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Troubleshooting: High Version Count Issues (文档 ID 296377.1)

Linux操作系统 作者:rongshiyuan 时间:2013-11-24 22:57:14 0 删除 编辑

In this Document

Purpose
  Ask Questions, Get Help, And Share Your Experiences With This Article
Troubleshooting Steps
  What is a 'High' Version Count?
  What is shared SQL ?
  What is 'SQL Metadata'?
  Why should I be concerned about 'High' Versions?
  How do I see the versions and why they are not shared ?
  What do the reasons given in v$SQL_SHARED_CURSOR mean?
  Version_rpt script:
  What further tracing is available.
  Are there any times when a high version count is expected even though BINDS are being used?
  Explanation:
  Enhancement to obsolete parent cursors if Version Count exceeds a threshold
  High Version Count with Adaptive Cursor Sharing
  Troubleshooting Other Issues
  Discuss High Version Count Issues
References

Applies to:

Oracle Database - Enterprise Edition - Version 10.2.0.1 and later
Oracle Database - Standard Edition - Version 10.2.0.1 and later
Oracle Database - Personal Edition - Version 10.2.0.1 and later
Information in this document applies to any platform.

Purpose

The Troubleshooting Guide is provided to assist in debugging SQL sharing issues. When possible, diagnostic tools are included in the document to assist in troubleshooting problems. This document does not contain bugs/patches as these topics are addressed in the articles referenced at the bottom of this document.

Ask Questions, Get Help, And Share Your Experiences With This Article

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Click here to join the discussion where you can ask questions, get help from others, and share your experiences with this specific article.
Discover discussions about other articles and helpful subjects by clicking here to access the main My Oracle Support Community page for Database Tuning.

Troubleshooting Steps

What is a 'High' Version Count?

There is no definitive definition of what a 'High' number of versions for a particular cursor is, different systems may be able to deal with different ranges of versions, However, AWR reports start reporting versions over 20 for a particular cursor and so that is as good an indicator of a potential problem as any.



Once you start getting into the hundreds or thousands range, then these are definitely 'High' counts and the causes should be examined and the numbers reduced so as to encourage the SQL to be shared.

What is shared SQL ?

The first thing to remember is that all SQL is implicitly sharable. When a SQL statement is entered, the RDBMS will create a hash value for text of the statement and that hash value then helps the RDBMS to easily find SQL already in the shared pool. It is not in the scope of this article to discuss this in any great detail, so let's just assume entering a series of text results in a hash value being created.

For instance :- 'select count(*) from emp' hashes to the value 4085390015

We now create a parent cursor for this sql and a single child. It does not matter that a SQL statement may never be shared - when it is first parsed a parent and a single child are created. The easy way to think of this is that the PARENT cursor is a representation of the hash value and the child cursor(s) represent the metadata for that SQL

What is 'SQL Metadata'?


Metadata is all the information which enables a statement to run. For instance, in the example I have given EMP is owned by scott and therefore has an OBJECT_ID which points to the EMP table owned by this user. When the user SCOTT logged in, optimizer parameters are initialised in that session for use by the statement, so this too is used by the optimizer and is therefore metadata. There are other examples of Metadata which will be mentioned further in this document.

Let's say this session logs out and back in again now. It then runs the same command again (as the same user). This time we already have the SQL in the shared pool (but we don't know this yet). What we do is hash the statement and then search for that hash value in the shared pool. If we find it, we can then search through the children to determine if any of them are usable by us (ie the metadata is the same). If it is, then we can share that SQL statement
I would still have one version of that SQL in the shared pool because the metadata enabled me to share the statement with the already existent child. The fundementals are that the parent is not shared, it is the children which determine shareability.

