Performance issues of `fetch first n rows only`
not sure i understand correctly. do you mean this is SLOWER or FASTER?

insert into new_table (new_column)
select column as new_column
from table
-- no WHERE clause
fetch first 1000 rows only;


in general, if there is a where-clause, any database engine must read records, and 'drop' those that don't match the where clause. the 'read-loop' must read from disk until 1000 matches are found. the best case will encounter NO non-matches; the worst case will read the complete dataset

without a where-clause, the database engine only has to read 'the first 1000 records', thus every IO operation can be used for the resultset. so, that should be faster.