Possible for Oracle to consider constraints when using bind variable?

Question

Consider the following table with a check constraint listing the possible values of the column
create table TEST_TABLE(
my_column varchar2(10));
insert into TEST_TABLE select 'VALUE1' from dba_objects;
alter table TEST_TABLE
add constraint TEST_TABLE_CHK1
check (my_column in ('VALUE1', 'VALUE2'));
begin dbms_stats.gather_table_stats(ownname=>user,tabname=>'TEST_TABLE');END;Let's see the number of logical I/O's needed for the following SQL statements.
(The value was obtained by the delta ofselect m.value from v$mystat m, v$statname n
where name='session logical reads' and m.statistic#=n.statistic#)
If string lateral is used:
declare
   n number;
begin
  select count(*) into n from test_table where my_column='VALUE1';
end;
Consistent Gets: 21
declare
   n number;
begin
  select count(*) into n from test_table where my_column='VALUE2';
end;
Consistent Gets: 21
declare
   n number;
begin
  select count(*) into n from test_table where my_column='VALUE3';
end;
Consistent Gets: 0Oracle can eliminate the table if it knows the queried value can't satisfy the constraint. Good.
(Actually, the execution plan for the last query included the 'FILTER' operation.)
However, if bind variable is used:
declare
   n number;
   x varchar2(10);
begin
  x := 'VALUE1';
  select count(*) into n from test_table where my_column=x;
end;
Consistent Gets: 21
declare
   n number;
   x varchar2(10);
begin
  x := 'VALUE3';
  select count(*) into n from test_table where my_column=x;
end;
Consistent Gets: 21Oracle can't eliminate the table using the constraint. I can understand that because bind variables are used, Oracle can't directly eliminate the table when generating the execution plan. However, is it possible to eliminate the table, or at least employ some shortcut in runtime? If not, will this be a performance loss compared with using values laterals when check constraint exists?
(And is it possible to use autotrace on PL/SQL block in sqlplus?)
Oracle:
     10.2.0.4 SE
     11.2.0.2 SE
OS:
     RHEL5

Answer 1

However, is it possible to eliminate the table, or at least employ some shortcut in runtime? I can't see how to do this. Oracle has a sqltext that has an embedded bind variable in it. And for this sqltext it has an execution plan in the shared pool that will be used irrespective of the actual bound values at runtime.
Maybe in 11G, with adaptive cursor sharing / plan bind awareness, Oracle might be smart enough to introduce a second execution plan for the VALUE3 case.
If not, will this be a performance loss compared with using values laterals when check constraint exists?Only if you submit the dumb query and search for records with VALUE3... Normally your application code would not hold/generate these queries.
Will it?
For columns whose values are bound by a CHECK constraint, one might even consider to never use bind variables, since very likely you will have few versions of queries that use these columns.
Not?
Edited by: Toon Koppelaars on Jun 22, 2011 1:20 PM