Now - another user 'TEST' has it's own version of EMP. If that user was to now run the select statement above then what would happen is :-

1. The statement is hashed - it is hashed to the value 4085390015
2. The SQL will be found in the shared pool as it already exists
3. The children are scanned (at this point we have one child)
4. Because the OBJECT_ID of the EMP table owned by TEST is different the OBJECT_ID owned by scott we have a 'mismatch'

(Essentially, what happens here is that we have a linked list of children which we move through in turn, comparing the metadata of the current SQL with that of all the children. If there were 100 children then we would scan each of them (looking for a possible mismatch and moving on) until we found one we could share. If we cannot share any (ie. have exhausted the list of children) then we need to create a new child)

5. We therefore have to create a new child - we now have 1 PARENT and 2 CHILDREN.

Why should I be concerned about 'High' Versions?

Unnecessary non-sharing of SQL, and the resultant versions of SQL, is a primary cause of library cache contention. Contention reduces the performance of your database and, in extreme cases, can cause it to appear to 'hang'. When you have unnecessary versions of a cursor, each time that cursor is executed, the parse engine has to search through the list of versions to see which is the cursor that you want. This wastes CPU cycles that you could be using on something else.

How do I see the versions and why they are not shared ?

The easiest way to get version information in a clear format is to use the script in the following article:

Document 438755.1 High SQL Version Counts - Script to determine reason(s)


To find the reasons for mismatches see the following section: What do the reasons given in v$SQL_SHARED_CURSOR mean?

If you are unable to use that script then you can select the same information from the base views as illustrated in the examples below.

Lets use the example above and take a look at what SQL we can use to see this in the shared pool.

SCOTT runs select count(*) from emp

I can now run the following to see the PARENT statement and it's hash value and address

select sql_text, hash_value,address from v$sqlarea where sql_text like 'select count(*) from emp%';

SQL_TEXT                 HASH_VALUE    ADDRESS
------------------------ ------------ ----------------
select count(*) from emp 4085390015   0000000386BC2E58


To see the CHILDREN (I expect to see 1 at this point) :-

  • Version 9.2.X.X and below :
    select * from v$sql_shared_cursor where kglhdpar = '0000000386BC2E58'
  • Version 10.0.X.X and above:
    select * from v$sql_shared_cursor where address = '0000000386BC2E58'

Output:

ADDRESS          KGLHDPAR         U S O O S L S E B P I S T A B D L T R I I R L I O S M U T N F 
---------------- ---------------- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
0000000386BC2D08 0000000386BC2E58 N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N 



We can see we have a single child (ADDRESS 0000000386BC2D08).
The mismatch information (U S O O S L etc) is all N because this is the first child. Now, if I log in as another user and run the same select (select count(*) from emp) and look again I will get the following output:-

ADDRESS          KGLHDPAR         U S O O S L S E B P I S T A B D L T R I I R L I O S M U T N F 
---------------- ---------------- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
0000000386BC2D08 0000000386BC2E58 N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N 
0000000386A91AA0 0000000386BC2E58 N N N N N N N N N N N N N Y N N N Y N N N N N N N N N N N N N 

We can now see the 2nd child ( 0000000386A91AA0) and also the reasons why it could not be shared with the first (The 'Y's denote a mismatch). The reasons are:

(1) AUTH_CHECK_MISMATCH and
(2) TRANSLATION_MISMATCH

This is because the objects under my new user do not map to those of SCOTT (the current child). A mismatch occurs because I cannot access SCOTTs objects and translation fails since we have different object_ids for the objects in each of our schemas. 

What do the reasons given in v$SQL_SHARED_CURSOR mean?

Below are the list of reasons as well as some worked examples (Those denoted by ** are the ones most often seen) :-

  • UNBOUND_CURSOR - The existing child cursor was not fully built (in other words, it was not optimized)
  • SQL_TYPE_MISMATCH - The SQL type does not match the existing child cursor
  • **OPTIMIZER_MISMATCH - The optimizer environment does not match the existing child cursor.
    For example:

    select count(*) from emp; ->> 1 PARENT, 1 CHILD
    alter session set optimizer_mode=ALL_ROWS
    select count(*) from emp; ->> 1 PARENT, 2 CHILDREN

        (The optimizer mode has changed and therefore
           the existing child cannot be reused)

    (The same applies with events - if I turned on tracing with 10046 than I would get the OPTIMIZER_MISMATCH again and a 3rd child)

          It is possible to see more details for the reason using cursortrace. Something like the following will be see:

 

Optimizer mismatch(12)

         The number is brackets gives the reason why

        1 = Degree used is not the default DOP

        2 = In (RAC) cases where instance count is not the same, or session CPU count is not the same, or thread count is not the same

        3 =  _parallel_syspls_obey_force is FALSE

        4 = The PQ mode does not match.

