model 'withDeleted()'

Hi, I'm have a weird problem, and I don't know how to fix it. I noticed that when using the withDeleted or onlyDeleted function of a model that has useSoftDeletes as true, it will add GROUP BY and ORDER BY to the query.
Here is the php code:

PHP Code:

$this->where('product_id', $productID)->selectSum('amount', 'total')->withDeleted()->first(); 


the query generated from this function is this:

Code:

SELECT SUM(`amount`) AS `total`
FROM `orders`
WHERE `product_id` = '40'
GROUP BY`orders`.`id`
ORDER BY `orders`.`id` ASC
LIMIT 1

but when I don't use the withDeleted function, the generated query is this:

Code:

SELECT SUM(`amount`) AS `total`
FROM `orders`
WHERE `product_id` = '40'
AND `orders`.`cancelled_at` IS NULL
LIMIT 1

is there any way to let the model build a query like this:

Code:

SELECT SUM(`amount`) AS `total`
FROM `orders`
WHERE `product_id` = '40'
LIMIT 1

$this->where('product_id', $productID)->selectSum('amount', 'total')->limit(1)->get()->getFirstRow();

I don't understand why GROUP BY and ORDER BY are added only when withDeleted() is used.

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@kenjis
ORDER BY gives a more predictable result when using LIMIT.
This is indicated by a comment in the code of the Model::doFirst() method with a reference to PostgreSQL.
The presence of GROUP BY, I cannot explain yet.