Understanding and Tuning the Shared Pool


Understanding and Tuning the Shared Pool

  1. 1.            Introduction

The aim of this article is to introduce the key issues involved in tuning the shared pool in Oracle 7 through 9. The notes here are particularly important if your system shows any of the following:

       Latch contention for the library cache latch/es

       Latch contention for the shared pool latch

       High CPU parse times

       High numbers of reloads in V$LIBRARYCACHE

       Lots of parse calls

       Frequent ORA-04031 errors

  1. 2.            What is the shared pool ?

Oracle keeps SQL statements, packages, object information and many other items in an area in the SGA known as the shared pool. This sharable area of memory is managed as a sophisticated cache and heap manager rolled into one. It has 3 fundamental problems to overcome:

  1. The unit of memory allocation is not a constant – memory allocations from the pool can be anything from a few bytes to many kilobytes
  2. Not all memory can be ‘freed’ when a user finishes with it (as is the case in a traditional heap manager) as the aim of the shared pool is to maximize sharability of information. The information in the memory may be useful to another session – Oracle cannot know in advance if the items will be of any use to anyone else or not.
  3. There is no disk area to page out to so this is not like a traditional cache where there is a file backing store. Only “recreatable” information can be discarded from the cache and it has to be re-created when it is next needed.

Given this background one can understand that management of the shared pool is a complex issue. The sections below list the key issues affecting the performance of the shared pool and its associated latches.

Items covered include:

       Terminology

       Benefits of Literal SQL ?

       Why Share SQL

       Reducing the load on the Shared Pool

   Parse Once / Execute Many

   Eliminating Literal SQL

   Avoid Invalidations

   CURSOR_SHARING parameter (8.1.6 onwards)

   SESSION_CACHED_CURSORS parameter

   CURSOR_SPACE_FOR_TIME parameter

   CLOSE_CACHED_OPEN_CURSORS parameter

   SHARED_POOL_RESERVED_SIZE parameter

   SHARED_POOL_RESERVED_MIN_ALLOC parameter

   SHARED_POOL_SIZE parameter

   _SQLEXEC_PROGRESSION_COST parameter (8.1.5 onwards)

   Precompiler HOLD_CURSOR and RELEASE_CURSOR Options

   DBMS_SHARED_POOL.KEEP

   Flushing the SHARED POOL

   Using V$ Views (V$SQL and V$SQLAREA)

   MTS & XA

       Useful SQL for looking at Shared Pool problems

       Issues in various Oracle Releases

  1. 3.            Terminology

Literal SQL

A literal SQL statement is considered as one which uses literals in the predicate/s rather than bind variables where the value of the literal is likely to differ between various executions of the statement.
Eg 1:

  SELECT * FROM emp WHERE ename=’CLARK’;

    is used by the application instead of

  SELECT * FROM emp WHERE ename=:bind1;

Eg 2:

  SELECT sysdate FROM dual;

    does not use bind variables but would not be considered as a literal

    SQL statement for this article as it can be shared.

Eg 3:

  SELECT version  FROM app_version WHERE version>2.0;

    If this same statement was used for checking the ‘version’ throughout

    the application then the literal value ‘2.0’ is always the same

    so this statement can be considered sharable.

Hard Parse

If a new SQL statement is issued which does not exist in the shared pool then this has to be parsed fully. Eg: Oracle has to allocate memory for the statement from the shared pool, check the statement syntactically and semantically etc… This is referred to as a hard parse and is very expensive in both terms of CPU used and in the number of latch gets performed.

Soft Parse

If a session issues a SQL statement which is already in the shared pool AND it can use an existing version of that statement then this is known as a ‘soft parse’. As far as the application is concerned it has asked to parse the statement.

Identical Statements ?

If two SQL statements mean the same thing but are not identical character for character then from an Oracle viewpoint they are different statements. Consider the following issued by SCOTT in a single session:

        SELECT ENAME from EMP;

        SELECT ename from emp;

Although both of these statements are really the same they are not identical as an upper case ‘E’ is not the same as a lower case ‘e’.

Sharable SQL

If two sessions issue identical SQL statements it does NOT mean that the statement is sharable. Consider the following:

        User SCOTT has a table called EMP and issues:

                SELECT ENAME from EMP;

        User FRED has his own table called EMP and also issues:

                SELECT ENAME from EMP;

Although the text of the statements are identical the EMP tables are different objects. Hence these are different versions of the same basic statement. There are many things that determine if two identical SQL strings are truely the same statement (and hence can be shared) including:

       All object names must resolve to the same actual objects

       The optimizer goal of the sessions issuing the statement should be the same

       The types and lengths of any bind variables should be “similar”. (We dont discuss the details of this here but different types or lengths of bind variables can cause statements to be classed as different versions)

       The NLS (National Language Support) environment which applies to the statement must be the same.

Versions of a statement

As described in ‘Sharable SQL’ if two statements are textually identical but cannot be shared then these are called ‘versions’ of the same statement. If Oracle matches to a statement with many versions it has to check each version in turn to see if it is truely identical to the statement currently being parsed. Hence high version counts are best avoided by:

       Standardising the maximum bind lengths specified by the client

       Avoid using identical SQL from lots of different schemas which use private objects. Eg: SELECT xx FROM MYTABLE; where each user has their own MYTABLE

       Setting _SQLEXEC_PROGRESSION_COST to ‘0’ in Oracle 8.1

Library Cache and Shared Pool latches

The shared pool latch is used to protect critical operations when allocating and freeing memory in the shared pool.

The library cache latches (and the library cache pin latch in Oracle 7.1) protect operations within the library cache itself.

