Category Archives: result_cache

RESULT_CACHE: run-time dependency tracking

Posted on by Posted in documentation, oracle, result_cache 1,881 Page views Leave a comment

As you know, since 11.2 “relies_on” clause was deprecated and oracle tracks dependencies at runtime now.

Test function and tables

[sourcecode language=”sql”]
create or replace function f_without_deps(p_tab varchar2) return varchar2
as
res varchar2(30);
begin
execute immediate ‘select ”’||p_tab||”’ from ‘||p_tab||’ where rownum=1′ into res;
return res;
end;
/
create table a as select ‘a’ a from dual;
create table b as select ‘b’ b from dual;
create view v_ab as select a,b from a,b;
[/sourcecode]

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And it works fine with normal tables:
v_ab

[sourcecode language=”sql”]
SQL> exec :p_tab:=’v_ab’;

PL/SQL procedure successfully completed.

SQL> call DBMS_RESULT_CACHE.flush();

Call completed.

SQL> select/*+ result_cache */ f_without_deps(:p_tab) result from dual;

RESULT
———-
v_ab

1 row selected.

SQL> select name,bucket_no, id,type,status,pin_count,scan_count,invalidations from v$result_cache_objects o order by type,id;

NAME BUCKET_NO ID TYPE STATUS PIN_COUNT SCAN_COUNT INVALID
—————————————————————– ———- — ———- ——— ———- ———- ——-
XTENDER.F_WITHOUT_DEPS 1579 0 Dependency Published 0 0 0
XTENDER.V_AB 3127 2 Dependency Published 0 0 0
XTENDER.B 778 3 Dependency Published 0 0 0
XTENDER.A 464 4 Dependency Published 0 0 0
select/*+ result_cache */ f_without_deps(:p_tab) result from dual 1749 1 Result Published 0 0 0
[/sourcecode]

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But don’t forget that the result_cache also caches such functions with the objects, that usually should not be cached, and such objects will not be listed in the result_cache dependencies list:
v$database

[sourcecode language=”sql”]
SQL> exec :p_tab:=’v$database’;

PL/SQL procedure successfully completed.

SQL> call DBMS_RESULT_CACHE.flush();

Call completed.

SQL> select/*+ result_cache */ f_without_deps(:p_tab) result from dual;

RESULT
———-
v$database

1 row selected.

SQL> select name,bucket_no, id,type,status,pin_count,scan_count,invalidations from v$result_cache_objects o order by type,id;

NAME BUCKET_NO ID TYPE STATUS PIN_COUNT SCAN_COUNT INVALID
—————————————————————– ———- — ———- ——— ———- ———- ——-
XTENDER.F_WITHOUT_DEPS 772 0 Dependency Published 0 0 0
PUBLIC.V$DATABASE 1363 2 Dependency Published 0 0 0
select/*+ result_cache */ f_without_deps(:p_tab) result from dual 2283 1 Result Published 0 0 0

3 rows selected.
[/sourcecode]

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As you can see, there is only dependency on public synonym V$DATABASE, but not real base fixed X$-tables.
SYS.OBJ$

[sourcecode language=”sql”]
SQL> exec :p_tab:=’sys.obj$’;

PL/SQL procedure successfully completed.

SQL> call DBMS_RESULT_CACHE.flush();

Call completed.

SQL> select/*+ result_cache */ f_without_deps(:p_tab) result from dual;

RESULT
———-
sys.obj$

1 row selected.

SQL> select name,bucket_no, id,type,status,pin_count,scan_count,invalidations from v$result_cache_objects o order by type,id;

NAME BUCKET_NO ID TYPE STATUS PIN_COUNT SCAN_COUNT INVALID
—————————————————————– ———- — ———- ——— ———- ———- ——-
XTENDER.F_WITHOUT_DEPS 3922 0 Dependency Published 0 0 0
select/*+ result_cache */ f_without_deps(:p_tab) result from dual 3753 1 Result Published 0 0 0

2 rows selected.
[/sourcecode]

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The results were cached and the dependencies do not include system objects.
We easily check that the queries with any table in SYS schema or with sysdate,systimestamp,current_date,current_timestamp,dbms_random will not be cached:
SYS tables

[sourcecode language=”sql”]
SQL> select/*+ result_cache */ current_scn result from v$database;

RESULT
———-
##########

1 row selected.

