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CAUSES AND SOLUTIONS TO ORA-1555 "SNAPSHOT TOO

CAUSES AND SOLUTIONS TO ORA-1555 "SNAPSHOT TOO OLD"
===================================================

Overview
~~~~~~~~

This article will discuss the circumstances under which a query can return the
Oracle error 'ORA-1555 "snapshot too old (rollback segment too small)". The
article will then proceed to discuss actions that can be taken to avoid the
error and finally will provide some simple PL/SQL scripts that
illustrate the issues discussed.

Terminology
~~~~~~~~~~~

It is assumed that the reader is familiar with standard Oracle terminology
such as 'rollback segment' and 'SCN'. If not, the reader should first read the
Oracle7 Server Concepts manual and related Oracle7 documentation.

In addition to this, two key concepts are briefly covered below which help in
the understanding of ORA-1555 :

READ CONSISTENCY

This is documented in the Oracle7 Server Concepts manual from page 10-6 to
10-9 and so will not be discussed further. However, for the purposes of this
article this should be read and understood if not understood already.

Oracle's ability to have multi-version read consistency is invaluable to the
database user so that they can guarantee that they are seeing a consistent
view of the data (no 'dirty reads').

DELAYED BLOCK CLEANOUT

This is best illustrated with an example: Consider a transaction that updates
a million row table. This obviously visits a large number of database blocks
to make the change to the data. When the user commits the transaction Oracle
does NOT go back and revisit these blocks to make the change permanent. It is
left for the next transaction that visits any block affected by the update to
'tidy up' the block (hence the term 'delayed block cleanout').

Whenever Oracle changes a database block (index, table, cluster) it stores
a pointer in the header of the data block which identifies the rollback
segment used to hold the rollback information for the changes made by the
transaction. (This is required if the user later elects to not commit the
changes and wishes to 'undo' the changes made).

Upon commit, the database simply marks the relevant rollback segment header
entry as committed. Now, when one of the changed blocks is revisited Oracle
examines the header of the data block which indicates that it has been changed
at some point. The database needs to confirm whether the change has been
committed or whether it is currently uncommitted. To do this, Oracle determines
the rollback segment used for the previous transaction (from the block's
header) and then determines whether the rollback header indicates whether it
has been committed or not.

If it is found that the block is committed then the header of the data block
is updated so that subsequent accesses to the block do not incur this
processing.

This behaviour is illustrated in a very simplified way below. Here we walk
through the stages involved in updating a data block.

STAGE 1 - No changes made

Description: This is the starting point. At the top of the data block we
have an area used to link active transactions to a rollback
segment (the 'tx' part), and the rollback segment header has a
table that stores information upon all the latest transactions
that have used that rollback segment.

In our example, we have two active transaction slots (01 and 02)
and the next free slot is slot 03. (Since we are free to
overwrite committed transactions).

Data Block 500 Rollback Segment Header 5
+----+--------------+ +----------------------+---------+
| tx | None | | transaction entry 01 |ACTIVE |
+----+--------------+ | transaction entry 02 |ACTIVE |
| row 1 | | transaction entry 03 |COMMITTED|
| row 2 | | transaction entry 04 |COMMITTED|
| ... .. | | ... ... .. | ... |
| row n | | transaction entry nn |COMMITTED|
+------------------+ +--------------------------------+

STAGE 2 - Row 2 is updated

Description: We have now updated row 2 of block 500. Note that the data block
header is updated to point to the rollback segment 5, transaction
slot 3 (5.3) and that it is marked uncommitted (Active).

Data Block 500 Rollback Segment Header 5
+----+--------------+ +----------------------+---------+
| tx |5.3uncommitted|-----+ | transaction entry 01 |ACTIVE |
+----+--------------+ | | transaction entry 02 |ACTIVE |
| row 1 | +----->| transaction entry 03 |ACTIVE |
| row 2 *changed* | | transaction entry 04 |COMMITTED|
| ... .. | | ... ... .. | ... |
| row n | | transaction entry nn |COMMITTED|
+------------------+ +--------------------------------+

STAGE 3 - The user issues a commit

Description: Next the user hits commit. Note that all that this does is it
updates the rollback segment header's corresponding transaction
slot as committed. It does *nothing* to the data block.

