What exactly is meant by "global statistics" on a partitioned table?

From the whitepaper "Understanding Optimizer Statistics":

"When dealing with partitioned tables the Optimizer relies on both the statistics for the entire table
(global statistics) as well as the statistics for the individual partitions (partition statistics) to select a
good execution plan for a SQL statement. If the query needs to access only a single partition, the
Optimizer uses only the statistics of the accessed partition. If the query access more than one partition,
it uses a combination of global and partition statistics."

How to manuall purge your unified auditing audit trail

This is how I purged my unified audit trail on an Oracle 12.2 test instance.

First, check the LAST_ARCHIVE_TS:

SELECT * FROM DBA_AUDIT_MGMT_LAST_ARCH_TS;

If you want to keep some of your audit records, set a timestamp before which all records should be purged. In this case, I want to purge everything, so I set it to SYSTIMESTAMP:

BEGIN
  DBMS_AUDIT_MGMT.SET_LAST_ARCHIVE_TIMESTAMP (
    AUDIT_TRAIL_TYPE => DBMS_AUDIT_MGMT.AUDIT_TRAIL_UNIFIED,
    LAST_ARCHIVE_TIME => SYSTIMESTAMP);
END;
/

Purge the audit trail:

BEGIN
  DBMS_AUDIT_MGMT.clean_audit_trail(
   audit_trail_type        => DBMS_AUDIT_MGMT.AUDIT_TRAIL_UNIFIED,
   use_last_arch_timestamp => TRUE);
END;
/

If you want to purge all records, run the same procedure but without the "use_last_arch_timestamp" directive:

BEGIN
  DBMS_AUDIT_MGMT.clean_audit_trail(
   audit_trail_type => DBMS_AUDIT_MGMT.AUDIT_TRAIL_UNIFIED
   );
END;
/

The dbms_audit_mgmt is documented here

How to create a unique constraint with an existing index (the USING-clause)

Create the index:

CREATE UNIQUE INDEX MYTABLE_IDX1 ON MYTABLE
(ID,CATEGORY)
LOCAL
TABLESPACE USERS
ONLINE;

Create a unique constraint:

ALTER TABLE MYTABLE ADD (
CONSTRAINT UK_MYTABLE
UNIQUE (ID,CATEGORY)
USING INDEX MYTABLE_IDX1
);

For a primary key constraint the syntax is similar.
In this example, an index named MYTABLE_PK will be automatically created on the fly:

ALTER TABLE MYTABLE ADD (
CONSTRAINT MYTABLE_PK
PRIMARY KEY (ID)
USING INDEX);

A workaround for ORA-54033: column to be modified is used in a virtual column expression

Applicable for Oracle 12.1.0.2.

My customer reports that an attempt to execute a simple DDL statement failed:

ALTER TABLE mytable
MODIFY (COL2 DATE );

The error thrown was: ORA-54033: column to be modified is used in a virtual column expression.

Quite correctly, a look at the table revealed a virtual column:

set lines 200
col owner format a15
col table_name format a30
col column_name format a30
col data_default format a60

select owner,table_name,column_name, data_Default
from dba_tab_cols
where table_name='MYTABLE'
and hidden_column='YES';

Output:

OWNER           TABLE_NAME      COLUMN_NAME                    DATA_DEFAULT                                                                    
--------------- --------------- ------------------------------ -----------------------------------------------
SCOTT           MYTABLE         SYS_STSC13O20ML6_5OD25YOF16STK SYS_OP_COMBINED_HASH("COL1","COL2","COL3","COL4")             
1 row selected.

You can also query the DBA_STAT_EXTENSION to get similar information:

set lines 200
col extension_name format a30
col extension format a50

SELECT EXTENSION_NAME, EXTENSION,creator,droppable
FROM DBA_STAT_EXTENSIONS
WHERE TABLE_NAME='MYTABLE';

Output:

EXTENSION_NAME                 EXTENSION                      CREATOR DROPPABLE
------------------------------ ------------------------------ ------- ---------
SYS_STSC13O20ML6_5OD25YOF16STK ("COL1","COL2","COL3","COL4")  SYSTEM  YES      
1 row selected.

So in order to modify the existing column, the extended statistics will have to be dropped and recreated:

set lines 200
set serveroutput on
set timing on

BEGIN
  dbms_stats.drop_extended_stats('SCOTT', 'MYTABLE', '(COL1,COL2,COL3,COL4)');
END;
/

ALTER TABLE mytable
MODIFY (COL2 DATE );

select dbms_stats.create_extended_stats('SCOTT', 'MYTABLE', '(COL1,COL2,COL3,COL4)') 
from dual;

How to audit specific users' actions on other schemas' objects

This was executed on Oracle 12.2

To audit actions by users on another schema's objects, execute this:

audit insert table,update table,select table, delete table by scott by access; 

Verify that the auditing is in place:

SELECT audit_option,success,failure
FROM DBA_STMT_AUDIT_OPTS
WHERE USER_NAME='SCOTT';

You can see that the actions we want to audit for user SCOTT is indeed set:

audit_option	success	failure
SELECT TABLE	BY ACCESS	BY ACCESS
INSERT TABLE	BY ACCESS	BY ACCESS
UPDATE TABLE	BY ACCESS	BY ACCESS
DELETE TABLE	BY ACCESS	BY ACCESS

This will generate audit entries whenever scott tries to perform queries or DML on other schemas' tables.

