A Persistence Framework for Scala and NoSQL

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offsets and limits

Longevity also supports offset and limit clauses for your queries. For example, suppose we want to display users by username, and we want to page the results. We construct a method that will return a query that will retrieve a single page of results:

def userPageQuery(pageNum: Int, pageSize: Int): Query[User] = {
  import User.queryDsl._
  import User.props._
  filterAll orderBy username offset pageNum * pageSize limit pageSize

The orderBy, offset, and limit clauses are all optional, and can occur whether or not the other two clauses appear. But the clauses have to occur in the right order, so variations such as these will not work:

// these combinations won't work:

filterAll offset pageNum * pageSize limit pageSize orderBy username

filterAll orderBy username limit pageSize offset pageNum * pageSize

The offset and limit clauses will affect performance. A large offset will see performance degrade, as the back end generally has to collect all the results with an offset less than the offset you supplied. The limit clause will only improve performace for unordered queries, or when the back end is able to collect the ordered results in place. See the section of ordered queries for a description of when this is possible.

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