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

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
{
  "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

Finer-Grained Event Trigger

You can refine the previous event trigger and only react to the events relevant to the schema cache. This also prevents unnecessary reloading when creating temporary tables(CREATE TEMP TABLE) inside functions.

-- watch create and alter
CREATE OR REPLACE FUNCTION pgrst_ddl_watch() RETURNS event_trigger AS $$
DECLARE
  cmd record;
BEGIN
  FOR cmd IN SELECT * FROM pg_event_trigger_ddl_commands()
  LOOP
    IF cmd.command_tag IN (
      'CREATE SCHEMA', 'ALTER SCHEMA'
    , 'CREATE TABLE', 'CREATE TABLE AS', 'SELECT INTO', 'ALTER TABLE'
    , 'CREATE FOREIGN TABLE', 'ALTER FOREIGN TABLE'
    , 'CREATE VIEW', 'ALTER VIEW'
    , 'CREATE MATERIALIZED VIEW', 'ALTER MATERIALIZED VIEW'
    , 'CREATE FUNCTION', 'ALTER FUNCTION'
    , 'CREATE TRIGGER'
    , 'CREATE TYPE'
    , 'CREATE RULE'
    , 'COMMENT'
    )
    -- don't notify in case of CREATE TEMP table or other objects created on pg_temp
    AND cmd.schema_name is distinct from 'pg_temp'
    THEN
      NOTIFY pgrst, 'reload schema';
    END IF;
  END LOOP;
END; $$ LANGUAGE plpgsql;

-- watch drop
CREATE OR REPLACE FUNCTION pgrst_drop_watch() RETURNS event_trigger AS $$
DECLARE
  obj record;
BEGIN
  FOR obj IN SELECT * FROM pg_event_trigger_dropped_objects()
  LOOP
    IF obj.object_type IN (
      'schema'
    , 'table'
    , 'foreign table'
    , 'view'
    , 'materialized view'
    , 'function'
    , 'trigger'
    , 'type'
    , 'rule'
    )
    AND obj.is_temporary IS false -- no pg_temp objects
    THEN
      NOTIFY pgrst, 'reload schema';
    END IF;
  END LOOP;
END; $$ LANGUAGE plpgsql;

CREATE EVENT TRIGGER pgrst_ddl_watch
  ON ddl_command_end
  EXECUTE PROCEDURE pgrst_ddl_watch();

CREATE EVENT TRIGGER pgrst_drop_watch
  ON sql_drop
  EXECUTE PROCEDURE pgrst_drop_watch();