I have a custom record that is 1:1 to transaction lines. On that record, I have a list/record-field for transaction and then a field for line_unique_key and line-id. To join correctly to this record, I need to do either

left join my_table MT on TL.uniquekey = MT.uniquekey

or

left join my_table MT on TL.id = MT.lineid and T.id = MT.transaction

I'm guessing that Netsuite doesn't index any custom columns. And to get the most performance out of the join, I guess I'd want to use an index to join? If I were to put the line unique key as the externalId, do you think externalId is indexed? Would it perform better? Or maybe Netsuites DB-optimizers create indexes automatically for commonly used joins? In searches, I need to use the real join to the transaction and then a formula criteria

decode({lineunquekey},{mytable.lineuniquekey},1,0) = 1