Database

Debugging performance issues

Debug slow-running queries using the Postgres execution planner.

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():

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-- enable explain
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alter role authenticator
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set pgrst.db_plan_enabled to 'true';
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-- reload the config
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notify pgrst, 'reload config';

Using explain()

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

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

Example data

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

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create table instruments (
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  id int8 primary key,
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  name text
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);
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insert into books
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  (id, name)
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values
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  (1, 'violin'),
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  (2, 'viola'),
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  (3, 'cello');

Expected response

The response would typically look like this:

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Aggregate  (cost=33.34..33.36 rows=1 width=112)
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  ->  Limit  (cost=0.00..18.33 rows=1000 width=40)
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        ->  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:

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

Replace '123.123.123.123' with your actual IP address.

Disabling explain

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

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-- disable explain
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alter role authenticator
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set pgrst.db_plan_enabled to 'false';
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-- if you used the above pre-request
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alter role authenticator
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set pgrst.db_pre_request to '';
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-- reload the config
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notify pgrst, 'reload config';