Eloquent chunk() missing half the results - php

Question

I have a problem with Laravel's ORM Eloquent chunk() method.
It misses some results.
Here is a test query :
$destinataires = Destinataire::where('statut', '<', 3)
->where('tokenized_at', '<', $date_active)
->chunk($this->chunk, function ($destinataires) {
foreach($destinataires as $destinataire) {
$this->i++;
}
}
echo $this->i;
It gives 124838 results.
But :
$num_dest = Destinataire::where('statut', '<', 3)
->where('tokenized_at', '<', $date_active)
->count();
echo $num_dest;
gives 249676, so just TWICE as the first code example.
My script is supposed to edit all matching records in the database. If I launch it multiple times, it just hands out half the remaining records, each time.
I tried with DB::table() instead of the Model.
I tried to add a ->take(20000) but it doesn't seem to be taken into account.
I echoed the query with ->toSql() and eveything seems to be fine (the LIMIT clause is added when I add the ->take() parameter).
Any suggestions ?

Answer 1

Imagine you are using chunk method to delete all of the records. The table has 2,000,000 records and you are going to delete all of them by 1000 chunks.
$query->orderBy('id')->chunk(1000, function ($items) {
foreach($items as $item) {
$item->delete();
}
});
It will delete the first 1000 records by getting first 1000 records in a query like this:
SELECT * FROM table ORDER BY id LIMIT 0,1000
And then the other query from chunk method is:
SELECT * FROM table ORDER BY id LIMIT 1000,2000
Our problem is here, that we delete 1000 records and then getting results from 1000 to 2000. Actually we are missing first 1000 records and this means that we are not going to delete 1000 records in first step of chunk! This scenario will be the same for other steps. In each step we are going to miss 1000 records and this is the reason that we are not getting best result in these situations.
I made an example for deletion because this way we could know the exact behavior of chunk method.
UPDATE:
You can use chunkById() for deleting safely.
Read more here:
http://laravel.at.jeffsbox.eu/laravel-5-eloquent-builder-chunk-chunkbyid
https://laravel.com/api/5.4/Illuminate/Database/Eloquent/Builder.html#method_chunkById

Answer 2

Quick answer: Use chunkById() instead of chunk().
When updating or deleting records while iterating over them, any changes to the primary key or foreign keys could affect the chunk query. This could potentially result in records not being included in the results.
The explanation can be found in the Laravel documentation:
If you are updating database records while chunking results, your chunk results could change in unexpected ways. If you plan to update the retrieved records while chunking, it is always best to use the chunkById method instead. This method will automatically paginate the results based on the record's primary key.
Example usage of chunkById():
DB::table('users')->where('active', false)
->chunkById(100, function ($users) {
foreach ($users as $user) {
DB::table('users')
->where('id', $user->id)
->update(['active' => true]);
}
});
(end of the update)
Below is the original answer which used the cursor() method instead of the chunk() method to solve the problem:
I had the same problem - only half of the total results were passed to the callback function of the chunk() method.
Here is the code which had the same problem - half of the transactions were not processed:
Transaction::whereNull('processed')->chunk(100, function ($transactions) {
$transactions->each(function($transaction){
$transaction->process();
});
});
I used Laravel 5.4 and managed to solve the problem replacing the chunk() method with cursor() method and changing the code accordingly:
foreach (Transaction::whereNull('processed')->cursor() as $transaction) {
$transaction->process();
}
Even though the answer doesn't address the problem itself, it provides a valuable solution.

Answer 3

For anyone looking for a bit of code that solves this, here you go:
while (Model::where('x', '>', 'y')->count() > 0)
{
Model::where('x', '>', 'y')->chunk(10, function ($models)
{
foreach ($models as $model)
{
$model->delete();
}
});
}
The problem is in the deletion / removal of the model while chunking away at the total. Including it in a while loop makes sure you get them all! This example works when deleting Models, change the while condition to suit your needs!

Answer 4

When you fetch data using chunk the same SQL query is being executed only the offset is different. Actually increasing as specified on the chunk method param. For example:
SELECT * FROM users WHERE status = 0;
Let's say there are 200 records(let's suppose that is a lot so we want to retrieve these data as chunks). So this looks like:
SELECT * FROM users WHERE status = 0 LIMIT 50 OFFSET 0(offset has a dynamic value
which means next time is 50, after that 100, and the last time 150).
The problem when using laravel chunk while updating is that we are only changing the offset. And this means the number of results is different each time we try to retrieve a chunk of data. So the first time there are 200 records that match the where condition. But if we update the status, for example to 1(status = 1) this means the next time when we try to fetch data we still execute the same query:
SELECT * FROM users WHERE status = 0 LIMIT 50 OFFSET 50(offset has a dynamic value
which means next time 100 and the last time 150).
We only have 150 records that match this query since we updated the table status = 1 for 50rows. Also we said the offset on the second time is going to be 50. And what is going to happen is that we skip 50 rows from 150rows since the offset is 50. And do the same update to these data. This means rows from 50->100 status is being updated to 1(status = 1) from the total of 150 rows.
The third time we run this query:
SELECT * FROM users WHERE status = 0 LIMIT 50 OFFSET 150(offset is going to be 150).
But the result of the query is 100 users in total that have status = 0. So no more data to go through.
This is not what you would expect to happen on the first thought. But this is how it works and why only half of data are being updated and the other part of data is being skipped.