relation "x" does not exist

PostgreSQL looked for a table (a “relation”, in its terms) with that name and didn’t find one. Sometimes the table really is missing. More often it exists, but the query names it in a way PostgreSQL doesn’t resolve to it.

Why it happens

Every PostgreSQL table lives in a schema. When you write a name without one, PostgreSQL checks only the schemas on your search_path, which by default is "$user", public: a schema named after your login role, then public. It doesn’t search any other schema.

The practice database keeps its tables in a schema called demo, so this fails even though demo.customers has ten rows:

SELECT first_name, country
FROM customers;

The message quotes the name exactly as PostgreSQL searched for it. relation "customers" means it looked for an unqualified customers on the search path. relation "demo.Customers" means it looked in demo for a table whose name has a capital C.

How to fix it

The table is in another schema. Qualify the name:

SELECT first_name, country
FROM demo.customers;

Or put the schema on the search path for the session, after which unqualified names resolve:

SET search_path TO demo, public;
SELECT first_name, country FROM customers;

SHOW search_path prints the current setting. To make the change permanent for a role, use ALTER ROLE ... SET search_path. Many ORMs and migration tools also have their own schema setting.

The name has capitals and was created with quotes. PostgreSQL lowercases every unquoted identifier, so demo.Customers and demo.CUSTOMERS both find customers. A quoted identifier keeps its case:

SELECT * FROM demo."Customers";     -- relation "demo.Customers" does not exist

The reverse catches people more often. A tool that generates DDL, or a migration written as CREATE TABLE "Customers", creates a table whose name really is Customers. That table can then only be reached by writing "Customers", quotes included, in every query.

The quotes span the schema and the table. "demo.customers" is a single identifier containing a dot, not a schema plus a table. Quote each part separately: "demo"."customers".

You’re connected to the wrong database. Schemas don’t cross databases. If the table is in analytics and your connection points at postgres, no search path will find it. Check the database name in your connection settings.

Finding out where the table actually is

When you don’t know the schema or the exact spelling, ask the catalog:

SELECT table_schema, table_name
FROM information_schema.tables
WHERE table_name ILIKE '%customer%';

This returns demo | customers. If it returns nothing, the table isn’t in this database, or your role has no privileges on it: information_schema only lists tables you have some privilege on.