Question

What is the difference between “TOP” and “SAMPLE” in TeraData SQL? Are they the same?

Answer 1

From TOP vs SAMPLE:

TOP 10 means “first 10 rows in sorted
order”. If you don’t have an ORDER BY,
then by extension it will be
interpreted as asking for “ANY 10
rows” in any order. The optimizer is
free to select the cheapest plan it
can find and stop processing as soon
as it has found enough rows to return.

If this query is the only thing
running on your system, TOP may appear
to always give you exactly the same
answer, but that behavior is NOT
guaranteed.

SAMPLE, as you have observed, does
extra processing to try to randomize
the result set yet maintain the same
approximate distribution. At a very
simple level, for example, it could
pick a random point at which to start
scanning the table and a number of
rows to skip between rows that are
returned.