Using optional variables in query-CodePudding

I have a search type which is used to query the database as follows,

type UserSearch = 
  { limit: int option
    skip: int option }

I'm using SQLProvider and would like to use limit and skip variables in my query such as

query {
  for user in Db.getDataContext().MyDb.Users do
  select user

  // if searchParams.limit.IsSome then
  //     take searchParams.limit.Value
  // if searchParams.skip.IsSome then
  //     skip searchParams.skip.Value
}

Is it possible?

My current approach is as follows;

let mutable search =
  query {
    for user in Db.getDataContext().MyDb.Users do
    select user
  }

if searchParams.limit.IsSome then
  search <-
    query {
      for user in search do
      select user
      take searchParams.limit.Value
    }

search |> Seq.toArray

Since queries are lazy, this should result a single query, am i correct on this assumption?

CodePudding user response：

Unfortunately, the query builder turns the result into seq<_> and forces it to run the SQL query, so you cannot easily compose queries this way. There may be more sophisticated tricks you could use, but for your specific case, it may be easier to just use a reasonable default value such as 0 for skip and Int32.MaxValue for take:

query {
  for user in Db.getDataContext().MyDb.Users do
  skip (defaultArg searchParams.skip 0)
  take (defaultArg searchParams.limit.Value Int32.MaxValue)
  select user }

This will generate a query with skip and take even when you do not need that, which may have some performance implications - I do not think it matters, but it may be best to test that.