mention/fast-doctrine-paginator

Fast Doctrine paginator for batching, infinite scrolling, Relay

dev-master 2019-12-10 09:20 UTC

This package is auto-updated.

Last update: 2021-04-10 12:37:48 UTC


README

This package provides a fast, no-offset, Doctrine paginator that's suitable for infinite scrolling, batch jobs, and GraphQL/Relay pagination.

Build Status Latest Version MIT License PHPStan Enabled

Install

composer require mention/fast-doctrine-paginator

Why use this paginator ?

It is fast

What makes this paginator fast is that it relies on seek/keyset pagination rather than limit/offset. Learn more at https://use-the-index-luke.com/no-offset.

In addition to that, it lets the user write its own optimised queries.

It is suitable for batch jobs

Batch jobs over Doctrine queries typically have two problems:

  • The query returns too many results at once and exhausts the memory (even when using iterate(), because the query result is still buffered locally)
  • The entities accumulate in the UnitOfWork and exhaust the memory

Using this paginator solves these two problems:

  • By using a paginated query, we avoid fetching too many items at once
  • By giving the opportunity to act before and after each page, the paginator gives a safe point where the entity manager can be cleared, in order to keep the number of managed entities under control

It is suitable for GraphQL/Relay

GraphQL/Relay style pagination requires fine-grained cursors that point to the beginning and end of a page, as well as to each item in a page. This paginator provides that.

It is suitable for infinite scrolling

Every page will be as fast as the previous one.

When not to use this paginator ?

This paginator is cursor-based, so it may not be suitable for you if you need to access a page by number (although this could be emulated).

Usage

Here is a typical example:

<?php

$query = $entityManager->createQuery('
    SELECT   u.id, u.name
    FROM     Acme\\Entity\\Users u
    WHERE    u.id > :idCursor
    ORDER    BY u.id ASC
');

// Number of items per page
$query->setMaxResults(3);

$paginator = DoctrinePaginatorBuilder::new()
    ->setQuery($query)
    ->setDiscriminators([
        new PageDiscriminator('idCursor', 'getId'),
    ])
    ->build();

foreach ($paginator as $page) {
    foreach ($page() as $user) {
        // ...
    }
    $em->clear();
}

A user-defined query is used to fetch the results for one page of results, and will be executed multiple times when fetching multiple pages. Pagination stops when the query returns no results.

The number of elements per page is defined by calling setMaxResults() on the query itself.

Pagination is keyset-based, rather than limit/offset-based: Instead of asking for the Nth rows in a query, we ask for the rows that are upper than the highest one of the previous page. When comparing rows, we only take into account one or a few columns, that we call the discriminators. We use the value of the discriminator columns of the last row of one page to create an internal cursor that can be used to fetch the next page.

For this to work effectively and flawlessly, the following conditions must be true:

  • The column used for discrimination must be unique (if it's not the case, a combination of multiple columns must be used)
  • The query must be ordered by the discrimination columns
  • The query must have a WHERE clause that selects only rows whose discriminators are higher than the higher discriminators of the previous page.

About the query

The query must be a Doctrine Query object. It must have a defined number of max results (setMaxResults()), because this defines the number of items per page.

It must be ordered by the discriminator columns.

It must have a WHERE clause that selects only the rows whose discriminators are higher than the ones of the previous page. The paginator calls setParameter() on the query to set these values.

Examples

Examples with a Users table:

id name
1 Jackson
2 Sophia
3 Aiden
4 Olivia
5 Lucas
6 Ava

If we sort by id, we can use id as discriminator, because it's unique.

Note the ORDER clause, that orders by our discriminator.

Note the WHERE clause, that discriminates by our discriminator. We use the query parameter :idCursor. The value of this parameter is automatically set by the paginator. By default, it's set to 0 when requesting the first page, and then it's automatically updated to the value found in the last row of the latest fetched page.

<?php

$query = $entityManager->createQuery('
    SELECT   u.id, u.name
    FROM     Users u
    WHERE    u.id > :idCursor
    ORDER    BY u.id ASC
');

// Max results per page
$query->setMaxResults(3);

$paginator = DoctrinePaginatorBuilder::new()
    ->setQuery($query)
    ->setDiscriminators([
        new PageDiscriminator('idCursor', 'getId'),
    ])
    ->build();

foreach ($paginator as $page) {
    foreach ($page() as $result) {
        // ...
    }
    $em->clear();
}

The first page will return this:

id name
1 Jackson
2 Sophia
3 Aiden

The paginator retains id=3 as cursor internally. Before requesting the next page, the paginator calls setParameter('idCursor', 3) on the query. As expected, the second page returns this:

id name
4 Olivia
5 Lucas
6 Ava

Sorting by name:

If we sort by name, we can not use it directly as discriminator, because it's not unique. If we sort by name and id, we can use name and id as discriminators, because they are unique together.

Notice how we use u.id as a fallback when the name equals to the current name cursor.

<?php

$query = $entityManager->createQuery('
    SELECT   u.id, u.name
    FROM     Users u
    WHERE    u.name > :nameCursor
    OR       (u.name = :nameCursor AND u.id > :idCursor)
    ORDER    BY u.name ASC, u.id ASC
');

$paginator = DoctrinePaginatorBuilder::new()
    ->setQuery($query)
    ->setDiscriminators([
        new PageDiscriminator('nameCursor', 'getName'),
        new PageDiscriminator('idCursor', 'getId'),
    ])
    ->build();

Resuming pagination

We can resume pagination on a new DoctrinePaginator instance by setting the cursor explicitly. This is useful when paginating through multiple requests, for example.

The end cursor of a page can be retrieved by calling getCursor() on a PaginatorItem object. Using this cursor will fetch the next page.

A paginator that will resume at this position can be built by calling setCursor() on a DoctrinePaginatorBuilder.

Batch jobs

The paginator is particularly suitable for batching, because it allows to keep the memory usage under control:

  • The query returns a controlled amount of rows
  • It gives an opportunity to act before and after every page

For example, the entity manager can be safely cleared between two pages:

<?php

foreach ($paginator as $page) {
    foreach ($page() as $result) {
        // ...
    }
    $em->clear();
}

GraphQL/Relay

The paginator is particularly suitable for GraphQL/Relay pagination, since it provides cursors for the pages and items.

  • For pages: Use firstCursor() / lastCursor() on the PaginatorPageInterface.
  • For items: Use getCursor() on the PaginatorPageItemInterface.
<?php

foreach ($paginator as $page) {
    $startCursor = $page->firstCursor();
    $endCursor = $page->lastCursor();
    foreach ($page->items() as $item) {
        $itemCursor = $item->getCursor();
    }
}

Authors

The Mention team and contributors