Tuesday, December 03, 2013

Oracle 12cR1, UDF Pragma and HyperLogLog

One interesting enhancement in 12cR1 PL/SQL is UDF pragma which has the following description:
The UDF pragma tells the compiler that the PL/SQL unit is a user defined function that is used primarily in SQL statements, which might improve its performance.
I though it would be very cool to try it out with my HyperLogLog post I did recently and see if it results in any measurable performance improvement.

Test Table

I'll use the same test table as I did in my original post:
SQL> create table z_hll_test as
  2   select dbms_random.string('x', 4)||rpad('x', 500, 'x') n
  3    from dual
  4    connect by level <= 1000000;
 
Table created

SQL> alter table z_hll_test cache;
 
Table altered
Note that I'm explicitly setting the table to cache in order to make in-memory PQ kick in and eliminate disk I/O as a factor from my test case. For each test I made sure that no physical I/O has happened (including temp I/O for native distinct test).

Regular Function
create or replace function num_zeroes(
 p_n binary_integer
 ) return binary_integer deterministic is
 l_t binary_integer;
 l_b binary_integer;
begin
 --assume 32-bit hash value, 10-bits for bucket
 if (p_n = 0) then return 22; end if;

 l_t := 1;
 l_b := 0;

 while ( bitand(p_n,l_t) = 0 )
 loop
  l_t := l_t*2;
  l_b := l_b+1;
 end loop;

  return l_b;

end num_zeroes;

SQL> select
  2    case
  3      when hll <= 2560 and zeroes > 0 then round(1024*ln(1024*1/zeroes))
  4      when hll >  1/30 * power(2,32) then round(-power(2,32) * ln(1 - hll/power(2,32)))
  5      else round(hll)
  6    end num_distinct
  7    from (
  8      select 0.72054 * (1048576/(current_sum+zeroes)) hll, zeroes
  9        from (
 10          select sum(1/power(2, val)) current_sum, (1024-count(*)) zeroes
 11            from (
 12              select /*+ parallel(z 4) */ mod(ora_hash(n), 1024) bucket,
 13                  max(num_zeroes(trunc(ora_hash(n)/1024)))+1 val
 14                from z_hll_test z
 15                group by mod(ora_hash(n), 1024)
 16            )
 17        )
 18  );
 
NUM_DISTINCT
------------
      748175
 
Executed in 0.889 seconds
Pragma UDF Function
create or replace function num_zeroes(
 p_n binary_integer
 ) return binary_integer deterministic is
 pragma udf;
 l_t binary_integer;
 l_b binary_integer;
begin
 --assume 32-bit hash value, 10-bits for bucket
 if (p_n = 0) then return 22; end if;

 l_t := 1;
 l_b := 0;

 while ( bitand(p_n,l_t) = 0 )
 loop
  l_t := l_t*2;
  l_b := l_b+1;
 end loop;

  return l_b;

end num_zeroes;

SQL> select
  2    case
  3      when hll <= 2560 and zeroes > 0 then round(1024*ln(1024*1/zeroes))
  4      when hll >  1/30 * power(2,32) then round(-power(2,32) * ln(1 - hll/power(2,32)))
  5      else round(hll)
  6    end num_distinct
  7    from (
  8      select 0.72054 * (1048576/(current_sum+zeroes)) hll, zeroes
  9        from (
 10          select sum(1/power(2, val)) current_sum, (1024-count(*)) zeroes
 11            from (
 12              select /*+ parallel(z 4) */ mod(ora_hash(n), 1024) bucket,
 13                  max(num_zeroes(trunc(ora_hash(n)/1024)))+1 val
 14                from z_hll_test z
 15                group by mod(ora_hash(n), 1024)
 16            )
 17        )
 18  );
 
NUM_DISTINCT
------------
      748175
 
Executed in 0.593 seconds
Pragma UDF gives us ~33% performance boost which is not too bad considering we didn't have to do anything else. However, that's not the most interesting part -- let's take a look at the native distinct next.

Native Distinct
SQL> select /*+ parallel(z 4) */ count(distinct n) from z_hll_test z;
 
COUNT(DISTINCTN)
----------------
          753204
 
Executed in 0.983 seconds

Note that this was an optimal execution!

Summary

Let's summarize results in a table:
Regular FunctionPragma UDF FunctionNative Distinct
0.8890.5930.983

As you can see pragma udf actually beats native implementation by a considerable margin which is very impressive given the fact that distict had an optimal execution.

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