I don't know if this error shows up in other cases, but I always see it when I'm about to do
x[y,b:=b] but first want to check how x[y] looks before creating or overwriting x$b. Here's an example: x <- data.table(a=rep(2:3,2),key='a') y <- data.table(a=1:4,b=4:1,key='a') x[y] # error x[y,nomatch=0] # ok x[y,b:=b] # ok I'd prefer to see the first attempt mapped to the second (with a suitable message), instead of erroring out. What do you all think? Is that reasonable/worthwhile? Best, Frank P.S. One other point, regarding the message itself (reproduced down below): I don't understand why repeated values in i are mentioned. -- For x[y] in my example, the problem seems to be coming from x having repeated rows, not i (y in this case); -- whereas y[x] works just fine (despite the repeated/duplicated values in i...which is x here). Error in vecseq(f__, len__, if (allow.cartesian) NULL else as.integer(max(nrow(x), : Join results in 6 rows; more than 4 = max(nrow(x),nrow(i)). Check for duplicate key values in i, each of which join to the same group in x over and over again. If that's ok, try including `j` and dropping `by` (by-without-by) so that j runs for each group to avoid the large allocation. If you are sure you wish to proceed, rerun with allow.cartesian=TRUE. Otherwise, please search for this error message in the FAQ, Wiki, Stack Overflow and datatable-help for advice.
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