SQL> select name,bucket_no, id,type,status,pin_count,scan_count,invalidations from v$result_cache_objects o order by type,id;

no rows selected

SQL> explain plan for select/*+ result_cache */ * from sys.obj$;

PLAN_TABLE_OUTPUT
————————————————————————–
Plan hash value: 2311451600

————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
————————————————————————–
| 0 | SELECT STATEMENT | | 87256 | 7328K| 296 (1)| 00:00:04 |
| 1 | TABLE ACCESS FULL| OBJ$ | 87256 | 7328K| 296 (1)| 00:00:04 |
————————————————————————–
[/sourcecode]

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Note that there is no “RESULT CACHE” line.
And even if create own tables in SYS schema(don’t do it 🙂), they will not be cached :
SYS.V_AB

[sourcecode language=”sql”]
SYS> create table a as select ‘a’ a from dual;
SYS> create table b as select ‘b’ b from dual;
SYS> create view v_ab as select a,b from a,b;
SYS> grant select on v_ab to xtender;

XTENDER> explain plan for select/*+ result_cache */ * from sys.v_ab;
PLAN_TABLE_OUTPUT
—————————————————————————–
Plan hash value: 215283502

—————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
—————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 6 | 4 (0)| 00:00:01 |
| 1 | MERGE JOIN CARTESIAN| | 1 | 6 | 4 (0)| 00:00:01 |
| 2 | TABLE ACCESS FULL | A | 1 | 3 | 2 (0)| 00:00:01 |
| 3 | BUFFER SORT | | 1 | 3 | 2 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL | B | 1 | 3 | 2 (0)| 00:00:01 |
—————————————————————————–

Note
—–
– dynamic sampling used for this statement (level=2)
[/sourcecode]

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But sys_context and userenv will be cached successbully:
sys_context

[sourcecode language=”sql”]
SQL> explain plan for select/*+ result_cache */ sys_context(‘userenv’,’os_user’) from dual;

PLAN_TABLE_OUTPUT
—————————————————————————————
Plan hash value: 1388734953

—————————————————————————————
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
—————————————————————————————
| 0 | SELECT STATEMENT | | 1 | 2 (0)| 00:00:01 |
| 1 | RESULT CACHE | 267m2hcwj08nq5kwxcb0nb2ka8 | | | |
| 2 | FAST DUAL | | 1 | 2 (0)| 00:00:01 |
—————————————————————————————

Result Cache Information (identified by operation id):
——————————————————

1 – column-count=1; attributes=(single-row); parameters=(sys_context);
name="select/*+ result_cache */ sys_context(‘userenv’,’os_user’) from dual"

14 rows selected.
[/sourcecode]

[collapse]
userenv

[sourcecode language=”sql”]
SQL> explain plan for select/*+ result_cache */ userenv(‘instance’) from dual;

PLAN_TABLE_OUTPUT
—————————————————————————————
Plan hash value: 1388734953

—————————————————————————————
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
—————————————————————————————
| 0 | SELECT STATEMENT | | 1 | 2 (0)| 00:00:01 |
| 1 | RESULT CACHE | dxzj3fks1sqfy35shbbst4332h | | | |
| 2 | FAST DUAL | | 1 | 2 (0)| 00:00:01 |
—————————————————————————————

Result Cache Information (identified by operation id):
——————————————————

1 – column-count=1; attributes=(single-row); parameters=(sys_context);
name="select/*+ result_cache */ userenv(‘instance’) from dual"
[/sourcecode]

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A function gets called twice if the result_cache is used

Posted on by Posted in curious, oracle, PL/SQL, result_cache 1,909 Page views Leave a comment

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

with result_cache_statistics

[sourcecode language=”sql”]
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
[/sourcecode]

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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!

Deterministic functions, result_cache and operators

Posted on by Posted in deterministic functions, oracle, result_cache 8,733 Page views Leave a comment

In previous posts about caching mechanism of determinstic functions I wrote that cached results are kept only between fetch calls, but there is one exception from this rule: if all function parameters are literals, cached result will not be flushed every fetch call.
Little example with difference:

SQL> create or replace function f_deterministic(p varchar2)
  2     return varchar2
  3     deterministic
  4  as
  5  begin
  6     dbms_output.put_line(p);
  7     return p;
  8  end;
  9  /
SQL> set arrays 2 feed on;
SQL> set serverout on;
SQL> select
  2     f_deterministic(x) a
  3    ,f_deterministic('literal') b
  4  from (select 'not literal' x
  5        from dual
  6        connect by level<=10
  7       );

A                              B
------------------------------ ------------------------------
not literal                    literal
not literal                    literal
not literal                    literal
not literal                    literal
not literal                    literal
not literal                    literal
not literal                    literal
not literal                    literal
not literal                    literal
not literal                    literal

10 rows selected.

not literal
literal
not literal
not literal
not literal
not literal
not literal

As you can see, ‘literal’ was printed once, but ‘not literal’ was printed 6 times, so it was returned from cache 4 times.

