> ## Documentation Index
> Fetch the complete documentation index at: https://docs.firebolt.io/llms.txt
> Use this file to discover all available pages before exploring further.

> You can mark a column as unique using a query hint comment.

# Unique columns

## Background: What are unique constraints?

When the optimizer knows that certain columns contain only unique values, it can make better estimates about the number of rows in certain situations, then leading to better plans.
You can learn more about how unique constraints improve performance in the documentation for [UNIQUE constraints](/performance-and-observability/query-planning/unique-constraint) at the table level.

## Using the unique columns hint

In contrast to specifying a unique constraint in the table definition, the `unique` hint allows you to specify uniqueness for a single query, without modifying the table definition.
This enables testing the performance impact of unique constraints before adding them to your table schema.
The hint has the same effect as declaring a column `UNIQUE` when [creating a table](/reference-sql/commands/data-definition/create-fact-dimension-table#column-constraints-and-the-default-expression), but applies only to the current query.
To make sure that all queries can benefit from the hint, we still recommend declaring the column as `UNIQUE` in the table definition.

<Warning>
  Since unique constraints are not enforced, ensure your data actually satisfies the uniqueness assumption. **Incorrect hints can lead to incorrect query results.**
</Warning>

## Example

```sql theme={"theme":{"light":"github-light","dark":"github-dark"}}
/*! unique(customers.customer_id) */
select c.customer_id, c.name, o.total_amount
from customers c
join orders o on c.customer_id = o.customer_id
```