f# sqlite sqlprovider minBy maxBy using float

Question

I have a sqlite table with a mix of integer and float columns. I'm trying to get the max and min values of each column. For integer columns the following code works but I get a cast error when using the same code on float columns:

let numCats = query{for row in db do minBy row.NumCats}

For float columns I'm using the following code but it's slow.

let CatHight = query{for row in db do select row.CatHeight} |> Seq.toArray |> Array.max

I have 8 integer columns and 9 float columns and the behavior has been consistent across all columns so that's why I think it's an issue with the column type. But I'm new to F# and don't know anything so I'm hoping you can help me.

Thank you for taking the time to help, it's much appreciated.

SQLProvider version: 1.0.41

System.Data.SQLite.Core version: 1.0.104

The error is: System.InvalidCastException occurred in FSharp.Core.dll

Added Information

I created a new table with one column of type float. I inserted the values 2.2 and 4.2. Using SQLProvider and System.Data.SQLite.Core I connected queried the database using minBy or maxBy and I get the cast exception. If the column type is integer it works correctly.

More Added Information

Exception detail:

System.Exception was unhandled Message: An unhandled exception of type 'System.Exception' occurred in >FSharp.Core.dll Additional information: Unsupported execution expression value(FSharp.Data.Sql.Runtime.QueryImplementation+SqlQueryable1[FSharp.>Data.Sql.Common.SqlEntity]).Min(row => >Convert(Convert(row.GetColumn("X"))))`

Code that fails:

open FSharp.Data.Sql

[<Literal>]
let ConnectionString =
"Data Source=c:\MyDB.db;" +
"Version=3;foreign keys=true"

type Sql = SqlDataProvider<Common.DatabaseProviderTypes.SQLITE,
ConnectionString,
//ResolutionPath = resolutionPath,
CaseSensitivityChange = Common.CaseSensitivityChange.ORIGINAL>

let ctx = Sql.GetDataContext()
let Db = ctx.Main.Test

let x = query{for row in Db do minBy row.X}
printfn "x: %A" x

Update 2/1/17

Another user was able to reproduce the issue so I filed an Issue with SQLProvider. I'm now looking at workarounds and while the following code works and is fast, I know there's a better way to do it - I just can't find the correct way. If somebody answers with better code I'll accept that answer. Thanks again for all the help.

let x = query {for row in db do
                sortBy row.Column
                take 1
                select row.Column } |> Seq.toArray |> Array.min

Answer 1

This is my workaround that @s952163 and good people in the SO f# chat room helped me with. Thanks again to everyone who helped.

let x = query {for row in db do
                sortBy row.Column
                take 1
                select row.Column } |> Seq.head

Answer 2

You need to coerce the output column to int or float (whichever you need or is giving trouble to you). You also need to take care in case any of your columns are nullable. The example below will coerce the column to float first (to take care of being nullable), then convert it to int, and finally get the minimum:

let x = query { for row in MYTABLE do
                minBy (int (float row.MYCOLUMN))}

You might want to change the order of course, or just say float Mycolumn.

Update: With Sqlite it indeed causes an error. You might want to do query { ... } |> Seq.minBy to extract the smallest number.