Monthly Archives: July 2012

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

Posted on by Posted in documentation, oracle, result_cache 2,109 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”?
Continue reading→

An interesting question from Valentin Nikotin

Posted on by Posted in Uncategorized 1,692 Page views Leave a comment

Test table:

create table tclob(c clob);

What is this code going to put out from the following blocks with and without “rollback”:

Task 1

[sourcecode language=”sql”]
declare
cl1 clob;
cl2 clob;
cl3 clob;
cl4 clob;
begin
cl1:=’1′;
insert into tclob values(cl1) returning c into cl2;
cl3:=cl2;
dbms_lob.append(cl3,’2′);
select c into cl4 from tclob;
— rollback;

dbms_output.put_line(cl1);
dbms_output.put_line(cl2);
dbms_output.put_line(cl3);
dbms_output.put_line(cl4);
end;
/
[/sourcecode]

[collapse]

Task 2

[sourcecode language=”sql”]
declare
cl1 clob;
cl2 clob;
cl3 clob;
cl4 clob;
begin
cl1 := ‘1’;
insert into tclob values (cl1) returning c into cl2;
cl3 := cl2;
dbms_lob.append(cl2, ‘2’);
select c into cl4 from tclob;
— rollback;

dbms_output.put_line(cl1);
dbms_output.put_line(cl2);
dbms_output.put_line(cl3);
dbms_output.put_line(cl4);
end;
/
[/sourcecode]

[collapse]
Task 3

[sourcecode language=”sql”]
declare
cl1 clob;
cl2 clob;
cl3 clob;
cl4 clob;
begin
cl1 := ‘1’;
insert into tclob values (cl1) returning c into cl2;
cl3 := cl2;
dbms_lob.append(cl2, ‘2’);
dbms_lob.append(cl3, ‘3’);
select c into cl4 from tclob;
— rollback;

dbms_output.put_line(cl1);
dbms_output.put_line(cl2);
dbms_output.put_line(cl3);
dbms_output.put_line(cl4);
end;
/
[/sourcecode]

[collapse]
Task 4

[sourcecode language=”sql”]
declare
cl1 clob;
cl2 clob;
cl3 clob;
cl4 clob;
begin
cl1 := ‘1’;
insert into tclob values (cl1) returning c into cl2;
cl3 := cl2;
dbms_lob.append(cl2, ’22’);
dbms_lob.append(cl3, ‘3’);
dbms_lob.append(cl2, ’44’);
select c into cl4 from tclob;
— rollback;
dbms_output.put_line(cl1);
dbms_output.put_line(cl2);
dbms_output.put_line(cl3);
dbms_output.put_line(cl4);
end;
/
[/sourcecode]

[collapse]

Check it both in Windows and Solaris/Linux 🙂

Explanation
It is clear that this bug is platform-depending and that the matter is in specifics of working with memory. The answer lies in the fact that cl3 and cl2 have unsynchronized lengths, which means that Oracle “forgets” to change the lengths of all remaining variables, that point to this “clob”, and as every operation of changing cl2/cl3 in fact changes the same thing, it turns out that “superfluous” becomes overwritten.