docs: add postgrest-js explain() (#18833)

spec/supabase_js_v2.yml

@@ -4378,6 +4378,89 @@ functions:
         hideCodeBlock: true
         isSpotlight: true
 
+  - id: using-explain
+    title: Using Explain
+    description: |
+      For debugging slow queries, you can get the [PostgreSQL EXPLAIN execution plan](https://www.postgresql.org/docs/current/sql-explain.html) of a query
+      using the `explain()` method. This works on any query, even for `rpc()` or writes.
+
+      Explain is not enabled by default as it can reveal sensitive information about your database.
+      It's best to only enable this for testing environments but if you wish to enable it for production you can provide additional protection by using a `pre-request` function.
+
+      To enable it:
+
+      ```sql
+      alter role authenticator set pgrst.db_plan_enabled to 'true';
+      notify pgrst, 'reload config';
+      ```
+
+      To only allow `explain()` from a fixed IP address you can do the following. Use this if you plan on enabling `explain()` on a production environment.
+
+      ```sql
+      create or replace function filter_plan_requests()
+      returns void as $$
+      declare
+        headers   json := current_setting('request.headers', true)::json;
+        client_ip text := coalesce(headers->>'cf-connecting-ip', '');
+        accept    text := coalesce(headers->>'accept', '');
+        your_ip   text := '123.123.123.123'; -- replace this by your ip
+      begin
+        if accept like 'application/vnd.pgrst.plan%' and client_ip != your_ip then
+          raise insufficient_privilege using
+            message = 'Not allowed to use application/vnd.pgrst.plan';
+        end if;
+      end; $$ language plpgsql;
+
+      alter role authenticator set pgrst.db_pre_request to 'filter_plan_requests';
+      notify pgrst, 'reload config';
+      ```
+
+      Once you're done using `explain()`, you can disable it with:
+
+      ```sql
+      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 '';
+
+      notify pgrst, 'reload config';
+      ```
+  - id: explain
+    $ref: '@supabase/postgrest-js.PostgrestTransformBuilder.explain'
+    title: explain()
+    examples:
+      - id: get-execution-plan
+        name: Get the execution plan
+        code: |
+          ```ts
+          const { data, error } = await supabase
+            .from('countries')
+            .select()
+            .explain()
+          ```
+        data:
+          sql: |
+            ```sql
+            create table
+              countries (id int8 primary key, name text);
+
+            insert into
+              countries (id, name)
+            values
+              (1, 'Afghanistan'),
+              (2, 'Albania'),
+              (3, 'Algeria');
+            ```
+        response: |
+          ```
+          Aggregate  (cost=33.34..33.36 rows=1 width=112)
+            ->  Limit  (cost=0.00..18.33 rows=1000 width=40)
+                  ->  Seq Scan on countries  (cost=0.00..22.00 rows=1200 width=40)
+          ```
+        description: |
+          By default, the data is returned in TEXT format, but can also be returned as JSON by using the `format` parameter.
+        hideCodeBlock: true
+        isSpotlight: true
+
   # NOTE: Not available on currently deployed PostgREST
   # db.geojson():
   #   $ref: '@supabase/postgrest-js.PostgrestTransformBuilder.geojson'