Pythian Blog: Technical Track

An UNDO in a PDB in Oracle 12c?

by Fred Denis on Feb 4, 2016 12:00:00 AM

According to the Oracle 12cR1 documentation and concepts, it is 100% clear that there can be only one UNDO tablespace in a multitenant architecture and it is at CDB level; thus, a PDB cannot have any UNDO tablespace. Are we really sure about that? Let's test it! First, we need a PDB with few tablespaces:   [code]FRED_PDB> select NAME, OPEN_MODE, CON_ID from v$pdbs ; NAME OPEN_MODE CON_ID -------------------------------------------------- ---------- ---------- FRED_PDB READ WRITE 4 FRED_PDB> select tablespace_name from dba_tablespaces ; TABLESPACE_NAME ------------------------------ SYSTEM SYSAUX TEMP USERS TBS_DATA 5 rows selected. FRED_PDB> show parameter undo NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ temp_undo_enabled boolean FALSE undo_management string AUTO undo_retention integer 900 undo_tablespace string UNDOTBS1 FRED_PDB>[/code]   There we have an UNDO tablespace named UNDOTBS1 at CDB level and no UNDO at PDB level. Let's try to create one : [code]FRED_CDB> create undo tablespace MY_PDB_UNDO ; Tablespace created. FRED_CDB>[/code]   It worked! Is the Oracle documentation wrong? Let's verify this weird successful UNDO tablespace creation: [code]FRED_PDB> select tablespace_name from dba_tablespaces where tablespace_name like '%UNDO%' ; no rows selected FRED_PDB> select tablespace_name from dba_tablespaces TABLESPACE_NAME ------------------------------ SYSTEM SYSAUX TEMP USERS TBS_DATA 5 rows selected. FRED_PDB>[/code]   No UNDO tablespace has in fact been created even if no error message has been raised by Oracle. Digging in the documentation, this is not a not a bug but a feature. Indeed, it is well specified that: [code]When the current container is a PDB, an attempt to create an undo tablespace fails without returning an error.[/code]   Please note that this is the behavior of the 12cR1 release; from my side, I think that this a "not yet feature" and we should see some real UNDO tablespaces in PDBs in the next release(s)! Discover more about our expertise in Oracle.

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