Creating Extended Statistics With Function-base Column Groups

There are nine restrictions to creating extended statistics. These restrictions are same for version 11.1, 11.2 and 12.1.. One of the constraints is "A column group can not contain expressions". Oracle documentation also provides an example what is a column group and what is an expression when it comes to extended statistics extension "An example column group can be "(c1, c2)" and an example expression can be "(c1 + c2)". In short if columns are comma separated then it will consider as a column group. However this causes a problem when creating extended statistics with functions. Give below is an example.

SQL> create table exstat (a number, b date);

SQL> select dbms_stats.create_extended_stats(user,'EXSTAT','(a,trunc(b))') from dual;
select dbms_stats.create_extended_stats(user,'EXSTAT','(a,trunc(b))') from dual
       *
ERROR at line 1:
ORA-20001: Invalid Extension: Column group can contain only columns seperated by comma

Trying to create extended statistics with column a and trunc(b) results in an error. What's clear from the error is that 1. Oracle was expecting a column group and 2. It must only contain columns separated by comma.
It is expecting a column group but the second portion of the extension is not recognized as a column hence the error. To overcome this create a function base index. For the above extended statistics extension following index was created

create index aidx on exstat(a,trunc(b));

After which the creation of the extended statistics works.

SQL> select dbms_stats.create_extended_stats(user,'EXSTAT','(a,trunc(b))') from dual;

DBMS_STATS.CREATE_EXTENDED_STATS(USER,'EXSTAT','(A,TRUNC(B))')
--------------------------------------------------------------------------------
SYS_STUE4B2X1G802ME0XHTBYWFY_Q

Simply create the index first and then extended statistics.



Trying to drop an extended statistics extension that uses function results in the following error

SQL>  exec dbms_stats.drop_extended_stats(user,'EXSTAT','(A,TRUNC(B))');
BEGIN dbms_stats.drop_extended_stats(user,'EXSTAT','(A,TRUNC(B))'); END;

*
ERROR at line 1:
ORA-20000: extension "(A,TRUNC(B))" does not exist
ORA-06512: at "SYS.DBMS_STATS", line 13055
ORA-06512: at "SYS.DBMS_STATS", line 45105
ORA-06512: at line 1

Even though the extension is present the error message says "does not exist". To resolve this drop the index created for the extended statistics extension

SQL> DROP INDEX AIDX;

Index dropped.

After which extended statistics is dropped without any issue.

SQL> exec dbms_stats.drop_extended_stats(user,'EXSTAT','(A,TRUNC(B))');

PL/SQL procedure successfully completed.

This was tested on 12.1, 11.2.0.4 and 11.2.0.3 and all exhibited the same behavior. But this test failed on 11.1.0.7. even after creating the index.

SQL> create table exstat (a number, b date);

SQL> select dbms_stats.create_extended_stats(user,'EXSTAT','(a,trunc(b))') from dual;
select dbms_stats.create_extended_stats(user,'EXSTAT','(a,trunc(b))') from dual
       *
ERROR at line 1:
ORA-20001: Invalid Extension: Column group can contain only columns seperated by comma

SQL> create index aidx on exstat(a,trunc(b));

SQL> select dbms_stats.create_extended_stats(user,'EXSTAT','(a,trunc(b))') from dual;
select dbms_stats.create_extended_stats(user,'EXSTAT','(a,trunc(b))') from dual
       *
ERROR at line 1:
ORA-20001: Invalid Extension: Column group can contain only columns seperated by comma

The index has created a hidden virtual column. Trying to add a virtual column complains of duplication

SQL>  alter table exstat add (c as (trunc(b)));
 alter table exstat add (c as (trunc(b)))
                         *
ERROR at line 1:
ORA-54015: Duplicate column expression was specified

But having a virtual column with the function expression didn't help either.

SQL> drop index aidx;

SQL>  alter table exstat add (c as (trunc(b)));

SQL>  select dbms_stats.create_extended_stats(user,'EXSTAT','(a,trunc(b))') from dual;
 select dbms_stats.create_extended_stats(user,'EXSTAT','(a,trunc(b))') from dual
        *
ERROR at line 1:
ORA-20001: Invalid Extension: Column group can contain only columns seperated by comma

It's temping to use the virtual column itself in the extended statistics column group. But "extension cannot contain a virtual column" is restriction number one!

SQL> select dbms_stats.create_extended_stats(user,'EXSTAT','(a,c)') from dual;
select dbms_stats.create_extended_stats(user,'EXSTAT','(a,c)') from dual
       *
ERROR at line 1:
ORA-20001: Error when processing extension -  virtual column is referenced in a column expression