Subject: Ora-1555:Snapshot too old
Last Revision Date: 11 September 1994
Authors: Chitra Mitra, Ziyad Dahbour, Rama Velpuri
ORA-1555 : SNAPSHOT TOO OLD
===========================
There are various reasons why customers can get the error ORA-1555. Sometimes
its due to Rollback segments being too small in size, but there are other
reasons. This bulletin is an attempt to give a complete summary of all the
situations which would cause an ORA-1555 and how to resolve them.
In order to understand the bulletin, one needs to understand some of the
internal mechanisms of Oracle, so we start by explaining briefly about
read consistency and block cleanouts.
Oracle always enforces statement-level read consistency. This guarantees
that the data returned by a single query is consistent with respect to time
when the query began. Therefore, a query never sees the data-changes made by
transactions that commit during the course of execution of the query.
Oracle uniquely identifies any given point in time by a set of numbers called
the System Change Numbers (SCN). So SCN can be defined as the state of the
database at any one given point in time. To produce read-consistency, Oracle
marks the current SCN as the query enters the execution phase. The query can
only see the snapshot of the records as they were at the time of marked SCN.
Oracle uses rollback segments to reconstruct the read-consistent snapshot
of the data. Whenever a transaction makes any changes, a snapshot of the
record before the changes were made is copied to a rollback segment and the
data block header is marked appropriately with the address of the rollback
segment block where the changes are recorded. The data block also maintains
the SCN of the last committed change to the block.
As the data blocks are read on behalf of the query, only blocks with lower
SCN than the query SCN will be read. If a block has uncommitted changes of
other transactions or changed data with more recent SCN, then the data is
reconstructed using the saved snapshot from the rollback segments. In some
rare situations, if RDBMS is not able to reconstruct the snapshot for a long
running query, the query results in ORA-1555 error.
A rollback segment maintains the snapshot of the changed data as long as the
transaction is still active (commit or rollback has not been issued). Once
a transaction is committed, RDBMS marks it with current SCN and the space used
by the snapshot becomes available for reuse.
Therefore, ORA-1555 will result if the query is looking for the snapshot which
is so old that rollback segment information could not be found becuase of
wrap around or overwrite.
SITUATIONS WHERE ORA-1555 ERRORS COMMONLY OCCUR
===============================================
1. Fewer and smaller rollback segments for a very actively changing database
If the database has many transactions changing data and commiting very
often, then chance of reusing the space used by a committed transaction
is higher. A long running query then may not be able to reconstruct the
snapshot due to wrap around and overwrite in rollback segments. Larger
rollback segments in this case will reduce the chance of reusing the
committed transaction slots.
2. Corrupted rollback segment
If the rollback segment is corrupted and could not be read, then a
statement needing to reconstruct a before image snapshot will result
in the error.
3. Fetch across commit
This is the situation when a query opens a cursor, then loops through
fetching, changing, and committing the records on the same table. In
this scenerio, very often ORA-1555 can result. Let's take the following
example to explain this.
A cursor was opened at SCN=10. The execution SCN of the query is then
marked as SCN=10. Every fetch by that cursor now need to get the
read-consistent data from SCN=10. The user program is now fetching
x numbers of records, changing them, and committing them. Let's say they
were committed with SCN=20. If a later fetch happens to retrieve a record
which is in one of the previously committed blocks, then the fetch will
see that the SCN there as 20. Since the fetch has to get the snapshot
from SCN=10 it will try to find it in the rollback segments. If it could
rollback sufficiently backwards as previously explained, then it could
reconstruct the snapshot from SCN=10. If not, then it will result in
ORA-1555 error.
Committing less often which will result in larger rollback segments will
REDUCE the probability of getting 'snapshot too old' error.
4. Fetch across commits with delayed block clean out
To complicate things, now we see how delayed block clean outs play an
important role in getting this error.
When a data or index block is modified in the database and the transaction
committed, oracle does a fast commit by marking the transaction as
committed in the rollback segment header but does not clean the datablocks
that were modified. The next transaction which does a select on the modified
blocks will do the actual cleanout of the block. This is known as a
delayed block cleanout.
Now, take the same scenario as described in previous section. But instead of
assuming one table, lets assume that there are two tables in question.
i.e: the cursor is opened and then in a loop, it fetches from one table and
changes records in another, and commits.
Even though the records are getting committed in another table it could
still cause ORA-1555 because cleanout has not been done on the table
from which the records are being fetched.
For this case, a full table scan before opening and fetching through the
cursor will help.
Summary:Fetches across commits as explained in last two cases are not
supported by ANSI standard. Accoding to ANSI standard a cursor
is invalidated when a commit is performed and should be closed
and reopened. Oracle allows users to do fetch across commits but
users should be aware that it might result in ORA-1555.