In your spreadsheet, you could also just use and avoid adding a new column!

=FILTER(all_items!A2:A, COUNTIF(out_of_stock!A2:A, all_items!A2:A) = 0)

If I do one of the following:

=ARRAYFORMULA(IF(LEN(A:A),DO_THE_THING,))

OR

= ARRAYFORMULA(IF(ISBLANK(A:A),,DO_THE_THING))

In both instances, the use of the IF should reduce the cost of the formula because it’s checking the content of the cell in question first compared to FILTER running against all cells in the range and returning only the ones that then meet the criteria?

Excellent! thank you very much Dave, I’m going ot review your new post, it will be really useful.

Alright, guys. Here’s what I’ve come up with. Google Sheets Formula to Create a Right Outer Join

..for me too

Makes sense, thank you, Dave!

Under the hood, I suspect that FILTER is effectively ARRAYFORMULA with an added “match” component.

ARRAYFORMULA loops through each row or column in a range. When using a non-array function such as VLOOKUP, ARRAYFORMULA iterates through each of the cells in the search_key range, looks for a matching record in the first column of the (search) range, and returns the value from the (return) index.

FILTER likely loops through each row or column in a range and applies nested functions just as ARRAYFORMULA does. However, it then reduces the returned range to only those rows or columns that match a specified set of criteria.

=FILTER(A:A,B:B=TRUE)

In this example, FILTER loops through the range A:A and checks to see if B:B=TRUE. It then returns the resulting range from A:A where B:B=TRUE.

=FILTER(VLOOKUP(A:A,data!A:B,2,TRUE),B:B=TRUE)

This formula loops through A:A and performs a VLOOKUP on each cell in A:A. The VLOOKUP returns the index (2) from the row where the value from first column from data!A:B matches that of the corresponding row in A:A. FILTER then reduces the range of A:A to only those rows where B:B=TRUE.

One thing that still seems like magic to me is how enclosing VLOOKUP — which really wants to just return a single value, even though it deceptively accepts a range (i.e. multiple rows) in its “search key” parameter, making me thing it’ll repeat for all values in that range — how just wrapping it in FILTER results in VLOOKUP _somehow_ now being applied for each result that FILTER returns. There’s a connection there I’m not seeing that’s probably key to understanding a bunch of other stuff with Sheets functions.

When I put VLOOKUP inside an array, inside ARRAYFORMULA, then it applies the VLOOKUP for all rows, and that makes sense to me, given the purpose of ARRAYFORMULA. However, it has that problem you discuss about empty rows and processing for all rows to the extent of the sheet, vs using FILTER that scopes it to only filled rows. Still, this makes more sense to me than FILTER in terms of the expected outcome.