Pagination

Pagination is configured at the action level. There are two kinds of pagination supported: keyset and offset. There are pros and cons to each. An action can support both at the same time, or only one (or none). A full count of records can be requested by passing page: [count: true], but it should be kept in mind that doing this requires running the same query twice, one of which is a count of all records. Ash does these in parallel, but it can still be quite expensive on large datasets. For more information on the options for configuring actions to support pagination, see the pagination section in Ash.Resource.Dsl.Read.read/2

Offset Pagination

Offset pagination is done via providing a limit and an offset. A limit is how many records that should be returned on the page. An offset is how many records from the beginning should be skipped. Using this, you might make requests like the following:

# Get the first ten records
Api.read(Resource, page: [limit: 10])
# Get the second ten records
Api.read(Resource, page: [limit: 10, offset: 10])
# No need to do this in practice, see `c:Ash.Api.page/2`

Offset Pros

  • Simple to think about
  • Possible to skip to a page by number. E.g the 5th page of 10 records is offset: 40
  • Easy to reason about what page you are currently on (if the total number of records is requested)
  • Can go to the last page (even though, if done by using the full count, the data could have changed)

Offset Cons

  • Does not perform well on large datasets
  • When moving between pages, if data was created or deleted, records may appear on multiple pages or be skipped

Keyset Pagination

Keyset pagination is done via providing an after or before option, as well as a limit. The value of this option should be a keyset that has been returned from a previous request. Keysets are returned when a request is made with a limit to an action that supports keyset pagination, and they are stored in the __metadata__ key of each record. The keyset is a special value that can be passed into the after or before options, to get records that occur after or before.

For example:

page = Api.read(Resource, page: [limit: 10])

last_record = List.last(page.results)

# No need to do this in practice, see `c:Ash.Api.page/2`
next_page = Api.read(Resource, page: [limit: 10, after: last_record.__metadata__.keyset])

Keyset Pros

  • Performs very well on large datasets (assuming indices exist on the columns being sorted on)
  • Behaves well as data changes. The record specified will always be the first or last item in the page

Keyset Cons

  • A bit more complex
  • Can't go to a specific page number
  • Going to the last page requires

For more information on keyset vs offset based pagination, see: