Sr ORACLE DBA: December 2010

Analyze Statement

The ANALYZE statement can be used to gather statistics for a specific table, index or cluster. The statistics can be computed exactly, or estimated based on a specific number of rows, or a percentage of rows:

ANALYZE TABLE employees COMPUTE STATISTICS;
ANALYZE INDEX employees_pk COMPUTE STATISTICS;

ANALYZE TABLE employees ESTIMATE STATISTICS SAMPLE 100 ROWS;
ANALYZE TABLE employees ESTIMATE STATISTICS SAMPLE 15 PERCENT;

DBMS_STATS

The DBMS_STATS package was introduced in Oracle 8i and is Oracles preferred method of gathering object statistics. Oracle list a number of benefits to using it including parallel execution, long term storage of statistics and transfer of statistics between servers. Once again, it follows a similar format to the other methods:

EXEC DBMS_STATS.gather_database_stats;
EXEC DBMS_STATS.gather_database_stats(estimate_percent => 15);

EXEC DBMS_STATS.gather_schema_stats('SCOTT');
EXEC DBMS_STATS.gather_schema_stats('SCOTT', estimate_percent => 15);

EXEC DBMS_STATS.gather_table_stats('SCOTT', 'EMPLOYEES');
EXEC DBMS_STATS.gather_table_stats('SCOTT', 'EMPLOYEES', estimate_percent => 15);

EXEC DBMS_STATS.gather_index_stats('SCOTT', 'EMPLOYEES_PK');
EXEC DBMS_STATS.gather_index_stats('SCOTT', 'EMPLOYEES_PK', estimate_percent => 15);
This package also gives you the ability to delete statistics:

EXEC DBMS_STATS.delete_database_stats;
EXEC DBMS_STATS.delete_schema_stats('SCOTT');
EXEC DBMS_STATS.delete_table_stats('SCOTT', 'EMPLOYEES');
EXEC DBMS_STATS.delete_index_stats('SCOTT', 'EMPLOYEES_PK');

Transfering Stats
=================

First the statistics must be collected into a statistics table. In the following examples the statistics for the APPS user are collected into a new table, STATS_TABLE, which is owned by scott:

SQL> EXEC DBMS_STATS.create_stat_table('SCOTT','STATS_TABLE');
SQL> EXEC DBMS_STATS.export_schema_stats('APPS','STATS_TABLE',NULL,'SCOTT');

This table can then be transfered to another server using your preferred method (Export/Import ) and the stats imported into the data dictionary as follows:

SQL> EXEC DBMS_STATS.import_schema_stats('APPS','STATS_TABLE',NULL,'SCOTT');
SQL> EXEC DBMS_STATS.drop_stat_table('APPS','STATS_TABLE');

Now that we understand why the ORA-1555 occurs and some aspects about how they can occur we need to examine the following:

How can we logically determine and resolve what has occurred to cause the ORA-1555?

1) Determine if UNDO_MANAGEMENT is MANUAL or AUTO

If set to MANUAL, it is best to move to AUM. If it is not feasible to switch to AUM see

Note 69464.1 Rollback Segment Configuration & Tips

to attempt to tune around the ORA-1555 using V$ROLLSTAT

If set to AUTO, proceed to #2

2) Gather the basic data

a) Acquire both the error message from the user / client ... and the message in the alert log

User / Client session example:

ORA-01555: snapshot too old: rollback segment number 9 with name "_SYSSMU1$" too small

Alert log example

ORA-01555 caused by SQL statement below (Query Duration=9999 sec, SCN:0x000.008a7c2d)

b) Determine the QUERY DURATION from the message in the alert log

From our example above ... this would be 9999

c) Determine the undo segment name from the user / client message

From our example above ... this would be _SYSSMU1$

d) Determine the UNDO_RETENTION of the undo tablespace

show parameter undo_retention

3) Determine if the ORA-1555 is occurring with an UNDO or a LOB segment

If the undo segment name is null ...

ORA-01555: snapshot too old: rollback segment number with name "" too small

or the undo segment is unknown

ORA-01555: snapshot too old: rollback segment number # with name "???" too small

then this means this is a read consistent failure on a LOB segment

If the segment_name or the undo segment is known the error is occurring with an UNDO segment.

==============================================================================================

What to do if an ORA-1555 is occurring with an UNDO segment
-------------------------------------------------------------------------------------------------

1) QUERY DURATION > UNDO_RETENTION

There are no guarantees that read consistency can be maintained after the transaction slot for the
committed row has expired (exceeded UNDO_RETENTION)

Why would one think that the transaction slot's time has exceeded UNDO_RETENTION?

Lets answer this with an example

If UNDO_RETENTION = 900 seconds ... but our QUERY DURATION is 2000 seconds ...