        5 = The degree does not match.

        6 = The parallel degree policy does not match.

        7 = The session limit is not the same as the cursor limit but the cursor limit is the same as the degree used.

        8 = The cursor limit is greater than the degree used and the session limit is less than the cursor limit

        9 = The cursor limit is less than the degree used and the session limit is not the same as the cursor limit

       10 = Optimizer mode difference

       11 =  Materialized View mismatch

       12 = Optimizer environment mismatch (ie an optimizer parameter is different)

       13 = Cardinality Feedback is use

  • OUTLINE_MISMATCH - The outlines do not match the existing child cursor

    If my user had created stored outlines previously for this command and they were stored in seperate categories (say "OUTLINES1" and "OUTLINES2") running:-
    alter session set use_stored_outlines = OUTLINES1;
    select count(*) from emp;
    alter session set use_stored_oulines= OUTLINES2;
    select count(*) from emp;

    --> Would create a 2nd child as the outline used is different than the first run.
  • STATS_ROW_MISMATCH - The existing statistics do not match the existing child cursor. Check that 10046/sql_trace is not set on all sessions as this can cause this.
  • LITERAL_MISMATCH - Non-data literal values do not match the existing child cursor
  • SEC_DEPTH_MISMATCH - Security level does not match the existing child cursor
  • EXPLAIN_PLAN_CURSOR - The child cursor is an explain plan cursor and should not be shared. Explain plan statements will generate a new child by default - the mismatch will be this.
  • BUFFERED_DML_MISMATCH - Buffered DML does not match the existing child cursor
  • PDML_ENV_MISMATCH - PDML environment does not match the existing child cursor
  • INST_DRTLD_MISMATCH - Insert direct load does not match the existing child cursor
  • SLAVE_QC_MISMATCH -The existing child cursor is a slave cursor and the new one was issued by the coordinator (or, the existing child cursor was issued by the coordinator and the new one is a slave cursor).
  • TYPECHECK_MISMATCH - The existing child cursor is not fully optimized
  • AUTH_CHECK_MISMATCH - Authorization/translation check failed for the existing child cursor
    The user does not have permission to access the object in any previous version of the cursor. A typical example would be where each user has it's own copy of a table
  • **BIND_MISMATCH - The bind metadata does not match the existing child cursor. For example:

    variable a varchar2(100);
    select count(*) from emp where ename = :a ->> 1 PARENT, 1 CHILD
    variable a varchar2(400);
    select count(*) from emp where ename = :a ->> 1 PARENT, 2 CHILDREN

       (The bind 'a' has now changed in definition)
  • DESCRIBE_MISMATCH - The typecheck heap is not present during the describe for the child cursor
  • LANGUAGE_MISMATCH - The language handle does not match the existing child cursor
  • TRANSLATION_MISMATCH - The base objects of the existing child cursor do not match.
    The definition of the object does not match any current version. Usually this is indicative of the same issue as "AUTH_CHECK_MISMATCH" where the object is different.
  • ROW_LEVEL_SEC_MISMATCH - The row level security policies do not match
  • INSUFF_PRIVS - Insufficient privileges on objects referenced by the existing child cursor
  • INSUFF_PRIVS_REM - Insufficient privileges on remote objects referenced by the existing child cursor
  • REMOTE_TRANS_MISMATCH - The remote base objects of the existing child cursor do not match
    USER1: select count(*) from table@remote_db
    USER2: select count(*) from table@remote_db

