How to fix Cannot attach partition in PostgreSQL

Ensure the table being attached has identical columns to the parent partitioned table, including column names, data types, and constraints.

-- Check parent table structure
SELECT column_name, data_type, is_nullable 
FROM information_schema.columns 
WHERE table_name = 'parent_table' 
ORDER BY ordinal_position;

-- Check partition table structure
SELECT column_name, data_type, is_nullable 
FROM information_schema.columns 
WHERE table_name = 'partition_table' 
ORDER BY ordinal_position;

If differences exist, alter the partition table to match or recreate it with the correct schema.

Create a CHECK constraint on the partition table that matches the partition bounds. This avoids a full table scan during attachment and ensures data validity.

ALTER TABLE measurement_y2008m02 
ADD CONSTRAINT y2008m02_check 
CHECK (logdate >= DATE '2008-02-01' AND logdate < DATE '2008-03-01');

For RANGE partitions, use >= for lower bound and < for upper bound. For LIST partitions, specify the exact values. This constraint tells PostgreSQL the data is pre-validated, avoiding the expensive scan.

Check that the bounds you are specifying for the new partition do not overlap with any existing partitions.

-- For RANGE partitions, view existing bounds
SELECT relname, pg_get_partkeydef(oid) as partition_key 
FROM pg_class 
WHERE relispartition AND relname LIKE 'parent_table_%';

-- Manually verify your new partition bounds fall in a gap
-- Example: if 2008-01 covers Jan, Feb covers Feb, then Mar must start at 2008-03-01

For LIST partitions, ensure the values do not duplicate existing partition values. Overlap causes the attachment to fail.

If the CHECK constraint reveals data outside partition bounds, you have two options:

Option 1: Remove out-of-range data

DELETE FROM measurement_y2008m02 
WHERE logdate < DATE '2008-02-01' OR logdate >= DATE '2008-03-01';

Option 2: Use a DEFAULT partition to catch misplaced rows

CREATE TABLE parent_table_default 
PARTITION OF parent_table DEFAULT;

After successful ATTACH PARTITION, you can drop the now-redundant CHECK constraint:

ALTER TABLE measurement_y2008m02 DROP CONSTRAINT y2008m02_check;

Once constraints are in place and data is verified, execute ATTACH PARTITION:

ALTER TABLE measurement ATTACH PARTITION measurement_y2008m02
  FOR VALUES FROM ('2008-02-01') TO ('2008-03-01');

If the CHECK constraint was present from earlier steps, this operation completes quickly. Without it, PostgreSQL must scan the entire table, which can be slow on large tables and holds an ACCESS EXCLUSIVE lock.