functional-programming - F# 查询表达式是否具有与 native SQL 相同的性能？

Question

我注意到 F# 查询表达式示例看起来好像在进行计算之前获取了表的全部内容。这似乎比本地 SQL 效率低。或者性能应该是一样的。

// A query expression.
let query1 =
    query {
        for customer in db.Customers do
            select customer
    }

Answer 1

F# 查询表达式非常智能。除了使用引号(类似于 System.Linq.Expressions )将代码转换为 SQL 之外，我并不完全理解它们是如何工作的。 ORM 有 measurable overhead ，原始查询会更快，但它们能够生成优化的查询
鉴于这些模型和 ef-core 上下文

[<CLIMutable>] type Address = { Id: int; City: string; Country: string }
[<CLIMutable>] type Person = { Id: int; Name: string; Age: int; Address: Address }

type PeopleContext() =
    inherit DbContext()

    [<FSharp.Core.DefaultValue>]
    val mutable _people : DbSet<Person>

    member db.People
        with get () = db._people
        and set value = db._people <- value

    override _.OnConfiguring options =
        options.UseSqlite("Data source=db.db3") |> ignore

我创建了这个查询

query {
    for person in ctx.People do
        where (person.Age > 21)
        select person.Address.City
        take 1
} |> Seq.toArray |> printfn "%A"

这被翻译成这个 sql

SELECT "a"."City"
FROM (
    SELECT "p"."AddressId"
    FROM "People" AS "p"
    WHERE "p"."Age" > 21
    LIMIT @__p_0
) AS "t"