from Release Notes v.1.0.1:

"
A Gotcha with SELECT FIRST
This

delete from TAB1 where PK1 in (select first 10 PK1 from TAB1);

will delete all of the rows in the table. Ouch! the sub-select is
evaluating each 10 candidate rows for deletion, deleting them, slipping
forward 10 more...ad infinitum, until there are no rows left. Beware!
"

Question: what is PK1 ?

TIA

Duilio Foschi