ask tom 关于01555的说明(备忘)

Jane -- Thanks for the question regarding "snapshot too old error", version oracle 8i

Submitted on 5-Jun-2000 15:48 Central time zone

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Last updated 20-Oct-2011 22:47

You Asked

Tom,
Would you tell me what snapshot too old error. When does it happen? What's the possible 
causes? How to fix it?

Thank you very much.

Jane 

and we said...

I think support note <Note:40689.1> covers this topic very well:

ORA-01555 "Snapshot too old" - Detailed Explanation
===================================================

Overview
~~~~~~~~

This article will discuss the circumstances under which a query can return the Oracle 
error ORA-01555 "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 Oracle Server 
Concepts manual and related Oracle documentation.

In addition to this, two key concepts are briefly covered below which help in the 
understanding of ORA-01555:
 
1. READ CONSISTENCY:
====================

This is documented in the Oracle Server Concepts manual 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 Server has the ability to have multi-version read consistency which is invaluable 
to you because it guarantees that you are seeing a consistent view of the data (no 'dirty 
reads').


2. 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. (i.e. 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-01555 Explanation
~~~~~~~~~~~~~~~~~~~~~

There are two fundamental causes of the error ORA-01555 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-01555. 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-01555 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-01555 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-01555 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: You 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: You 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 - 
rem * Example of getting ora-1555 "Snapshot too old" by
rem * 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-01555. 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 Oracle discrete 
transaction facility then it will return ORA-01555.

 o It is feasible that a rollback segment created with the OPTIMAL clause maycause a 
query to return ORA-01555 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-01555 "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.