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dbms_random in parallel

Posted on June 13, 2012 by Sayan Malakshinov Posted in documentation, oracle, parallel

The documentation for dbms_random states:

It will automatically initialize with the date, user ID, and process ID if no explicit initialization is performed.

This phrase does not answer the question, which “process id” is going to be used for initialization in case of parallel execution. That’s why I decided to give a vivid example that shows independence of “dbms_random” generator from “process id” of slave, that is generating identical values in parallel:

with
 t  as ( select/*+ materialize */ level n from dual connect by level<=4000)
,t1 as (
         select--+ materialize parallel(t 4)
            dbms_random.string('x',4)
            ||';'
            ||(select sid||';'||process||';'||pid
               from v$session, v$process
               where sid=sys_context('USERENV','SID')
                 and PADDR=ADDR
                 and n>0
              ) f
         from t
)
,t2 as (
         select
            t1.f
           ,count(*) over(partition by regexp_substr(f,'^[^;]+')) cnt
         from t1
)
select f
      ,regexp_substr(f,'[^;]+') rnd
      ,regexp_substr(f,'[^;]+',1,2) sid
      ,regexp_substr(f,'[^;]+',1,3) process
      ,regexp_substr(f,'[^;]+',1,4) pid
from t2 
where cnt>1
order by f

Result:


F RND SID PROCESS PID
AARV;130;5472;30 AARV 130 5472 30
AARV;68;2228;29 AARV 68 2228 29
AC2R;130;5472;30 AC2R 130 5472 30
AC2R;68;2228;29 AC2R 68 2228 29
AC8O;130;5472;30 AC8O 130 5472 30
AC8O;68;2228;29 AC8O 68 2228 29
AKVZ;130;5472;30 AKVZ 130 5472 30
AKVZ;68;2228;29 AKVZ 68 2228 29
ALTQ;130;5472;30 ALTQ 130 5472 30
ALTQ;68;2228;29 ALTQ 68 2228 29
… … … … …

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