How to fix Undefined table in PostgreSQL

Connect to PostgreSQL and run the \dt command in psql to see all tables:

psql -U username -d database_name
\dt

This will display all relations in the public schema. If your table is not listed, verify you're in the correct database and schema.

Verify which database and schema you're currently in:

SELECT current_database();
SHOW search_path;

The search_path determines which schemas PostgreSQL searches for unqualified table names. By default, it's "$user", public. If your table is in a different schema, it won't be found.

If you created the table with double quotes (e.g., CREATE TABLE "MyTable"), PostgreSQL preserves the case and treats it as case-sensitive. To query it, use the exact case with quotes:

SELECT * FROM "MyTable";  -- Correct
SELECT * FROM mytable;     -- Will fail with 42P01

Best practice: Create tables without quotes in lowercase to avoid this issue:

CREATE TABLE mytable (id INT);  -- Recommended
SELECT * FROM mytable;          -- Works fine

If your table is in a non-public schema, specify the schema explicitly:

SELECT * FROM myschema.mytable;  -- Correct
SELECT * FROM mytable;            -- May fail if not in search_path

You can also permanently update the search_path for your session:

SET search_path TO myschema, public;
SELECT * FROM mytable;  -- Now searches myschema first

Use the information_schema to check if the table actually exists:

SELECT table_name 
FROM information_schema.tables 
WHERE table_schema = 'public' 
AND table_name = 'mytable';

For a specific schema:

SELECT table_name 
FROM information_schema.tables 
WHERE table_schema = 'myschema';

If no results are returned, the table truly does not exist and needs to be created.

Ensure your database user has the necessary permissions on both the schema and table:

-- Check schema permissions
GRANT USAGE ON SCHEMA myschema TO myuser;

-- Check table permissions
GRANT SELECT ON myschema.mytable TO myuser;

Without USAGE on the schema, PostgreSQL will not find tables within it, even if they exist.

Connect to different databases and verify the table location:

psql -U username -d database1
\dt table_name

psql -U username -d database2
\dt table_name

If connecting programmatically, verify your connection string includes the correct database name:

postgresql://user:password@localhost:5432/correct_database