ERROR

How to Fix PostgreSQL 'Object Not in Prerequisite State' Error (2025)

Quick Fix Summary

TL;DR

Identify and resolve the specific constraint or dependency blocking your DDL operation, such as a foreign key, index, or view.

PostgreSQL throws ERROR: 55000 'object not in prerequisite state' when a Data Definition Language (DDL) command, like ALTER TABLE, cannot be executed due to an existing constraint or dependency. The operation is blocked to prevent data corruption or logical inconsistencies.

Diagnosis & Causes

Attempting to ALTER TABLE with active foreign key constraints.

Changing a column type referenced by a view or materialized view.

Dropping a column that is part of a UNIQUE or PRIMARY KEY constraint.

Modifying a table with dependent CHECK constraints.

Renaming a column used in an index or trigger.

Recovery Steps

Step 1: Identify the Blocking Dependency

First, query the system catalog to find all objects depending on the table or column you're trying to modify. This pinpoints the exact conflict.

sql

SELECT pg_describe_object(classid, objid, objsubid) AS dependent_object,
       pg_describe_object(refclassid, refobjid, refobjsubid) AS referenced_object
FROM pg_depend
WHERE refobjid = 'your_schema.your_table'::regclass;

Step 2: Handle Foreign Key Constraints

If a foreign key is blocking an ALTER TABLE, you must drop it before the operation and recreate it afterward. Use a transaction for safety.

sql

BEGIN;
ALTER TABLE child_table DROP CONSTRAINT fk_constraint_name;
ALTER TABLE parent_table ALTER COLUMN id TYPE bigint; -- Your intended DDL
ALTER TABLE child_table ADD CONSTRAINT fk_constraint_name FOREIGN KEY (parent_id) REFERENCES parent_table(id);
COMMIT;

Step 3: Manage Dependent Views

For columns referenced by views, you must drop the view, perform your ALTER, and then recreate the view. Capture the view definition first.

sql

-- 1. Get view definition
SELECT pg_get_viewdef('your_schema.your_view'::regclass, true);
-- 2. Drop the view
DROP VIEW your_schema.your_view;
-- 3. Perform your ALTER TABLE operation
ALTER TABLE your_schema.your_table ...;
-- 4. Recreate the view using the saved definition

Step 4: Deal with Indexes and Unique Constraints

To change a column's data type when it's part of a UNIQUE/PRIMARY KEY or index, drop the constraint/index, alter the column, and rebuild.

sql

BEGIN;
ALTER TABLE your_table DROP CONSTRAINT your_unique_constraint;
ALTER TABLE your_table ALTER COLUMN your_column TYPE new_data_type USING your_column::new_data_type;
ALTER TABLE your_table ADD CONSTRAINT your_unique_constraint UNIQUE (your_column);
COMMIT;

Step 5: Use VALIDATE CONSTRAINT for Large Tables

After recreating foreign keys on large tables, defer the validation to avoid a long exclusive lock during the COMMIT.

sql

ALTER TABLE child_table ADD CONSTRAINT fk_constraint_name
FOREIGN KEY (parent_id) REFERENCES parent_table(id)
NOT VALID; -- Creation is fast
ALTER TABLE child_table VALIDATE CONSTRAINT fk_constraint_name; -- Validation runs with a share lock

Architect's Pro Tip

"Use `SET lock_timeout = '5s';` in your session before running diagnostic queries on a busy production system to prevent being blocked by, or contributing to, lock contention."

Frequently Asked Questions

Can I ignore this error and force the change?

No. This error is a safeguard. Forcing a change with tools like pg_dump/restore or unsafe casts can corrupt data or break application logic. Always resolve the dependency.

Does this error occur with ALTER COLUMN SET NOT NULL?

Yes, if existing NULL values violate the new constraint. You must first update NULLs to a valid value or add a DEFAULT clause before applying NOT NULL.

Is there a way to see all dependencies for a whole schema?

Yes. Use: `SELECT * FROM pg_depend;` combined with `\d+ your_table` in psql, or use the `pgAdmin` GUI's 'Dependencies' tab for a visual graph.