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select
    username,
...
and gmtdatetime >= '$min_date'
and gmtdatetime < '$max_date'
group by username;

select
    username,
...
where username in $user_string
and gmtdatetime >= '$min_date'
and gmtdatetime < '$max_date'

while(my @row = $res->fetchrow_array()) {

...
    }
    
}

foreach my $i_user (keys(%stats)) {
    $stats{$i_user}{'UNIQUE_PRODUCTS'} = keys(%{$user_products{$i_user
+}});
}