Data Block 500 Rollback Segment Header 5
+----+--------------+ +----------------------+---------+
| tx |5.3uncommitted|-----+ | transaction entry 01 |ACTIVE |
+----+--------------+ | | transaction entry 02 |ACTIVE |
| row 1 | +----->| transaction entry 03 |COMMITTED|
| row 2 *changed* | | transaction entry 04 |COMMITTED|
| ... .. | | ... ... .. | ... |
| row n | | transaction entry nn |COMMITTED|
+------------------+ +--------------------------------+

STAGE 4 - Another user selects data block 500

Description: Some time later another user (or the same user) revisits data
block 500. We can see that there is an uncommitted change in the
data block according to the data block's header.

Oracle then uses the data block header to look up the
corresponding rollback segment transaction table slot, sees that
it has been committed, and changes data block 500 to reflect the
true state of the datablock. (Ie, it performs delayed cleanout).

Data Block 500 Rollback Segment Header 5
+----+--------------+ +----------------------+---------+
| tx | None | | transaction entry 01 |ACTIVE |
+----+--------------+ | transaction entry 02 |ACTIVE |
| row 1 | | transaction entry 03 |COMMITTED|
| row 2 | | transaction entry 04 |COMMITTED|
| ... .. | | ... ... .. | ... |
| row n | | transaction entry nn |COMMITTED|
+------------------+ +--------------------------------+

ORA-1555 Explanation
~~~~~~~~~~~~~~~~~~~~

There are two fundamental causes of the error ORA-1555 that are a result of
Oracle trying to attain a 'read consistent' image. These are :

o The rollback information itself is overwritten so that Oracle is unable to
rollback the (committed) transaction entries to attain a sufficiently
old enough version of the block.

o The transaction slot in the rollback segment's transaction table (stored
in the rollback segment's header) is overwritten, and Oracle cannot
rollback the transaction header sufficiently to derive the original
rollback segment transaction slot.

Both of these situations are discussed below with the series of steps that
cause the ORA-1555. In the steps, reference is made to 'QENV'. 'QENV' is short
for 'Query Environment', which can be thought of as the environment that
existed when a query is first started and to which Oracle is trying to attain a
read consistent image. Associated with this environment is the SCN (System
Change Number) at that time and hence, QENV 50 is the query environment with
SCN 50.

CASE 1 - ROLLBACK OVERWRITTEN

This breaks down into two cases: another session overwriting the rollback
that the current session requires or the case where the current session
overwrites the rollback information that it requires. The latter is discussed
in this article because this is usually the harder one to understand.

Steps:

1. Session 1 starts query at time T1 and QENV 50

2. Session 1 selects block B1 during this query

3. Session 1 updates the block at SCN 51

4. Session 1 does some other work that generates rollback information.

5. Session 1 commits the changes made in steps '3' and '4'.
(Now other transactions are free to overwrite this rollback information)

6. Session 1 revisits the same block B1 (perhaps for a different row).

Now, Oracle can see from the block's header that it has been changed and
it is later than the required QENV (which was 50). Therefore we need to
get an image of the block as of this QENV.

If an old enough version of the block can be found in the buffer cache
then we will use this, otherwise we need to rollback the current block
to generate another version of the block as at the required QENV.

It is under this condition that Oracle may not be able to get the
required rollback information because Session 1's changes have generated
rollback information that has overwritten it and returns the ORA-1555
error.

CASE 2 - ROLLBACK TRANSACTION SLOT OVERWRITTEN

1. Session 1 starts query at time T1 and QENV 50

2. Session 1 selects block B1 during this query

3. Session 1 updates the block at SCN 51

4. Session 1 commits the changes
(Now other transactions are free to overwrite this rollback information)

5. A session (Session 1, another session or a number of other sessions)
then use the same rollback segment for a series of committed
transactions.

These transactions each consume a slot in the rollback segment
transaction table such that it eventually wraps around (the slots are
written to in a circular fashion) and overwrites all the slots. Note
that Oracle is free to reuse these slots since all transactions are
committed.

6. Session 1's query then visits a block that has been changed since the
initial QENV was established. Oracle therefore needs to derive an image
of the block as at that point in time.

Next Oracle attempts to lookup the rollback segment header's transaction
slot pointed to by the top of the data block. It then realises that this
has been overwritten and attempts to rollback the changes made to the
rollback segment header to get the original transaction slot entry.

If it cannot rollback the rollback segment transaction table
sufficiently it will return ORA-1555 since Oracle can no longer derive
the required version of the data block.


It is also possible to encounter a variant of the transaction slot being
overwritten when using block cleanout. This is briefly described below :

Session 1 starts a query at QENV 50. After this another process updates
the blocks that Session 1 will require. When Session 1 encounters these
blocks it determines that the blocks have changed and have not yet been
cleaned out (via delayed block cleanout). Session 1 must determine whether the
rows in the block existed at QENV 50, were subsequently changed,

In order to do this, Oracle must look at the relevant rollback segment
transaction table slot to determine the committed SCN. If this SCN is after
the QENV then Oracle must try to construct an older version of the block and
if it is before then the block just needs clean out to be good enough for
the QENV.

If the transaction slot has been overwritten and the transaction table
cannot be rolled back to a sufficiently old enough version then Oracle
cannot derive the block image and will return ORA-1555.

(Note: Normally Oracle can use an algorithm for determining a block's SCN
during block cleanout even when the rollback segment slot has been
overwritten. But in this case Oracle cannot guarantee that the
version of the block has not changed since the start of the query).

Solutions
~~~~~~~~~