This says that our query has most likely encountered a row that was committed more than 900
seconds ago ... and has been overwritten as we KNOW that the transaction slot being examined
no longer matches the row we are looking for

The reason we say "most likely" is that it is possible that an unexpired committed transaction slot was
overwritten due to either space pressure on the undo segment or this is a bug

SOLUTION:

The best solution is to tune the query can to reduce its duration. If that cannot be done then increase
UNDO_RETENTION based on QUERY DURATION to allow it to protect the committed
transaction slots for a longer period of time

NOTE: Increasing UNDO_RETENTION requires additional space in the UNDO tablespace. Make
sure to accommodate for this space. One method of doing this is to set AUTOEXTEND on one or
more of the UNDO tablespace datafiles for a period of time to allow for the increased space. Once the
size has stabilized, AUTOEXTEND can be removed.

See the solution for #2 below for more options

2) QUERY DURATION <= UNDO_RETENTION

This case is most often due to the UNDO tablespace becoming full sometime during the time when the
query was running

How do we tell if the UNDO tablespace has become full during the query?

Examine V$UNDOSTAT.UNXPSTEALCNT for the period while the query that generated the
ORA-1555 occurred.

This column shows committed transaction slots that have not exceeded UNDO_RETENTION but
were overwritten due to space pressure on the undo tablespace (IE became full).

If UNEXPSTEACNT > 0 for the time period during which the query was running then this shows
that the undo tablespace was too small to be able to maintain UNDO_RETENTION. Unexpired
blocks were over written, thus ending read consistency for those blocks for that time period.
set pagesize 25
set linesize 120

select inst_id,
to_char(begin_time,'MM/DD/YYYY HH24:MI') begin_time,
UNXPSTEALCNT "# Unexpired|Stolen",
EXPSTEALCNT "# Expired|Reused",
SSOLDERRCNT "ORA-1555|Error",
NOSPACEERRCNT "Out-Of-space|Error",
MAXQUERYLEN "Max Query|Length"
from gv$undostat
where begin_time between
to_date('','MM/DD/YYYY HH24:MI:SS')
and
to_date('','MM/DD/YYYY HH24:MI:SS')
order by inst_id, begin_time;

NOTE: Start time of the query can be determined by subtracting the query duration from the timestamp
of the ORA-1555
EXAMPLE:

Tue May 26 16:16:57 2009
ORA-01555 caused by SQL statement below (SQL ID: 54yn3n36w24ft, Query Duration=922 sec, SCN: 0x0007.8a55f4e3)

922 seconds is 15 min 22 seconds

So the start time of the query would be May 26 16:01:35

If after researching, you find no existing bugs seem to match your case, then additional info will be
needed to file a bug.

Starting with release 9.2.0.4 an event, 10442, can be set to dump all the necessary diagnostics
information to one trace file. NOTE: Event 10442 does not trigger diagnostics in cases where the
ORA-1555 error is associated with out-of-row LOB undo.

The event can be enabled using the following settting in init.ora

event="10442 trace name context forever, level 10"

or

Alter system set events '10442 trace name context forever , level 10';
Reproduce the ORA-1555 error with a new connection to trigger the trace diagnostics file.
NOTE: There have been issues with 10g and 11g on RAC using 10442. The event can contribute to factors leading to a hang on one or more nodes on the RAC. Refer to Bug 10051817 for more fixes for these problems on RAC.

Another thing to consider is whether Oracle is behaving as designed as far as "steal logic" is concerned

Here is the logic for allocating space for a new transaction--based on Bug:4070480

* Allocate new extent in the undo tablespace
* If failed, steal an expired extent from another undo segment. This involves shrinking from the
other segment and adding to the current undo segment.
* If failed, autoextend a datafile (if enabled)
* If failed, reuse an unexpired extent from the current undo segment.
* If failed, steal an unexpired extent from another undo segment.
* If failed, report an "Out-Of-Space" error.

What to do if an ORA-1555 is occurring with a LOB segment
---------------------------------------------------------------------------------------------

1) Often the ORA-1555 with the null undo segment is accompanied by an ORA-22924

Error: ORA-22924
Text: snapshot too old
---------------------------------------------------------------------------
Cause: The version of the LOB value needed for the consistent read was already overwritten by another
writer.

2) Determine if PCTVERSION or RETENTION is used

Examine DBA_LOBS.PCTVERSION and DBA_LOBS.RETENTION
(or investigate through the data dictionary--see below)

NOTE: Only one of these may be set, so this will determine which method of undo management is
used for this LOB

There is a bug with these two columns prior to 10.2 and as such the data dictionary must be
examined to determine which of these is being used

Note 422826.1 How To Identify LOB Segment Use PCTVERSION Or RETENTION From
Data Dictionary

If PCTVERSION is used
------------------------------------

This method of LOB undo management is akin to use of rollback segments and as such there is little
control to tune undo usage.

The only parameter we have control over with this method is PCTVERSION itself.

