Recently I showed simple example how result_cache works with non-deterministic functions and observed strange behaviour: a function gets fired once in the normal query, but twice with the result_cache hint.
Moreover, only third and subsequent query executions return same cached results as second query execution.
I didn’t want to investigate such behavior, just because 1) we should not cache the results of non-deterministic functions and 2) it doesn’t matter if we use deterministic functions.
But later I was asked to explain this, so this post is just a short description with test case.
Look at the simple function that just returns random values:
create or replace function f_nondeterministic(p int:=100) return int as res number; begin res:=round(dbms_random.value(0,p)); return res; end;
SQL> exec dbms_result_cache.flush;
PL/SQL procedure successfully completed.
SQL> select/*+ result_cache */ f_nondeterministic(1000) nondeter from dual;
NONDETER
----------
481
SQL> select/*+ result_cache */ f_nondeterministic(1000) nondeter from dual;
NONDETER
----------
689
SQL> select/*+ result_cache */ f_nondeterministic(1000) nondeter from dual;
NONDETER
----------
689
SQL> exec dbms_result_cache.flush;
PL/SQL procedure successfully completed.
SQL> select/*+ result_cache */ f_nondeterministic(1000) nondeter from dual;
NONDETER
----------
481
SQL> select name,value from v$result_cache_statistics where name in ( 'Create Count Success', 'Find Count');
NAME VALUE
-------------------------------------------------- ----------
Create Count Success 1
Find Count 0
SQL> select/*+ result_cache */ f_nondeterministic(1000) nondeter from dual;
NONDETER
----------
689
SQL> select name,value from v$result_cache_statistics where name in ( 'Create Count Success', 'Find Count');
NAME VALUE
-------------------------------------------------- ----------
Create Count Success 1
Find Count 1
SQL> select/*+ result_cache */ f_nondeterministic(1000) nondeter from dual;
NONDETER
----------
689
SQL> select name,value from v$result_cache_statistics where name in ( 'Create Count Success', 'Find Count');
NAME VALUE
-------------------------------------------------- ----------
Create Count Success 1
Find Count 2
SQL> select name,bucket_no, id,type,status,pin_count,scan_count,invalidations from v$result_cache_objects o;
NAME BUCKET_NO ID TYPE STATUS PIN_COUNT SCAN_COUNT INVALIDATIONS
-------------------------------------------------- ---------- ---------- ---------- --------- ---------- ---------- -------------
XTENDER.F_NONDETERMINISTIC 552 0 Dependency Published 0 0 0
select/*+ result_cache */ f_nondeterministic(1000) 2102 1 Result Published 0 2 0
nondeter from dual
As you can see, second execution returns different result than first one.
If we change this function:
create or replace function f_nondeterministic(p int:=100) return int
as
res number;
begin
res:=round(dbms_random.value(0,p));
dbms_output.put_line('fired! ('||res||')');
return res;
end;
and repeat this test-case:
SQL> select/*+ result_cache */ f_nondeterministic(1000) nondeter from dual;
NONDETER
----------
943 -- << (2)
1 row selected.
fired! (607) -- << (1)
fired! (943) -- << (2)
SQL> /
NONDETER
----------
607 -- << (1)
1 row selected.
SQL> /
NONDETER
----------
607 -- << (1)
1 row selected.
SQL> /
NONDETER
----------
607 -- << (1)
1 row selected.
we will see that there were 2 function executions: first result was cached, and the second was fetched!