In my case, I tested this by logging on to the database as user scott, and executed a delete statement against a test table called "yy", owned by the SALES schema:

sqlplus / as sysdba
grant create synonym to scott;

sqlplus scott/tiger@testdb01
SQL> create synonym yy for sales.yy;

Synonym created.

SQL> delete from yy where antall in (1,2);

7 rows deleted.

SQL> commit;

Commit complete.

This particular database writes its audit information to the operating system, as indicated by the audit_trail parameter:

sqlplus / as sysdba
SQL> show parameter audit_trail

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
audit_trail                          string      OS

My audit_file_dest resides inside the ADR directory structure:

show parameter audit_file_dest

NAME                                 TYPE        VALUE
------------------------------------ ----------- --------------------------------
audit_file_dest                      string      /u01/oracle/admin/testdb01/adump

Checking the /u01/oracle/admin/testdb01/adump I found the file and it showed:

Thu Apr  4 16:32:56 2019 +02:00
LENGTH: "276"
SESSIONID:[8] "14060442" 
ENTRYID:[2] "67" 
STATEMENT:[2] "14" 
USERID:[6] "SCOTT" 
USERHOST:[18] "MYDOMAIN\MYPC123" 
TERMINAL:[12] "MYPC123" 
ACTION:[1] "7" 
RETURNCODE:[1] "0" 
OBJ$CREATOR:[11] "SALES" 
OBJ$NAME:[2] "YY" 
OS$USERID:[6] "vegardk" 
DBID:[10] "2425899399"

The action codes are important. Number 7 indicates a DELETE statement, which is what I expected to find.
The entire list can be found here

How to gather histograms on a column of a table

Use the METHOD_OPT directive in DBMS_STATS.GATHER_TABLE_STATS to collect histograms for column ARTICLE_NAME in table SALES_Q1_RESULTS. In this example, 6 buckets are defined.
Check the view DBA_TAB_COLUMNS to confirm whether or not there are histograms collected on the column:

SELECT  NUM_DISTINCT,DENSITY,NUM_NULLS,NUM_BUCKETS,HISTOGRAM
FROM    DBA_TAB_COLUMNS 
WHERE   TABLE_NAME = 'SALES_Q1_RESULTS' 
AND     COLUMN_NAME = 'ARTICLE_NAME';

NUM_DISTINCT	DENSITY	NUM_NULLS	NUM_BUCKETS	HISTOGRAM
0	0	0	0	NONE

BEGIN
  DBMS_STATS.GATHER_TABLE_STATS (OWNNAME => 'SH', TABNAME => 'SALES_Q1_RESULTS', METHOD_OPT => 'FOR COLUMNS SIZE 6 ARTICLE_NAME');
END;
/

SELECT  NUM_DISTINCT,DENSITY,NUM_NULLS,NUM_BUCKETS,HISTOGRAM
FROM    DBA_TAB_COLUMNS 
WHERE   TABLE_NAME = 'SALES_Q1_RESULTS' 
AND     COLUMN_NAME = 'ARTICLE_NAME';

NUM_DISTINCT	DENSITY	NUM_NULLS	NUM_BUCKETS	HISTOGRAM
7	0.166666666666667	0	6	HEIGHT BALANCED

Read more in Oracles Documentation

How to remove the schema OWF_MGR from a database

Applicable for Oracle version 12.1.0.2.0.

Workflow manager is a component used for Orchestrating of Oracle Warehouse Builder (OWB).
If you have a schema called OWF_MGR dangling in your database, it can be removed. I did it the following way:

Find the number of grants per user from OWF_MGR:

SELECT UNIQUE GRANTEE,COUNT(*)
FROM DBA_TAB_PRIVS
WHERE OWNER='OWF_MGR'
GROUP BY GRANTEE;

This query gave me to grantees: PUBLIC and the role WF_PLSQL_UI.

Revoke the privileges granted. Generate the revoke statements:

select 'revoke execute on '|| owner ||'.' || table_name || ' from WF_PLSQL_UI;' 
from  dba_tab_privs
where owner='OWF_MGR'
AND grantee='WF_PLSQL_UI';

Generate the same statements for PUBLIC.

Drop the role:

drop role WF_PLSQL_UI;

Finally, drop the user with the cascade option:

drop user owf_mgr cascade;

I found this information here