I've seen ORA-1555 (snapshot too old) lots of times in the past and they always
seemed to be caused by 1 of 2 scenarios.
Either an image of a block needed by a long running query is not available
because another session has modified and committed a change to the block and
the 'before modification' image of the block is no longer available in the
rollback segments. (common) Or it's a delayed block cleanout issue where the
ITL in the block header needs maintenance. (uncommon, but not exceedingly rare)
I'm now getting an ORA-1555 error that doesn't seem to fit either case.
This particular table is roughly 11 million rows in a database 10 terrabytes in
size and not terrible busy. There's 300 Gig of undo tablespace and
UNDO_RETENTION is set for 40 hours. (don't ask why 40 hours. I didn't set it
up that way and nobody seems to remember why it was set that way)
Running the following query returns an error in anywhere from 2 to 10 seconds.
SQL> select count(*) from app_owner.type_history;
select count(*) from hri1_owner.type_history
ERROR at line 1:
ORA-01555: snapshot too old: rollback segment number 491 with name
"_SYSSMU491_93190984$" too small
The error is repeatable, with the same rollback segment (number 491) listed
Since I'm getting this error in sometimes as little as 2 seconds or so, the
first scenario I described above doesn't seem to apply.
Usually, the recommended action to eliminate problems with delayed block
cleanout is to either gather stats on the table or do a 'select * from table'
to force oracle to visit every block. That doesn't seem to be working here.
All attempts to analyze the table or select count(*) from the table get the
same ORA-1555 error
This is v 188.8.131.52 (EE) on solaris.
Searches on the web and metalink (yes, I still call it that) have proven
fruitless so far.
Anybody have any thoughts on why this might be occurring?
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