      (Although the SQL is identical, the dblink pointed to
       by remote_db may be a private dblink which resolves
       to a different object altogether)
  • LOGMINER_SESSION_MISMATCH
  • INCOMP_LTRL_MISMATCH
  • OVERLAP_TIME_MISMATCH - error_on_overlap_time mismatch
  • SQL_REDIRECT_MISMATCH - sql redirection mismatch
  • MV_QUERY_GEN_MISMATCH - materialized view query generation
  • USER_BIND_PEEK_MISMATCH - user bind peek mismatch
  • TYPCHK_DEP_MISMATCH - cursor has typecheck dependencies
  • NO_TRIGGER_MISMATCH - no trigger mismatch
  • FLASHBACK_CURSOR - No cursor sharing for flashback
  • ANYDATA_TRANSFORMATION - anydata transformation change
  • INCOMPLETE_CURSOR - incomplete cursor.
    When bind length is upgradeable (i.e. we found a child cursor that matches everything
    else except that the bind length is not long enough), we mark the old cursor is not usable
    and build a new one.  This means the version can be ignored.
  • TOP_LEVEL_RPI_CURSOR - top level/rpi cursor
    In a Parallel Query invocation this is expected behaviour (we purposely do not share)
  • DIFFERENT_LONG_LENGTH - different long length
  • LOGICAL_STANDBY_APPLY - logical standby apply mismatch
  • DIFF_CALL_DURN - different call duration
  • BIND_UACS_DIFF - bind uacs mismatch
  • PLSQL_CMP_SWITCHS_DIFF - plsql compiler switches mismatch
  • CURSOR_PARTS_MISMATCH - cursor-parts executed mismatch
  • STB_OBJECT_MISMATCH - STB object different (now exists)
  • ROW_SHIP_MISMATCH - row shipping capability mismatch
  • PQ_SLAVE_MISMATCH - PQ slave mismatch
    Check you want to be using PX with this reason code, as the problem could be caused by running lots of small SQL statements which do not really need PX. If you are on < 11i you may be hitting Bug:4367986
  • TOP_LEVEL_DDL_MISMATCH - top-level DDL cursor
  • MULTI_PX_MISMATCH - multi-px and slave-compiled cursor
  • BIND_PEEKED_PQ_MISMATCH - bind-peeked PQ cursor
  • MV_REWRITE_MISMATCH - MV rewrite cursor
  • ROLL_INVALID_MISMATCH - rolling invalidation window exceeded
    This is caused by the rolling invalidation capability in DBMS_STATS. The child cannot be shared as it's invalidation window is exceeded. See:

    Document 557661.1  Rolling Cursor Invalidations with DBMS_STATS in Oracle10g
  • OPTIMIZER_MODE_MISMATCH - optimizer mode mismatch
  • PX_MISMATCH - parallel query mismatch
    If running 11.1.0.6 and RAC see Bug:7352775. Check that if (on each instance) parallel_instance_groups is set then instance_groups is set to the same.
  • MV_STALEOBJ_MISMATCH - mv stale object mismatch
  • FLASHBACK_TABLE_MISMATCH - flashback table mismatch
  • LITREP_COMP_MISMATCH - literal replacement compilation mismatch 

New in 11g :

  • PLSQL_DEBUG - debug mismatch Session has debugging parameter plsql_debug set to true
  • LOAD_OPTIMIZER_STATS  - Load optimizer stats for cursor sharing
  • ACL_MISMATCH   -  Check ACL mismatch
  • FLASHBACK_ARCHIVE_MISMATCH  - Flashback archive mismatch
  • LOCK_USER_SCHEMA_FAILED  - Failed to lock user and schema
  • REMOTE_MAPPING_MISMATCH  - Remote mapping mismatch
  • LOAD_RUNTIME_HEAP_FAILED  - Runtime heap mismatch
  • HASH_MATCH_FAILED  - Hash mismatch
    Set to "Y" if sharing fails due to a hash mismatch, such as the case with mismatched histogram data or a range predicate marked as unsafe by literal replacement (See Bug 3461251)

New in 11.2  :

  • PURGED_CURSOR - cursor marked for purging

         The cursor has been marked for purging with dbms_shared_pool.purge

  • BIND_LENGTH_UPGRADEABLE - bind length upgradeable

          Could not be shared because a bind variable size was smaller than the new value beiing inserted    (marked as BIND_MISMATCH in earlier versions).