This section lists some of the solutions that can be used to avoid the
ORA-1555 problems discussed in this article. It addresses the cases where
rollback segment information is overwritten by the same session and when the
rollback segment transaction table entry is overwritten.

It is worth highlighting that if a single session experiences the ORA-1555
and it is not one of the special cases listed at the end of this article, then
the session must be using an Oracle extension whereby fetches across commits are
tolerated. This does not follow the ANSI model and in the rare cases where
ORA-1555 is returned one of the solutions below must be used.

CASE 1 - ROLLBACK OVERWRITTEN

1. Increase size of rollback segment which will reduce the likelihood of
overwriting rollback information that is needed.

2. Reduce the number of commits (same reason as 1).

3. Run the processing against a range of data rather than the whole table.
(Same reason as 1).

4. Add additional rollback segments. This will allow the updates etc. to be
spread across more rollback segments thereby reducing the chances of
overwriting required rollback information.

5. If fetching across commits, the code can be changed so that this is not
done.

6. Ensure that the outer select does not revisit the same block at different
times during the processing. This can be achieved by :

- Using a full table scan rather than an index lookup
- Introducing a dummy sort so that we retrieve all the data, sort it
and then sequentially visit these data blocks.

CASE 2 - ROLLBACK TRANSACTION SLOT OVERWRITTEN

1. Use any of the methods outlined above except for '6'. This will allow
transactions to spread their work across multiple rollback segments
therefore reducing the likelihood or rollback segment transaction table
slots being consumed.

2. If it is suspected that the block cleanout variant is the cause, then
force block cleanout to occur prior to the transaction that returns the
ORA-1555. This can be achieved by issuing the following in SQL*Plus,
SQL*DBA or Server Manager :

alter session set optimizer_goal = rule;
select count(*) from table_name;

If indexes are being accessed then the problem may be an index block and
clean out can be forced by ensuring that all the index is traversed. Eg,
if the index is on a numeric column with a minimum value of 25 then the
following query will force cleanout of the index :

select index_column from table_name where index_column > 24;

