GROUP BY

Group of records instead of processing many individual records.

SELECT LeadSource
FROM Lead
GROUP BY LeadSource

public inherited sharing class LeadSelector implements SOQL.Selector {

    public static SOQL query() {
        return SOQL.of(Lead.SObjectType);
    }
}

public with sharing class MyController {

    public static List<AggregateResult> getGroupedLeads() {
        return LeadSelector.query()
                .with(Lead.LeadSource)
                .groupBy(Lead.LeadSource)
                .toAggregated();
    }
}