Also i want to show the differences in consistency between:
1. Calling a function with determinstic and result_cache;
2. Calling an operator for function with result_cache;
3. Calling an operator for function with deterministic and result_cache;

In this example I will do updates in autonomouse transactions to emulate updates in another session during query execution:

Tables and procedures with updates

[sourcecode language=”sql”]
drop table t1 purge;
drop table t2 purge;
drop table t3 purge;

create table t1 as select 1 id from dual;
create table t2 as select 1 id from dual;
create table t3 as select 1 id from dual;

create or replace procedure p1_update as
pragma autonomous_transaction;
begin
update t1 set id=id+1;
commit;
end;
/
create or replace procedure p2_update as
pragma autonomous_transaction;
begin
update t2 set id=id+1;
commit;
end;
/
create or replace procedure p3_update as
pragma autonomous_transaction;
begin
update t3 set id=id+1;
commit;
end;
/
[/sourcecode]

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Variant 1

[sourcecode language=”sql”]
create or replace function f1(x varchar2) return number result_cache deterministic
as
r number;
begin
select id into r from t1;
p1_update;
return r;
end;
/
[/sourcecode]

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Variant 2

[sourcecode language=”sql”]
create or replace function f2(x varchar2) return number result_cache
as
r number;
begin
select id into r from t2;
p2_update;
return r;
end;
/
create or replace operator o2
binding(varchar2)
return number
using f2
/
[/sourcecode]

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Variant 3

[sourcecode language=”sql”]
create or replace function f3(x varchar2) return number result_cache deterministic
as
r number;
begin
select id into r from t3;
p3_update;
return r;
end;
/
create or replace operator o3
binding(varchar2)
return number
using f3
/
[/sourcecode]

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Test:

SQL> set arrays 2;
SQL> select
  2     f1(dummy) variant1
  3    ,o2(dummy) variant2
  4    ,o3(dummy) variant3
  5  from dual
  6  connect by level<=10;

  VARIANT1   VARIANT2   VARIANT3
---------- ---------- ----------
         1          1          1
         2          1          1
         2          1          1
         3          1          1
         3          1          1
         4          1          1
         4          1          1
         5          1          1
         5          1          1
         6          1          1

10 rows selected.

SQL> /

  VARIANT1   VARIANT2   VARIANT3
---------- ---------- ----------
         7         11         11
         8         11         11
         8         11         11
         9         11         11
         9         11         11
        10         11         11
        10         11         11
        11         11         11
        11         11         11
        12         11         11

10 rows selected.

We can see that function F1 returns same results every 2 execution – it is equal to fetch size(“set arraysize 2”),
operator O2 and O3 return same results for all rows in first query execution, but in the second query executions we can see that they are incremented by 10 – it’s equal to number of rows.
What we can learn from that:
1. The use of the function F1 with result_cache and deterministic reduces function executions, but all function results are inconsistent with query;
2. Operator O2 returns consistent results, but function is always executed because we invalidating result_cache every execution;
3. Operator O3 works as well as operator O2, without considering that function is deterministic.

All tests scripts: tests.zip

When oracle invalidates result_cache function results without any changes in objects on which depends

Posted on by Posted in documentation, oracle, result_cache 2,066 Page views Leave a comment

On our production servers we have simple function with result_cache, like this:

create or replace function f_rc(p_id number) return number result_cache
is
  ret number;
begin
  select t.val into ret from rc_table t where t.id=p_id;
  return ret;
exception 
  when no_data_found then 
     return null;
end;
/

And oracle invalidates its results very frequently without any changes in table or function. I found only 2 cases when oracle invalidates result_cache results without any changes in table:
1. “select for update” from this table with commit; (strictly speaking, select-for-update is DML and it changes blocks, so it’s ok)
2. deleting rows from parent table that has no child records if there is unindexed foreign key with “on delete cascade”.
I test it on 11.2.0.1, 11.2.0.3, on solaris x64 and windows. Test cases for this i will show below.
But none of them can be the cause of our situation: we have no unindexed fk, and even if i lock all rows with “select for update”, it still does not stop invalidating.
In what other cases this happens? Am I right that the oracle does not track any changes, but the captures of the locks and “commits”?
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