Examples
~~~~~~~~

Listed below are some PL/SQL examples that can be used to illustrate the
ORA-1555 cases given above. Before these PL/SQL examples will return this
error the database must be configured as follows :

o Use a small buffer cache (db_block_buffers).

REASON: We do not want the session executing the script to be able to
find old versions of the block in the buffer cache which can be
used to satisfy a block visit without requiring the rollback
information.

o Use one rollback segment other than SYSTEM.

REASON: We need to ensure that the work being done is generating rollback
information that will overwrite the rollback information required.

o Ensure that the rollback segment is small.

REASON: See the reason for using one rollback segment.

ROLLBACK OVERWRITTEN

rem * 1555_a.sql - Example of getting ora-1555 "Snapshot too old" by
rem * a session overwriting the rollback information required
rem * by the same session.

drop table bigemp;
create table bigemp (a number, b varchar2(30), done char(1));

drop table dummy1;
create table dummy1 (a varchar2(200));

rem * Populate the example tables.
begin
for i in 1..4000 loop
insert into bigemp values (mod(i,20), to_char(i), 'N');
if mod(i,100) = 0 then
insert into dummy1 values ('ssssssssssss');
commit;
end if;
end loop;
commit;
end;
/

rem * Ensure that table is 'cleaned out'.
select count(*) from bigemp;

declare
-- Must use a predicate so that we revisit a changed block at a different
-- time.

-- If another tx is updating the table then we may not need the predicate
cursor c1 is select rowid, bigemp.* from bigemp where a < 20;

begin
for c1rec in c1 loop

update dummy1 set a = 'aaaaaaaa';
update dummy1 set a = 'bbbbbbbb';
update dummy1 set a = 'cccccccc';
update bigemp set done='Y' where c1rec.rowid = rowid;
commit;
end loop;
end;
/

ROLLBACK TRANSACTION SLOT OVERWRITTEN

rem * 1555_b.sql - Example of getting ora-1555 "Snapshot too old" by
rem * overwriting the transaction slot in the rollback
rem * segment header. This just uses one session.

drop table bigemp;
create table bigemp (a number, b varchar2(30), done char(1));

rem * Populate demo table.
begin
for i in 1..200 loop
insert into bigemp values (mod(i,20), to_char(i), 'N');
if mod(i,100) = 0 then
commit;
end if;
end loop;
commit;
end;
/

drop table mydual;
create table mydual (a number);
insert into mydual values (1);
commit;

rem * Cleanout demo table.
select count(*) from bigemp;

declare

cursor c1 is select * from bigemp;

begin

-- The following update is required to illustrate the problem if block
-- cleanout has been done on 'bigemp'. If the cleanout (above) is commented
-- out then the update and commit statements can be commented and the
-- script will fail with ORA-1555 for the block cleanout variant.
update bigemp set b = 'aaaaa';
commit;

for c1rec in c1 loop
for i in 1..20 loop
update mydual set a=a;
commit;
end loop;
end loop;
end;
/

Special Cases
~~~~~~~~~~~~~

There are other special cases that may result in an ORA-1555. These are given
below but are rare and so not discussed in this article :

o Trusted Oracle can return this if configured in OS MAC mode. Decreasing
LOG_CHECKPOINT_INTERVAL on the secondary database may overcome the problem.

o If a query visits a data block that has been changed by using the Oracle7
discrete transaction facility then it will return ORA-1555.

o It is feasible that a rollback segment created with the OPTIMAL clause may
cause a query to return ORA-1555 if it has shrunk during the life of the
query causing rollback segment information required to generate consistent
read versions of blocks to be lost.

Summary
~~~~~~~

This article has discussed the reasons behind the error ORA-1555 "Snapshot
too old", has provided a list of possible methods to avoid the error when it
is encountered, and has provided simple PL/SQL scripts that illustrate the
cases discussed.

+==+

Diagnostics and References:

* {975.6,Y,100} ORA-01555: SNAPSHOT TOO OLD (ROLLBACK SEGMENT TOO SMALL)
* {3366.6,Y,100} ROLLBACK