Translate index match in Excel Power Query

Using a similar algorithm as m_dekorte , and generating a CSV file with 800,000 rows, and only single matches between Columns B & C, the PQ took about 15 seconds to run. (It took longer to write the results back to the Excel worksheet). I did not get any out of memory errors.

M-Code: Paste into Advanced Editor

let

//Change next line to reflect actual file path
    Source = Csv.Document(File.Contents("C:\Users\ron\Documents\test.csv"),[Delimiter=",", Columns=3, Encoding=1252, QuoteStyle=QuoteStyle.None]),
    #"Promoted Headers" = Table.PromoteHeaders(Source, [PromoteAllScalars=true]),
    #"Changed Type" = Table.TransformColumnTypes(#"Promoted Headers",{{"Column A", type text}, {"Column B", Int64.Type}, {"Column C", Int64.Type}}),

//Add index column to be able to re-sort into original order
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),

//Join the table to itself, using different columns for the "key"
    #"Self-Join" = Table.NestedJoin(#"Added Index","Column B",#"Added Index","Column C","Index Match",JoinKind.LeftOuter),

//Expand the joined table Column A
    #"Expanded Index Match" = Table.ExpandTableColumn(#"Self-Join", "Index Match", {"Column A"}, {"Index Match.Column A"}),

//Sort back to original order
    #"Sorted Rows" = Table.Sort(#"Expanded Index Match",{{"Index", Order.Ascending}}),

//Remopve Index column
    #"Removed Columns" = Table.RemoveColumns(#"Sorted Rows",{"Index"})
in
    #"Removed Columns"

Partial Results

How much memory do you have?

Are you running Excel 32 bit or 64 bit?