Estimate undo

Question

UNDO tablespace.
Provides transaction consistency and roll back.  If transaction consistency cant be maintained as we've run out of undio segments we get snapshot too old.  Well documented.
If we're getting transactions that are running out of undo  we can extend the undo tablespace and investigate the transaction to see what its doing.   At this point it may be too late, it might be a critical transaction.    Is there a way of
A) reporting on UNDO usage?   (At 4am last night transaction X took 80% of all available undo space available)
B) alerting when a transaction is reaching max available UNDO space?

Answer 1

A) reporting on UNDO usage?   (At 4am last night transaction X took 80% of all available undo space available)
SELECT /* + RULE */ df.tablespace_name "Tablespace",
df.bytes / (1024 * 1024) "Size (MB)",
SUM(fs.bytes) / (1024 * 1024) "Free (MB)",
Nvl(Round(SUM(fs.bytes) * 100 / df.bytes),1) "% Free",
Round((df.bytes - SUM(fs.bytes)) * 100 / df.bytes) "% Used"
FROM dba_free_space fs,
(SELECT tablespace_name,SUM(bytes) bytes
FROM dba_data_files
GROUP BY tablespace_name) df
WHERE fs.tablespace_name = df.tablespace_name
GROUP BY df.tablespace_name,df.bytes
UNION ALL
SELECT /* + RULE */ df.tablespace_name tspace,
fs.bytes / (1024 * 1024),
SUM(df.bytes_free) / (1024 * 1024),
Nvl(Round((SUM(fs.bytes) - df.bytes_used) * 100 / fs.bytes), 1),
Round((SUM(fs.bytes) - df.bytes_free) * 100 / fs.bytes)
FROM dba_temp_files fs,
(SELECT tablespace_name,bytes_free,bytes_used
FROM v$temp_space_header
GROUP BY tablespace_name,bytes_free,bytes_used) df
WHERE fs.tablespace_name = df.tablespace_name
GROUP BY df.tablespace_name,fs.bytes,df.bytes_free,df.bytes_used
ORDER BY 4 DESC;
B) alerting when a transaction is reaching max available UNDO space?
You can use Grid Control.