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Possible causes of the Oracle error ORA-01555.

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Users requiring an overview of the causes of the Oracle error ORA-01555.


There are various reasons why customers can get the error ORA-01555. Sometimes it is 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-01555 error 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 guarantee 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 an ORA-01555 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-01555 will result if the query is looking for the snapshot which is so old that rollback segment information could not be found because of wrap around or overwrite.




SITUATIONS WHERE ORA-01555 ERRORS COMMONLY OCCUR:
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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 the 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 an ORA-01555 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 needs 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 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 an ORA-01555 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 data blocks 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, let us 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-01555 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. According 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 commit but users should be aware that it might result in ORA-01555.


Search Words:
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ORA-1555

Notes from Oracle Metalink

Thanks
SasiKumar