  • USE_FEEDBACK_STATS - cardinality feedback

         Cardinality feedback is being used and therefore a new plan could be formed for the current execution.

  • BIND_EQUIV_FAILURE - The bind value's selectivity does not match that used to optimize the existing child cursor

There is no longer  ROW_LEVEL_SEC_MISMATCH in 11.2.

Version_rpt script:

The script version_rpt can also be run to produce a summary report of the v$sql_shared_cursor view with additional diagnostic information. The script can be found in:

Document 438755.1 High SQL Version Counts - Script to determine reason(s) 


Running the Script:
Generate reports for all cursors with more than 100 versions using SQL_ID (10g and up):

select b.* from v$sqlarea a ,table(version_rpt(a.sql_id)) b where loaded_versions >=100;

-- Generate reports for all cursors with more than 100 versions using HASH_VALUE:

select b.* from v$sqlarea a ,table(version_rpt(null,a.hash_value)) b where loaded_versions>=100;

-- Generate the report for cursor with sql_id cyzznbykb509s:

select * from table(version_rpt('cyzznbykb509s'));

What further tracing is available.

In 10G it is possible to use CURSORTRACE to aid the investigation of why cursors are not being shared. This event should only be used under the guidance of support and the resultant trace file is undocumented. To get the trace for a particular SQL statement you first of all need to get the hash_value (See the above select from v$sqlarea). You then set the trace on using:-  

alter system set events
'immediate trace name cursortrace level 577, address hash_value';

(level 578/580 can be used for high level tracing (577=level 1, 578=level 2, 580=level 3)

This will write a trace file to user_dump_dest each time we try to reuse the cursor.
To turn off tracing use:-

alter system set events
'immediate trace name cursortrace level 2147483648, address 1';

Please note: Bug 5555371 exists in 10.2 (fixed in 10.2.0.4) where cursor trace cannot fully be turned off and single line entries will still be made to the trace file as a result. The w/a is to restart the instance. How invasive this BUG is depends on the executions of the cursor (and the size of the resultant trace file additions)

In 11.2 there is also cursordump:

alter system set events 'immediate trace name cursordump level 16'

 (please ensure system , not session, is used as the level meaning changes)

This dumps some additional information such as expanding on the parameters for 'optimizer_mismatch' issues.

Are there any times when a high version count is expected even though BINDS are being used?

Consider the following where cursor_sharing=SIMILAR

select /* TEST */ * from emp where sal > 100;
select /* TEST */ * from emp where sal > 101;
select /* TEST */ * from emp where sal > 102;
select /* TEST */ * from emp where sal > 103;
select /* TEST */ * from emp where sal > 104;

SELECT sql_text,version_count,address
FROM V$SQLAREA
WHERE sql_text like 'select /* TEST */%';

SELECT * FROM V$SQL_SHARED_CURSOR WHERE kglhdpar = '&my_addr';

You will see several versions , each with no obvious reason for not being shared

Explanation:

One of the cursor sharing criteria when literal replacement is enabled with cursor_sharing as similar is that bind value should match initial bind value if the execution plan is going to change depending on the value of the literal. The reason for this is we _might_ get a sub optimal plan if we use the same cursor. This would typically happen when depending on the value of the literal optimizer is going to chose a different plan. Thus in this test case we have a predicate with > , if this was a equality we would always share the same child cursor. If application developers are ready to live with a sub-optimal plan and save on memory , then they need to set the parameter to force.

"The difference between SIMILAR and FORCE is that SIMILAR forces similar statements to share the SQL area without deteriorating execution plans.
Setting CURSOR_SHARING to FORCE forces similar statements to share the SQL area potentially deteriorating execution plans."

