensure you protect your database by restricting access to the explain() feature. You can do so by using a pre-request function that filters requests based on the IP address:

create or replace function filter_plan_requests()
returns void as $$
declare
  headers   json := current_setting('request.headers', true)::json;
  client_ip text := coalesce(headers->>'cf-connecting-ip', '');
  accept    text := coalesce(headers->>'accept', '');
  your_ip   text := '123.123.123.123'; -- replace this with your IP
begin
  if accept like 'application/vnd.pgrst.plan%' and client_ip != your_ip then
    raise insufficient_privilege using
      message = 'Not allowed to use application/vnd.pgrst.plan';
  end if;
end; $$ language plpgsql;
alter role authenticator set pgrst.db_pre_request to 'filter_plan_requests';
notify pgrst, 'reload config';

Caution

The pgrst.db_pre_request configuration only works with the Data API (PostgREST). It does not work with Realtime, Storage, or other Supabase products.

If you're using db_pre_request to call a function (like set_information()) that sets up context or performs checks on every request, and you need similar behavior for other Supabase products, you must call the function directly in your Row Level Security (RLS) policies instead.

Example:

If you have a db_pre_request function that calls set_information() that returns true to set up context or perform checks, and you have an RLS policy like:

create policy "Individuals can view their own todos."
on todos for select
using ( (select auth.uid()) = user_id );

To achieve the same behavior with other Supabase products, you need to call the function directly in your RLS policy:

create policy "Individuals can view their own todos."
on todos for select
using ( set_information() AND (select auth.uid()) = user_id );

This ensures the function is called when evaluating RLS policies for all products, not only Data API requests.

Performance consideration:

Be aware that calling functions directly in RLS policies can impact database performance, as the function is evaluated for each row when the policy is checked. Consider optimizing your function or using caching strategies if performance becomes an issue.

Replace '123.123.123.123' with your actual IP address.

Disabling explain

To disable the explain() method after use, execute the following SQL commands:

-- disable explain
alter role authenticator
set pgrst.db_plan_enabled to 'false';

-- if you used the above pre-request
alter role authenticator
set pgrst.db_pre_request to '';

-- reload the config
notify pgrst, 'reload config';