Estimate Undo Retention , Undo Optimization

undo_retention parameter will not guarantee a RETENTION unless you define a RETENTION GUARANTEE CLAUSE on tablespace level

Actual Undo Size

SELECT SUM(a.bytes) “UNDO_SIZE”
FROM v$datafile a,
v$tablespace b,
dba_tablespaces c
WHERE c.contents = ‘UNDO’
AND c.status = ‘ONLINE’
AND b.name = c.tablespace_name
AND a.ts# = b.ts#;

UNDO_SIZE
———-
209715200

Undo Blocks per Second

SELECT MAX(undoblks/((end_time-begin_time)*3600*24))
“UNDO_BLOCK_PER_SEC”
FROM v$undostat;

UNDO_BLOCK_PER_SEC
——————
3.12166667

DB Block Size
SELECT TO_NUMBER(value) “DB_BLOCK_SIZE [KByte]”
FROM v$parameter
WHERE name = ‘db_block_size’;

DB_BLOCK_SIZE [Byte]
——————–
4096

Calculation for Optimal Undo Retention
209’715’200 / (3.12166667 * 4’096) = 16’401 [Sec]  Using Inline Views, you can do all in one query!

SELECT d.undo_size/(1024*1024) “ACTUAL UNDO SIZE [MByte]”,
SUBSTR(e.value,1,25) “UNDO RETENTION [Sec]”,
ROUND((d.undo_size / (to_number(f.value) *
g.undo_block_per_sec))) “OPTIMAL UNDO RETENTION [Sec]”
FROM (
SELECT SUM(a.bytes) undo_size
FROM v$datafile a,
v$tablespace b,
dba_tablespaces c
WHERE c.contents = ‘UNDO’
AND c.status = ‘ONLINE’
AND b.name = c.tablespace_name
AND a.ts# = b.ts#
) d,
v$parameter e,
v$parameter f,
(
SELECT MAX(undoblks/((end_time-begin_time)*3600*24))
undo_block_per_sec
FROM v$undostat
) g
WHERE e.name = ‘undo_retention’
AND f.name = ‘db_block_size’
/
ACTUAL UNDO SIZE [MByte]
————————
200
UNDO RETENTION [Sec]
——————–
10800
OPTIMAL UNDO RETENTION [Sec]
—————————-
16401

Calculate Needed UNDO Size for given Database Activity

SELECT d.undo_size/(1024*1024) “ACTUAL UNDO SIZE [MByte]”,
SUBSTR(e.value,1,25) “UNDO RETENTION [Sec]”,
(TO_NUMBER(e.value) * TO_NUMBER(f.value) *
g.undo_block_per_sec) / (1024*1024)
“NEEDED UNDO SIZE [MByte]”
FROM (
SELECT SUM(a.bytes) undo_size
FROM v$datafile a,
v$tablespace b,
dba_tablespaces c
WHERE c.contents = ‘UNDO’
AND c.status = ‘ONLINE’
AND b.name = c.tablespace_name
AND a.ts# = b.ts#
) d,
v$parameter e,
v$parameter f,
(
SELECT MAX(undoblks/((end_time-begin_time)*3600*24))
undo_block_per_sec
FROM v$undostat
) g
WHERE e.name = ‘undo_retention’
AND f.name = ‘db_block_size’
/
ACTUAL UNDO SIZE [MByte]
————————
200
UNDO RETENTION [Sec]
——————–
10800
NEEDED UNDO SIZE [MByte]
————————
131.695313