Count Distinct

Count Distinct is an aggregate function that counts the number of distinct values in a specified property. Analytics requires the postgresql-hll extension to be installed in the database to use this function.

Example:

use Doctrine\ORM\Mapping as ORM;
use Rekalogika\Analytics\PostgreSQLHll\AggregateFunction\CountDistinct;
use Rekalogika\Analytics\Core\Entity\BaseSummary;
use Rekalogika\Analytics\Metadata\Attribute as Analytics;
use Rekalogika\Analytics\Core\ValueResolver\IdentifierValue;

class OrderSummary extends BaseSummary
{
    #[ORM\Column(type: 'rekalogika_hll')]
    #[Analytics\Measure(
        function: new CountDistinct(new IdentifierValue('customer')),
    )]
    private ?int $uniqueCustomers = null;
}

warning

As with other implementations of Count Distinct in data warehousing and parallel computing, this function uses the Hyperloglog algorithm to return an approximate count of distinct values, not an exact count.