Thread: integer square root function proposed

Dear PostgreSQL colleagues:

I have just joined this, my first PG mailing list.

Reading the documentation I found no built-in function for integer square root, requiring a sequence of:

floor(sqrt(foo))::integer

to go from an integer to the integer square root as an integer.

If PG leaders smile on this suggestion, could we have isqrt(foo) where foo has a numeric type and the result type is the same numeric type, with the result type being the integer square root for foo >= 0 and producing an error if negative? (And with infinities and NaNs processed as described in 8.1 Numeric Types).

I did not see an explicit enhancement request list in the PG mailing lists. Recommendations for a more specific list to post this to are welcome.

I suspect the majority are comfortable getting a non-integer result for the square root of a number.  What environment have you which needs (sqrt(a) * sqrt(a)) < a-epsilon?

Dear Rob and all readers:

Generating prime numbers is one example where you use integer square root in the inner loop, going from integer to integer.

Calculating an integer square root from an integer input may have a more efficient algorithm than doing so in floating-point, with the caveat that an underlying processor architecture may provide floating-point square root instructions but not integer square root instructions. In that particular case an implementation could use the floating-point instructions internally.

Some but not all programming languages provide isqrt directly, math.isqrt in Python or isqrt in Common Lisp for example.

It would be a useful and convenient function and would not, I believe, impair the other features of PostgreSQL in any way.

That said, as a PG novice (2+ years now), I completely defer to the greater wisdom of those much more involved in PostgreSQL. So something for you all to think about.

Seasons greetings to you and welcome to the list.  Here we generally bottom post.

I have zero say in whether or not isqrt might get added to postgres but in the meantime you might craft your own function (sql or plpgsql) which does the necessary casting.  Or are you specifically after a faster function?  And would a plpythonu function using their isqrt be fast enough.  (Note the "u" in plpythonu means "untrusted")

"Martin L. Buchanan" <martinlbuchanan@gmail.com> writes:
> Reading the documentation I found no built-in function for integer square
> root, requiring a sequence of:
> floor(sqrt(foo))::integer
> to go from an integer to the integer square root as an integer.

I'm a little down on this idea, mainly because I doubt there is any
principled argument as to whether the rounding ought to be down, up,
or to nearest.  Specific applications probably want a specific one
of those, but how likely is it that a built-in choice would satisfy
many users?

I'm not buying the performance angle too much either.  Even granting
that your platform has an integer square root instruction that's faster
than its floating-point one, and that we can get at that from C, the
difference would likely be swamped by the overhead of the SQL function
call.

> (And with infinities and NaNs processed as described in
> 8.1 Numeric Types).

Our integers do not have infinities, nor NaNs.

Anyway, I don't mean to slam the door on this idea completely,
but I think you need to make a much better-supported argument
for it.

            regards, tom lane