"...If retention guarantee is enabled, the specified minimum undo retention is guaranteed; the database never overwrites unexpired undo data even if it means that transactions fail due to lack of space in the undo tablespace. If retention guarantee is not enabled, the database can overwrite unexpired undo when space is low, thus lowering the undo retention for the system..." Acoording to this, correct answers should be C and E.
By definition, undo extents that still have to overlive the retention period are unexpired. So, when undo_retention is set and retention is guaranteed, unexpired undo is always retained.
As opposed to the case in which retention is noguarantee, where also unexpired extents can be overwritten IF and when needed.
In this particolar case, so, I would say C-D are the correct answers.
After reading the oracle docs, section 16.2.2.3 Retention Guarantee. I can say the correct answer shall be C,E. retention guarantee & undo_retention are about unexpired undo information.
https://docs.oracle.com/database/121/ADMIN/undo.htm#ADMIN11466
I Agree.
Undo retention is the threshold after which UNDO go from unexpired to expired. So it's pointless to talk about undo retention that keep some kind of undo. UNDO RETENTION defines what kind of undo it is.
So C and D
1. Active UNDO:
- UNDO data generated by currently executing transactions.
- Retained until the transaction is committed or rolled back.
- Not affected by the UNDO_RETENTION parameter.
- Always retained because it is essential to maintain transaction consistency.
2. UNDO for Committed Transactions:
- UNDO data generated by committed or rolled back transactions.
- Primarily used for features such as Consistent Read (CR) and Flashback Query.
- Retention period is specified by the UNDO_RETENTION parameter.
- After the period specified by UNDO_RETENTION has elapsed, this UNDO data becomes expired and may be overwritten.
The UNDO_RETENTION parameter only specifies the retention period for UNDO data of committed transactions. The retention period of active UNDO is not affected by UNDO_RETENTION.
Therefore, option B is incorrect. UNDO_RETENTION does not specify the retention period for all types of UNDO; it only applies to UNDO data of committed transactions. Active UNDO is always retained regardless of UNDO_RETENTION.
Additionally, Oracle has another type of UNDO called System UNDO, which is used to track changes in the data dictionary, but users cannot directly control it.
The reason why option E is incorrect:
According to the Oracle Database documentation, the UNDO_RETENTION parameter specifies the minimum duration in seconds for which UNDO information for committed transactions is retained.
In other words, this parameter specifies the duration for which Oracle attempts to retain UNDO data for committed transactions without expiring it.
The expression "unexpired UNDO" in option E is not appropriate.
The correct expressions would be "UNDO data for committed transactions" or "retaining UNDO data without expiring it."
The English expression in option E, "UNDO_RETENTION specifies for how long Oracle attempts to keep unexpired UNDO," is also not appropriate.
A more accurate expression would be, "UNDO_RETENTION specifies the minimum duration in seconds for which Oracle attempts to retain UNDO information for committed transactions."
Therefore, option E contains problems and is not a correct expression.
A FALSE: Old (committed) undo information that is older than the current undo retention period is said
to be expired and its space is available to be overwritten by new transactions.
B FALSE :Undo retention period, which is the minimum amount of time that Oracle Database
attempts to retain "old undo" (not all types) information before overwriting it.
C TRUE: it is retained for consistent read and Oracle Flashback operations.To guarantee the success of long-running queries or Oracle Flashback operations,
you can enable retention guarantee.
D FALSE:When automatic undo management is enabled, there is always a current undo
retention period, which is the minimum amount of time that Oracle Database
attempts to retain "old undo"( not Active undo) information before overwriting it.
E TRUE: If retention guarantee is enabled, then the specified minimum undo retention is
guaranteed; the database never overwrites unexpired undo data even if it means that
transactions fail due to lack of space in the undo tablespace.
Unexpired undo can be reused if not enough space in undo tbs ,
The UNDO_RETENTION parameter can only be honored if the current undo tablespace has enough space. If an active transaction requires undo space and the undo tablespace does not have available space, then the system starts reusing unexpired undo space. This action can potentially cause some queries to fail with a "snapshot too old" message.
I go with D,E
I bet on A, C
When automatic undo management is enabled, there is always a current undo retention period, which is the minimum amount of time that Oracle Database attempts to retain old undo information before overwriting it. Old (committed) undo information that is older than the current undo retention period is said to be expired. Old undo information with an age that is less than the current undo retention period is said to be unexpired.
this is real question asked in the exam
the one more question is
which two statments are true about UNDO and UNDO tablespace?
A) There can be only one UNDO tablespace created in database.
B) UNDO segements are owned by SYSBACKUP.
C)UNDO Segments are owned by SYSTEM
D)An instance will crash if the active undo tablespace is lost.
E) An undo tablespace may be owned by only one instance.
correct answer is A & D. any suggestion for this answer? this question is asked in real exam
E is correct
UNDO_RETENTION
(seconds)
Once a transaction commits the undo data for that transaction stays in the undo tablespace until space is required in which case it will be over written.
committed undo information (unexpired) :
also known as unexpired undo, required to support undo_retention interval
after undo_retention period or undo tablespace space pressure unless guaranteed option is set (see below)
http://datadisk.org.uk/html_docs/oracle/undo.htm
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