How to calculate time difference between concecutive rows efficiently

[Anonymous]

I'm using Power Query Online and I have about 60000 rows of data with columns session_user_email and session_start_time.

I need to calculate the time difference between concecutive rows for the same email between start_times. If the emails are different, value should be null. Data is already sorted.

I'm using the Index method but it is slow. This is my code:

#"Added index" = Table.AddIndexColumn(#"Renamed columns", "Index", 0, 1, Int64.Type),
  #"Added custom" =
Table.AddColumn(#"Added index", "session_end_time", each try
if 
#"Added index"{[Index]}[session_user_email] = 
#"Added index"{[Index] + 1}[session_user_email] then
#"Added index"{[Index] + 1}[session_start_time]
else
null
otherwise
null),

 How can I make this more efficient?

[AlienSx · Accepted solution]

@Anonymous 

end_time = (tbl as table) as table =>
    Table.FromColumns(
        Table.ToColumns(tbl) & {List.RemoveFirstN(tbl[session_start_time], 1) & {null}},
        Table.ColumnNames(tbl) & {"session_end_time"}
    ),
g = Table.Group(#"Renamed columns", "session_user_email", {"upd", (x) => end_time(Table.Sort(x, "session_start_time"))}),
expand = Table.ExpandTableColumn(g, "upd", {"session_start_time", "session_end_time"})

[Anonymous]

This works very well, thank you!

[Syndicate_Admin]

Slap a Table.Buffer around your code.