Schema Cache

Certain PostgREST features require metadata from the database schema. Getting this metadata requires executing expensive queries, so in order to avoid repeating this work, PostgREST uses a schema cache.

Feature	Required Metadata
Resource Embedding	Foreign key constraints
Stored Functions	Function signature (parameters, return type, volatility and overloading)
Upserts	Primary keys
Insertions	Primary keys (optional: only if the Location header is requested)
OPTIONS requests	View INSTEAD OF TRIGGERS and primary keys
OpenAPI Support	Table columns, primary keys and foreign keys
	View columns and INSTEAD OF TRIGGERS
	Function signature

The Stale Schema Cache

When you make changes on the metadata mentioned above, the schema cache will turn stale on a running PostgREST. Future requests that use the above features will need the schema cache to be reloaded; otherwise, you’ll get an error instead of the expected result.

For instance, let’s see what would happen if you have a stale schema cache for foreign key relationships and function signatures.

Stale Foreign Key Relationships

Suppose you add a cities table to your database and define a foreign key that references an existing countries table. Then, you make a request to get the cities and their belonging countries.

GET /cities?select=name,country:countries(id,name) HTTP/1.1

curl "http://localhost:3000/cities?select=name,country:countries(id,name)"

The result will be an error:

{
  "hint": "If a new foreign key between these entities was created in the database, try reloading the schema cache.",
  "message": "Could not find a relationship between cities and countries in the schema cache"
}

As you can see, PostgREST couldn’t find the newly created foreign key in the schema cache. See Schema Cache Reloading and Automatic Schema Cache Reloading to solve this issue.

Stale Function Signature

The same issue will occur on newly created functions on a running PostgREST.

CREATE FUNCTION plus_one(num integer)
RETURNS integer AS $$
 SELECT num + 1;
$$ LANGUAGE SQL IMMUTABLE;

GET /rpc/plus_one?num=1 HTTP/1.1

curl "http://localhost:3000/rpc/plus_one?num=1"

{
  "hint": "If a new function was created in the database with this name and arguments, try reloading the schema cache.",
  "message": "Could not find the api.plus_one(num) function in the schema cache"
}

Here, PostgREST tries to find the function on the stale schema to no avail. See Schema Cache Reloading and Automatic Schema Cache Reloading to solve this issue.

Schema Cache Reloading

To reload the cache without restarting the PostgREST server, send a SIGUSR1 signal to the server process.

killall -SIGUSR1 postgrest

For docker you can do:

docker kill -s SIGUSR1 <container>

# or in docker-compose
docker-compose kill -s SIGUSR1 <service>

There’s no downtime when reloading the schema cache. The reloading will happen on a background thread while requests keep being served.

Reloading with NOTIFY

There are environments where you can’t send the SIGUSR1 Unix Signal (like on managed containers in cloud services or on Windows systems). For this reason, PostgREST also allows you to reload its schema cache through PostgreSQL NOTIFY as follows:

NOTIFY pgrst, 'reload schema'

The "pgrst" notification channel is enabled by default. For configuring the channel, see db-channel and db-channel-enabled.

Automatic Schema Cache Reloading

You can do automatic schema cache reloading in a pure SQL way and forget about stale schema cache errors with an event trigger and NOTIFY.

-- Create an event trigger function
CREATE OR REPLACE FUNCTION public.pgrst_watch() RETURNS event_trigger
  LANGUAGE plpgsql
  AS $$
BEGIN
  NOTIFY pgrst, 'reload schema';
END;
$$;

-- This event trigger will fire after every ddl_command_end event
CREATE EVENT TRIGGER pgrst_watch
  ON ddl_command_end
  EXECUTE PROCEDURE public.pgrst_watch();

Now, whenever the pgrst_watch trigger is fired in the database, PostgREST will automatically reload the schema cache.

To disable auto reloading, drop the trigger:

DROP EVENT TRIGGER pgrst_watch