Handle backend-specific empty IN list at runtime by sniffing connection type.

This isn't pretty, but definitely simpler than giving the command batch access
to the backend that was used to generate the given command fragments.

Author: rspeele

src/Rezoom.SQL.Mapping/CommandBatch.fs

@@ -1,21 +1,78 @@
 namespace Rezoom.SQL.Mapping
 open System
+open System.Data
 open System.Data.Common
 open System.Collections.Generic
 open System.Text
 open System.Threading
 open FSharp.Control.Tasks.ContextInsensitive
 open Rezoom.SQL
 
-type private CommandBatchBuilder(conn : DbConnection, tran : DbTransaction) =
-    let maxParameters =
-        match conn.GetType().Namespace with
-        | "System.Data.SqlClient" -> 2100 // SQL server
+type private CommandBatchRuntimeBackend =
+    private
+    | SQLServer
+    | Oracle
+    | Postgres
+    | MySQL
+    | SQLite
+    | Other
+    static member OfNamespace(ns : string) =
+        match ns with
+        | "System.Data.SqlClient" -> SQLServer
         | "System.Data.OracleClient"
-        | "Oracle.DataAccess.Client" -> 2000 // Oracle
-        | "Npgsql" -> 10000 // Postgres -- can support more but it's probably a bad idea
-        | "MySql.Data.MySqlClient" -> 10000 // MySQL -- can support more but it's probably a bad idea
-        | _ -> 999 // SQLite default, and probably the lowest of any DB
+        | "Oracle.DataAccess.Client" -> Oracle
+        | "Npgsql" -> Postgres
+        | "MySql.Data.MySqlClient" -> MySQL
+        | "System.Data.SQLite"
+        | "Devart.Data.SQLite" -> SQLite
+        | _ -> Other
+    member this.MaxParameters() =
+        match this with
+        | SQLServer -> 2100
+        | Oracle -> 2000
+        | Postgres
+        | MySQL -> 10_000
+        | SQLite 
+        | Other -> 999
+    static member private PgType(ty : DbType) =
+        match ty with
+        | DbType.String
+        | DbType.StringFixedLength
+        | DbType.AnsiString
+        | DbType.AnsiStringFixedLength -> "text"
+        | DbType.Byte
+        | DbType.SByte
+        | DbType.UInt16
+        | DbType.UInt32
+        | DbType.UInt64
+        | DbType.Int16
+        | DbType.Int32
+        | DbType.Int64
+        | DbType.VarNumeric
+        | DbType.Decimal
+        | DbType.Single
+        | DbType.Double -> "numeric"
+        | DbType.Binary -> "bytea"
+        | DbType.Guid -> "uuid"
+        | DbType.DateTime
+        | DbType.DateTime2
+        | DbType.DateTimeOffset -> "timestamptz"
+        | _ -> "unknown"
+    member this.EmptyInList(ty : DbType) =
+        match this with
+        | Postgres ->
+            // PG has to be difficult and demand a type specifier matching the input
+            "(SELECT NULL::" + CommandBatchRuntimeBackend.PgType(ty) + " WHERE FALSE)"
+        | SQLite ->
+            // SQLite is cool and accepts the simple approach. This might be faster than the empty subquery.
+            "()"
+        | _ ->
+            "(SELECT NULL WHERE 1=0)"
+
+type private CommandBatchBuilder(conn : DbConnection, tran : DbTransaction) =
+    let runtimeBackend = CommandBatchRuntimeBackend.OfNamespace(conn.GetType().Namespace)
+    let maxParameters = runtimeBackend.MaxParameters()
+
     static let terminatorColumn i = "RZSQL_TERMINATOR_" + string i
     static let terminator i = ";--'*/;SELECT NULL AS " + terminatorColumn i
     static let parameterName i = "@RZSQL_" + string i
@@ -51,12 +108,15 @@ type private CommandBatchBuilder(conn : DbConnection, tran : DbTransaction) =
                 | CommandText str -> str
                 | Parameter i ->
                     match command.Parameters.[i] with
-                    | ListParameter(_, os) ->
-                        let parNames =
-                            seq {
-                                for j = 0 to os.Length - 1 do yield parameterNameArray (parameterOffset + i) j
-                            }
-                        "(" + String.concat "," parNames + ")"
+                    | ListParameter(dbTy, os) ->
+                        if os.Length = 0 then
+                            runtimeBackend.EmptyInList(dbTy)
+                        else
+                            let parNames =
+                                seq {
+                                    for j = 0 to os.Length - 1 do yield parameterNameArray (parameterOffset + i) j
+                                }
+                            "(" + String.concat "," parNames + ")"
                     | ScalarParameter _ -> parameterName (parameterOffset + i)
                     | RawSQLParameter frags ->
                         for frag in frags do