Remove uniqueness of index in PostgreSQL

sql

database-management

postgresql

index-management

byAlex Kataev·Oct 29, 2024

You can perform this operation with a series of SQL commands. Just remember to fill in your specific data where needed:

BEGIN;
DROP INDEX CONCURRENTLY your_unique_index;
CREATE INDEX CONCURRENTLY your_new_index ON your_table(your_column);
COMMIT;

Fasten your seatbelts! We're dropping the existing unique index and introducing a new index, sans UNIQUE, using the CONCURRENTLY keyword to avoid locking the table. Ensure your placeholders (your_unique_index, your_new_index, your_table, and your_column) meet your database. Fear not, as we're not renaming anything. Also, encapsulate everything in a transaction for atomicity, safeguarding against half-operations.

Understanding the guts of unique index removal

Indexes and constraints serve different purposes in PostgreSQL. It's crucial to distinguish them to properly perform operations. Specifically, a unique constraint is upheld by an automatically created unique index. The index itself doesn't enforce a table-level constraint.

To see if your target is an index or a constraint, execute:

SELECT constraint_name
FROM information_schema.table_constraints
WHERE table_name = 'your_table' AND constraint_type = 'UNIQUE';

To drop a unique constraint, make this dance move:

ALTER TABLE your_table DROP CONSTRAINT your_constraint;

From an efficiency standpoint, index re-creation, without the UNIQUE attribute, is the way to go when you want to retain an index but drop its uniqueness. DROP INDEX, despite the name, won't alter an index's uniqueness.

Heads up! Dependencies and unique constraints

Be careful when you stumble upon dependencies linked to your unique constraint. You might accidentally step on some toes (or worse), like affecting foreign keys. Dance carefully around these, else you may perform a destructive move.

Preparing for cases of non-alterable unique constraints

1. Bit the bullet ― create a new index

This is our bread and butter, depicted in the fast answer. Destructive changes to your table's data aren't welcomed here.

2. Backup, if drastic changes are to follow

When dealing with key operations, particularly dropping and recreating indexes, a backup is your guardian angel. Always have one before performing risky moves.

3. Mind the performance

Index rebuilding with large tables can be resource-intensive. Plan your maintenance windows or utilize the CONCURRENTLY keyword to reduce bottlenecks.

Extra techniques & precautions

Unique constraint with pgAdmin III

Let's say you're comfortable with pgAdmin III but can't modify the Unique property because it’s grayed out - kind of like trying to run on cheat mode but it's not allowed. Fret not, our above SQL commands will come to your rescue.

Understanding NOT NULL and unique constraints

Speaking of constraints, don't mix up NOT NULL and unique constraints. No one likes a salad when they asked for fries. NOT NULL ensures a column doesn't accept null values, while a “unique constraint” ensures that all meal orders (values), are distinctly different.