Add column with list.generate

Hello Anonymous 

I would suggest you to do this with List.Accumulate. After you created the list that represents your index-column you have to integrate it your current table with Table.ToColumns and Table.FromColumns. Check out this example.

Give it a try and if you have performance issue we can check if we can enhance it a little bit

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlCK1YlWMgSTBkgkXCQWAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [A = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"A", Int64.Type}}),
    CreateNewIndexColumn = List.RemoveFirstN
    (
        List.Accumulate
        (
            #"Changed Type"[A],
            {1},
            (state,current)=> state & {List.Last(state) + current}
        )
    )
,

    AddIndexColumnToTable = Table.FromColumns
    (
        Table.ToColumns(#"Changed Type" ) & {CreateNewIndexColumn},
        Table.ColumnNames(#"Changed Type") & {"Index"}
    )
in
    AddIndexColumnToTable

 Copy paste this code to the advanced editor in a new blank query to see how the solution works.

If this post helps or solves your problem, please mark it as solution (to help other users find useful content and to acknowledge the work of users that helped you)
Kudoes are nice too

Have fun

Jimmy

ten millions rows worked in less than1'

let
    Source = List.Transform(List.Random(10000000), each Number.Round(_)),
    #"Converted to Table" = Table.FromList(Source, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Grouped Rows" = Table.Group(#"Converted to Table", {"Column1"}, {{"Count", each Table.RowCount(_), Int64.Type}},GroupKind.Local),
    #"Added Index" = Table.AddIndexColumn(#"Grouped Rows", "Index", 1, 1, Int64.Type),
    #"Added Custom" = Table.AddColumn(#"Added Index", "idx", each List.Repeat({[Index]},[Count])),
    #"Expanded idx" = Table.ExpandListColumn(#"Added Custom", "idx")
in
    #"Expanded idx"