It is also possible to tell from 10046 trace (level 4/12 - BINDS) if a bind is considered to be unsafe
The flag oacfl2 in 9i and fl2 in 10g will show if a variable is unsafe.

BINDS #2:
bind 0: dty=2 mxl=22(04) mal=00 scl=00 pre=00 oacflg=10 oacfl2=500 size=24
offset=0
bfp=1036d6408 bln=22 avl=04 flg=09
value=16064
bind 1: dty=2 mxl=22(04) mal=00 scl=00 pre=00 oacflg=10 oacfl2=500 size=24
offset=0
bfp=1036d4340 bln=22 avl=04 flg=09


In 10g (10.2.0.5) and 11g using the example query above this looks like:

alter session set cursor_sharing=force;
alter session set events '10046 trace name context forever,level 12';


select /* TEST */ * from emp where sal > :"SYS_B_0"
END OF STMT
..
BINDS #3071441600:
Bind#0
oacdty=02 mxl=22(03) mxlc=00 mal=00 scl=00 pre=00
oacflg=10 fl2=0300 frm=00 csi=00 siz=24 off=0
kxsbbbfp=295c96f0 bln=22 avl=03 flg=09
value=103


The "fl2=0300" entry indicates that this is and Unsafe literal and the bind was generated by replacement :

#define UACFBLTR 0x00000100 /* Bind was generated by LiTeRal replacement */
#define UACFUNSL 0x00000200 /* UNSafe Literal */


The 0x200 entry being the important flag for determination of literal 'safety'.
For additional details on this topic see:

Document 377847.1 Unsafe Literals or Peeked Bind Variables
Document 261020.1 High Version Count with CURSOR_SHARING = SIMILAR or FORCE

Enhancement to obsolete parent cursors if Version Count exceeds a threshold

In 11gr2, an issue of Child cursors growing very long was introduced.  An enhancement request was filed to address this issue Bug 10187168. When the child cursors grow beyond certain count be it 20 or 100, it obsoletes the parent cursors.  In order to activate this enhancement bug set following:

1. If 11.2.0.3 and above, set the following parameters:

"_cursor_obsolete_threshold"  to 100 (this is the number of child cursor after which we obsolete it) 

2. If  11.2.0.2.2, then set: 

"_cursor_features_enabled" to 1026
event 106001 with value 100 (as the parameter _cursor_obsolete_threshold is not present) 

For more information, please read the following article regarding the enhancement involved:

Document 10187168.8 Enhancement to obsolete parent cursors if Version Count exceeds a threshold

High Version Count with Adaptive Cursor Sharing

With introduction of adaptive cursor sharing in 11g, there may be increased version count due to more child cursors.  The adaptive cursor sharing is meant to adapt execution plans, depending on the selectivity of the bind variable.  For more information on adaptive cursor sharing, please review following note:

Document 740052.1 Adaptive Cursor Sharing Overview

Some known issues with Adaptive Cursor Sharing Overview:

Document 7213010.8 Bug 7213010 - Adaptive cursor sharing generates lots of child cursors
Document 8491399.8 Bug 8491399 - Adaptive Cursor Sharing does not match the correct cursor version for queries using CHAR datatype 

Troubleshooting Other Issues

For guidance troubleshooting other performance issues take a look at:

Document 1377446.1 Troubleshooting Performance Issues

 

Discuss High Version Count Issues

The window below is a live discussion of this article (not a screenshot). We encourage you to join the discussion by clicking the "Reply" link below for the entry you would like to provide feedback on. If you have questions or implementation issues with the information in the article above, please share that below.

References

NOTE:377847.1 - Unsafe Literals or Peeked Bind Variables




NOTE:438755.1 - High SQL Version Counts - Script to determine reason(s)
NOTE:557661.1 - Rolling Cursor Invalidations with DBMS_STATS.AUTO_INVALIDATE
NOTE:261020.1 - High Version Count with CURSOR_SHARING = SIMILAR or FORCE

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