SOLUTION:

Increase PCTVERSION
ALTER TABLE **table name** MODIFY LOB (**lob column name**) (PCTVERSION 100)

If the parameter is at 100% then the only solution will be to move to use of RETENTION
ALTER TABLE **table name** MODIFY LOB (**lob column name**) (RETENTION)

If RETENTION is used
---------------------------------

You may have RETENTION in place, but it may not be working as you expect.

See Note 800386.1 ORA-1555 - UNDO_RETENTION is silently ignored if the LOB
resides in a MSSM tablespace
Note 563470.1 Lob retention not changing when undo_retention is changed

If RETENTION is used the same rules apply for the LOB segments as do the UNDO segments

1) QUERY DURATION > UNDO_RETENTION

There are no guarantees that read consistency can be maintained after the old lob segment for
the committed row has expired (exceeded UNDO_RETENTION)

SOLUTION:

The best solution is to tune the query to reduce its duration. If that cannot be done then
increase UNDO_RETENTION based on QUERY DURATION to allow it to protect the
committed transaction slots for a longer period of time. A change to UNDO_RETENTION
does not automatically change RETENTION. See Note 563470.1 Lob retention not
changing when undo_retention is changed.

NOTE 1: Increasing UNDO_RETENTION requires additional space in the LOB segments
tablespace. Make sure to accommodate for this. One method of doing this is to set
AUTOEXTEND on one of more of the lob segments tablespace datafiles. There may or
may not be a stabilization of size like the UNDO tablespace. If it does stabilize then the
AUTOEXTEND can be removed. See the solution for #2 below for more options.

NOTE 2: RETENTION for LOBs does not function the same as it does for undo. LOB segments
will not automatically extend to allow for retention of read consistent copies of a lob. As
such, at times like these, PCTVERSION 100 is a better solution.

2) QUERY DURATION <= UNDO_RETENTION
This case is most often due to the LOB segments tablespace becoming full sometime during the time
when the query was running.

How do we tell if the LOB segment tablespace has become full during the query?

Unfortunately there is no easy way to do this. V$UNDOSTAT does not contain info about the LOB
segments. There may be a way to use Enterprise Manager and examine the high watermark
(HWM) of the tablespace in question.

There is an enhancement request for this info to be included in the future

Bug:3200789 Abstract: VISIBILITY OF LOB SEGMENT USAGE FOR UNDO

SOLUTION:

Provide more space in the LOB segments tablespace so that transaction slots do not have to be
stolen. This can be done by:

* Using AUTOEXTEND on an existing LOB segments tablespace datafile
* Increase the size of an existing LOB segments tablespace datafile
* Add a datafile to the LOB segments tablespace

3) Occasionally we encounter situations that do not fit into either #1 or #2 above. We have to consider the possibility of a bug in those cases.

EXAMPLES:

Note 253131.1 Concurrent Writes May Corrupt LOB Segment When Using Auto Segment
Space Management (ORA-1555)

Bug:5636728 FALSE ORA-1555 WHEN READING LOBS AFTER SHRINK

NOTE: If RAC is in use, changes must be made to AT LEAST the instance that raised the ORA-1555

The following query will determine the instance on which the ORA-1555 occurred
select stat.inst_id, seg.tablespace_name
from dba_rollback_segs seg, gv$rollstat stat
where seg.segment_id = stat.usn
and seg.segment_name='';

@ Former title: ORA-1555 - YET STILL ANOTHER APPROACH

References
BUG:5636728 - FALSE ORA-1555 WHEN READING LOBS AFTER SHRINK
BUG:6919859 - ORA-22924, ORA-1555: SNAPSHOT TOO OLD WITH RETENTION BASED LOB
NOTE:104008.1 - ORA-01555: "SNAPSHOT TOO OLD" - Additional Solutions
NOTE:10630.1 - ORA-01555: "Snapshot too old" - Overview
NOTE:253131.1 - Concurrent Writes May Corrupt LOB Segment When Using Auto Segment Space Management (ORA-1555)
NOTE:389554.1 - ORA-1555 Using Automatic Undo Management - How to troubleshoot
NOTE:40689.1 - ORA-01555 "Snapshot too old" - Detailed Explanation
NOTE:422826.1 - How To Identify LOB Segment Use PCTVERSION Or RETENTION From Data Dictionary
NOTE:452341.1 - ORA-01555 And Other Errors while Exporting Table With LOBs, How To Detect Lob Corruption.
NOTE:563470.1 - Lob retention not changing when undo_retention is changed
NOTE:800386.1 - ORA-1555 - UNDO_RETENTION is silently ignored if the LOB resides in a MSSM tablespace

Sr ORACLE DBA

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Thursday, December 30, 2010

Analyze Statement and Transferring Statistics.

TSM for ORACLE Installation

ora-01555 snapshot too old error.

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