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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →For ordinary uniqueness over table columns, use a UNIQUE constraint: PostgreSQL enforces it with an automatically created unique index, while recording the rule as part of the table’s schema. Use a standalone unique index when you need index-specific behavior such as a partial predicate or an expression key. On a live table, you can reduce prolonged write blocking by creating an eligible unique index concurrently and then attaching it as a constraint—but the process is not lock-free.
The examples below follow PostgreSQL 18 documentation, accessed October 7, 2026. Check the documentation for your server’s major version before running a production migration.
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What is the difference between a unique constraint and a unique index?
Both prevent duplicate key values. A UNIQUE constraint is a named table rule; PostgreSQL implements it with an associated unique index. A standalone unique index expresses the uniqueness requirement through the index object itself.
For a plain column or set of columns, the constraint is usually the clearer schema choice. PostgreSQL explicitly cautions against manually creating another index on columns already covered by a unique constraint: that would duplicate the automatically created index. PostgreSQL currently permits only B-tree indexes to be declared unique. See the PostgreSQL 18 documentation on unique indexes.
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When a constraint fits
Choose a constraint when the rule is simply that one or more table columns must be unique and you want that rule represented as a constraint in the schema. PostgreSQL creates the enforcing index for you.
When a standalone unique index fits
Choose a standalone index when uniqueness applies only to rows matching a predicate, or when the key is an expression rather than just table columns. These are index-specific forms; they cannot be attached as ordinary unique constraints with UNIQUE USING INDEX.
Which should you use?
| Need | Use | Important detail |
|---|---|---|
| Uniqueness across ordinary column values | UNIQUE constraint |
PostgreSQL creates the supporting unique index; do not add a duplicate. |
| Uniqueness only for rows matching a condition | Partial unique index | It cannot be attached using UNIQUE USING INDEX and cannot be the referenced key for a foreign key. |
| Uniqueness based on an expression | Expression-based unique index | It cannot be attached using UNIQUE USING INDEX. |
| A foreign key must reference the key | Primary key, unique constraint, or non-partial unique index | A partial unique index is not eligible. PostgreSQL does not automatically index the foreign-key columns on the referencing table. |
PostgreSQL’s constraints documentation describes which keys can be referenced by a foreign key.
How NULLs and multi-column keys affect uniqueness
NULL behavior
By default, PostgreSQL treats NULL values as distinct in a unique index, so multiple rows can have NULL in a unique key. If NULLs should count as equal, use NULLS NOT DISTINCT where supported by your intended server version and index definition. Make this decision before building the index: the default may not match the business rule.
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Composite keys
For a unique index over multiple columns, PostgreSQL rejects a duplicate only when all indexed values match. For example, a unique key on (tenant_id, email) permits the same email for different tenants but not twice for the same tenant, subject to the NULL behavior above.
How to add a unique constraint to a live table with less write blocking
For an eligible ordinary key on a non-partitioned table, build the unique index concurrently first, then attach it as a constraint:
CREATE UNIQUE INDEX CONCURRENTLY users_email_key_idx
ON users (email);
ALTER TABLE users
ADD CONSTRAINT users_email_key
UNIQUE USING INDEX users_email_key_idx;Replace the example table and object names with your own. This sequence is designed to avoid a prolonged period in which writes are blocked, not to guarantee zero locking or zero operational impact. PostgreSQL says the concurrent build does not take locks that prevent concurrent inserts, updates, or deletes, unlike a standard index build. The CREATE INDEX documentation also notes that the concurrent operation performs extra work and has transaction-wait phases.
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1. Confirm the rule and check existing data
Decide which columns form the key, whether NULLs may repeat, and whether all key columns together must be unique. Check for existing duplicates and resolve them before migration. A unique build validates existing rows, and concurrent application writes can also encounter uniqueness errors before the index is marked ready.
2. Run the concurrent build outside a transaction
Run the CREATE UNIQUE INDEX CONCURRENTLY statement on its own, outside any transaction block. It cannot run inside one, so configure migration tooling to execute this step without wrapping it in a transaction. Only one concurrent index build can run on a given table at a time, and schema modification of that table is not allowed while the index is being built.
The build allows writes during its scans, but it performs two table scans and waits for relevant transactions. It therefore takes longer and more work than a normal build; CPU and I/O use can still slow other activity.
3. Attach the index as a constraint
After the index build succeeds, run the ALTER TABLE statement separately. The index must be a B-tree with default sort ordering and must not contain expression columns or a partial predicate. PostgreSQL describes attaching an existing index as useful when a constraint needs to be added without blocking table updates for a long time. The operation still acquires a table lock. See ALTER TABLE.
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Once attached, the constraint takes ownership of the index. Dropping the constraint also removes that index.
4. Verify the result and handle failures deliberately
After migration, verify that the constraint exists and that its associated index is valid. If concurrent creation fails, PostgreSQL may leave an invalid index. It is ignored for query planning but can still add update overhead. The documented recovery options are to drop it and retry, or rebuild it with REINDEX INDEX CONCURRENTLY.
Do not blindly rerun the create statement after a failed build: a failure during the second scan can leave an invalid unique index that continues enforcing uniqueness. Inspect the index state and account for any lingering uniqueness enforcement before choosing a recovery path.
What “without locking the table” really means
A normal index build blocks writes until it finishes. A concurrent build avoids locks that prevent concurrent inserts, updates, and deletes during its scans, but it takes longer, waits for relevant transactions, and still consumes resources. Attaching the index uses ALTER TABLE and acquires a lock. The practical goal is to avoid prolonged write blocking, not to perform a lock-free migration.
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Partitioned tables
The documented UNIQUE USING INDEX operation is unsupported on partitioned tables, and concurrent creation for partitioned indexes is not directly supported. PostgreSQL’s index documentation describes building indexes on individual partitions and creating the partitioned index separately to reduce the write-locking period. Treat this as a separate migration design and consult documentation for the exact server major version.
Turning the index into a primary key
If you attach an index as a primary key and its columns are not already NOT NULL, PostgreSQL attempts to set them to NOT NULL, which requires a table scan. This can materially change the migration’s operational profile.
Using NOT VALID
NOT VALID is not a shortcut for adding a unique constraint. PostgreSQL currently permits that option only for foreign-key, CHECK, and not-null constraints, not UNIQUE constraints.
Foreign keys and indexing the referencing side
A foreign key may reference a primary key, a unique constraint, or the columns of a non-partial unique index. A partial unique index cannot serve as the referenced key. PostgreSQL does not automatically create an index on the referencing columns, so consider whether the workload needs one for lookups or updates involving the foreign key.
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For production use, validate the intended uniqueness rule against your actual PostgreSQL major version and migration framework. The PostgreSQL 18 documentation used here is available under unique indexes, CREATE INDEX, ALTER TABLE, and constraints.
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