Grouped running total with Power Query M - powerbi

Question

This is how my table looks like (1.7 million rows):
I'm trying to build a running total per customer ID and date.
This is easy to express using DAX, but unfortunately I don't have enough memory on my machine (16GB RAM).
So, I'm trying to find an alternative with Power Query M using buffered tables, etc. but that is too complicated for me.
Can anyone help? Thank you so much in advance!
EDIT: After sorting by Date and CustomerID, added index and added a custom column with:
= Table.AddColumn(#"Added Index", "Personalizado", each (i)=>List.Sum(Table.SelectRows(#"Added Index", each [CustomerID]=i[CustomerID] and [Index]<=i[Index]) [Sales]))
I get the following:
EDIT2:
The whole code:
let
Origem = dataset,
#"Linhas Agrupadas" = Table.Group(Origem, {"Date", "CustomerID"}, {{"Sales", each List.Sum([Sales]), type nullable number}}),
#"Linhas Ordenadas" = Table.Sort(#"Linhas Agrupadas",{{"Date", Order.Ascending}, {"CustomerID", Order.Ascending}}),
#"Linhas Filtradas" = Table.SelectRows(#"Linhas Ordenadas", each [Sales] <> 0),
#"Added Index" = Table.AddIndexColumn(#"Linhas Filtradas", "Index", 0, 1, Int64.Type),
#"Personalizado Adicionado" = Table.AddColumn(#"Added Index","CumSum",(i)=>List.Sum(Table.SelectRows(#"Added Index", each [CustomerID]=i[CustomerID] and [Index]<=i[Index]) [Sales]), type number )
in
#"Personalizado Adicionado"

Answer 1

Method1
Sort your data to start with, perhaps on the date column and CustomerID column. However it appears on screen is the row order it is going to accumulate the total
Add column .. index column...
Add column .. custom column with formula
= (i)=>List.Sum(Table.SelectRows(#"Added Index", each [CustomerID]=i[CustomerID] and [Index]<=i[Index]) [Sales])
Right click index column and remove it
Likely adding a Table.Buffer() around the index step will help speed things up
Sample full code:
let Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
#"Sorted Rows" = Table.Sort(Source,{{"CustomerID", Order.Ascending}, {"Date", Order.Ascending}}),
#"Added Index" = Table.Buffer(Table.AddIndexColumn(#"Sorted Rows", "Index", 0, 1)),
#"Added Custom" = Table.AddColumn(#"Added Index","CumSum",(i)=>List.Sum(Table.SelectRows(#"Added Index", each [CustomerID]=i[CustomerID] and [Index]<=i[Index]) [Sales]), type number ),
#"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Index"})
in #"Removed Columns"
Method 2:
Create function fn_cum_total
(Input) =>
let withindex = Table.AddIndexColumn(Input, "Index", 1, 1),
cum = Table.AddColumn(withindex, "Total",each List.Sum(List.Range(withindex[Sales],0,[Index])))[Total]
in cum
Create query that uses that function to add cumulative totals to Sales column after grouping on CustomerID
let Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
#"Sorted Rows" = Table.Buffer(Table.Sort(Source,{{"CustomerID", Order.Ascending}, {"Date", Order.Ascending}})),
Running_Total = Table.Group(#"Sorted Rows",{"CustomerID"},{{"Data",
(Input as table) as table => let zz = fn_cum_total(Input),
result = Table.FromColumns(Table.ToColumns(Input)&{zz}, Value.Type(Table.AddColumn(Input, "total", each null, type number))) in result, type table}} ),
#"Expanded Data" = Table.ExpandTableColumn(Running_Total, "Data", {"Date", "Sales", "total"}, {"Date", "Sales", "total"})
in #"Expanded Data"
I cannot take credit for method 2, borrowed long ago, but do not recall source