Paginator

Cursor based pagination for Elixir Ecto.

Why?

There are several ways to implement pagination in a project and they all have pros and cons depending on your situation.

Limit-offset

This is the easiest method to use and implement: you just have to set

LIMIT

OFFSET

• Inconsistent results: if the dataset changes while you are querying, the results in the page will shift and your user might end seeing records they have already seen and missing new ones.

• Inefficiency:

  OFFSET N

  instructs the database to skip the first N results of a query. However, the database must still fetch these rows from disk and order them before it can returns the ones requested. If the dataset you are querying is large this will result in significant slowdowns.

Cursor-based (a.k.a keyset pagination)

This method relies on opaque cursor to figure out where to start selecting records. It is more performant than

LIMIT-OFFSET

It's also consistent, any insertions/deletions before the current page will leave results unaffected.

It has some limitations though: for instance you can't jump directly to a specific page. This may not be an issue for an API or if you use infinite scrolling on your website.

Learn more

• http://use-the-index-luke.com/no-offset
• http://use-the-index-luke.com/sql/partial-results/fetch-next-page
• https://www.citusdata.com/blog/2016/03/30/five-ways-to-paginate/
• https://developer.twitter.com/en/docs/tweets/timelines/guides/working-with-timelines

Getting started

defmodule MyApp.Repo do
  use Ecto.Repo,
    otp_app: :my_app,
    adapter: Ecto.Adapters.Postgres

  use Paginator
end
query = from(p in Post, order_by: [asc: p.inserted_at, asc: p.id])
page = MyApp.Repo.paginate(query, cursor_fields: [:inserted_at, :id], limit: 50)
page.entries contains all the entries for this page.
page.metadata contains the metadata associated with this page (cursors, limit, total count)

Installation

Add

:paginator

mix.exs

def deps do
  [
    {:paginator, "~> 1.0.4"}
  ]
end

Usage

Add

Paginator

defmodule MyApp.Repo do
  use Ecto.Repo,
    otp_app: :my_app,
    adapter: Ecto.Adapters.Postgres

  use Paginator
end

Use the

paginate

query = from(p in Post, order_by: [asc: p.inserted_at, asc: p.id])

return the first 50 posts
%{entries: entries, metadata: metadata}
  = Repo.paginate(
    query,
    cursor_fields: [:inserted_at, :id],
    limit: 50
  )
assign the after cursor to a variable
cursor_after = metadata.after
return the next 50 posts
%{entries: entries, metadata: metadata}
  = Repo.paginate(
    query,
    after: cursor_after,
    cursor_fields: [{:inserted_at, :asc}, {:id, :asc}],
    limit: 50
  )
assign the before cursor to a variable
cursor_before = metadata.before
return the previous 50 posts (if no post was created in between it should be
the same list as in our first call to paginate)
%{entries: entries, metadata: metadata}
  = Repo.paginate(
    query,
    before: cursor_before,
    cursor_fields: [:inserted_at, :id],
    limit: 50
  )
return total count
NOTE: this will issue a separate SELECT COUNT(*) FROM table query to the
database.
%{entries: entries, metadata: metadata}
  = Repo.paginate(
    query,
    include_total_count: true,
    cursor_fields: [:inserted_at, :id],
    limit: 50
  )
IO.puts "total count: #{metadata.total_count}"

Security Considerations

Repo.paginate/4

ArgumentError

before

after

Indexes

If you want to reap all the benefits of this method it is better that you create indexes on the columns you are using as cursor fields.

Example

# If your cursor fields are: [:inserted_at, :id]
# Add the following in a migration

create index("posts", [:inserted_at, :id])

Caveats

• This method requires a deterministic sort order. If the columns you are currently using for sorting don't match that definition, just add any unique column and extend your index accordingly.
• You need to add

  :order_by

  clauses yourself before passing your query to

  paginate/2

  . In the future we might do that for you automatically based on the fields specified in

  :cursor_fields

  .
• There is an outstanding issue where Postgrex fails to properly builds the query if it includes custom PostgreSQL types.
• This library has only be tested with PostgreSQL.

Documentation

Documentation is written into the library, you will find it in the source code, accessible from

iex

Contributing

Running tests

Clone the repo and fetch its dependencies:

$ git clone https://github.com/duffelhq/paginator.git
$ cd paginator
$ mix deps.get
$ mix test

Building docs

$ mix docs

Copyright and License

Copyright (c) 2017 Steve Domin.

This software is licensed under the MIT license.