All of these latches are potential points of contention. The number of latch gets occurring is influenced directly by the amount activity in the shared pool, especially parse operations. Anything that can minimise the number of latch gets and indeed the amount of activity in the shared pool is helpful to both performance and scalability.

  1. 4.            Literal SQL versus Shared SQL

To give a balanced picture this short section describes the benefits of both literal SQL and sharable SQL.

Literal SQL

The Cost Based Optimizer (CBO) works best when it has full statistics and when statements use literals in their predicates. Consider the following:

        SELECT distinct cust_ref FROM orders WHERE total_cost < 10000.0;

    versus

        SELECT distinct cust_ref FROM orders WHERE total_cost < :bindA;

For the first statement the CBO could use histogram statistics that have been gathered to decide if it would be fastest to do a full table scan of ORDERS or to use an index scan on TOTAL_COST (assuming there is one). In the second statement CBO has no idea what percentage of rows fall below “:bindA” as it has no value for this bind variable to determine an execution plan . Eg: “:bindA” could be 0.0 or 99999999999999999.9

There could be orders of magnitude difference in the response time between the two execution paths so using the literal statement is preferable if you want CBO to work out the best execution plan for you. This is typical of Decision Support Systems where there may not be any ‘standard’ statements which are issued repeatedly so the chance of sharing a statement is small. Also the amount of CPU spent on parsing is typically only a small percentage of that used to execute each statement so it is probably more important to give the optimizer as much information as possible than to minimize parse times.

  1. 5.            Sharable SQL

If an application makes use of literal (unshared) SQL then this can severely limit scalability and throughput. The cost of parsing a new SQL statement is expensive both in terms of CPU requirements and the number of times the library cache and shared pool latches may need to be acquired and released.

Eg: Even parsing a simple SQL statement may need to acquire a library cache latch 20 or 30 times.

The best approach to take is that all SQL should be sharable unless it is adhoc or infrequently used SQL where it is important to give CBO as much information as possible in order for it to produce a good execution plan.

  1. 6.            Reducing the load on the Shared Pool

Parse Once / Execute Many

By far the best approach to use in OLTP type applications is to parse a statement only once and hold the cursor open, executing it as required. This results in only the initial parse for each statement (either soft or hard). Obviously there will be some statements which are rarely executed and so maintaining an open cursor for them is a wasteful overhead.

Note that a session only has <Parameter:OPEN_CURSORS> cursors available and holding cursors open is likely to increase the total number of concurrently open cursors.

In precompilers the HOLD_CURSOR parameter controls whether cursors are held open or not while in OCI developers have direct control over cursors .

Eliminating Literal SQL

If you have an existing application it is unlikely that you could eliminate all literal SQL but you should be prepared to eliminate some if it is causing problems. By looking at the V$SQLAREA view it is possible to see which literal statements are good candidates for converting to use bind variables. The following query shows SQL in the SGA where there are a large number of similar statements:

  SELECT substr(sql_text,1,40) “SQL”,

         count(*) ,

         sum(executions) “TotExecs”

    FROM v$sqlarea

   WHERE executions < 5

   GROUP BY substr(sql_text,1,40)

  HAVING count(*) > 30

   ORDER BY 2

  ;

  Note: If there is latch contention for the library cache latches the above

        statement may cause yet further contention problems.

The values 40,5 and 30 are example values so this query is looking for different statements whose first 40 characters are the same which have only been executed a few times each and there are at least 30 different occurrances in the shared pool. This query uses the idea it is common for literal statements to begin “SELECT col1,col2,col3 FROM table WHERE …” with the leading portion of each statement being the same.

Note:There is often some degree of resistance to converting literal SQL to use bind variables. Be assured that it has been proven time and time again that performing this conversion for the most frequently occurring statements can eliminate problems with the shared pool and improve scalability greatly.

See the documentation on the tool/s you are using in your application to determine how to use bind variables in statements.

Avoid Invalidations

Some specific orders will change the state of cursors to INVALIDATE. These orders modify directly the context of related objects associated
with cursors. That’s orders are TRUNCATE, ANALYZE or DBMS_STATS.GATHER_XXX on tables or indexes, grants changes on underlying objects. The associated cursors will stay in the SQLAREA but when it will be reference next time, it should be reloaded and reparsed fully, so the global performance will be impacted.

The following query could help us to better identify the concerned cursors:

 SELECT substr(sql_text, 1, 40) “SQL”, invalidations from v$sqlarea

 order by invalidations DESC;

To get more details on this, consult Note 115656.1 and Note 123214.1.

CURSOR_SHARING parameter (8.1.6 onwards)

<Parameter:CURSOR_SHARING> is a new parameter introduced in Oracle8.1.6. It should be used with caution in this release. If this parameter is set to FORCE then literals will be replaced by system generated bind variables where possible. For multiple similar statements which differ only in the literals used this allows the cursors to be shared even though the application supplied SQL uses literals. The parameter can be set dynamically at the system or session level thus:

        ALTER SESSION SET cursor_sharing = FORCE;

        or

        ALTER SYSTEM SET cursor_sharing = FORCE;

or it can be set in the init.ora file.
Note: As the FORCE setting causes system generated bind variables to be used in place of literals, a different execution plan may be chosen by the cost based optimizer (CBO) as it no longer has the literal values available to it when costing the best execution plan.

In Oracle9i, it is possible to set CURSOR_SHARING=SIMILAR. SIMILAR causes statements that may differ in some literals, but are otherwise identical, to share a cursor, unless the literals affect either the meaning of the statement or the degree to which the plan is optimized. This enhancement improves the usability of the parameter for situations where FORCE would normally cause a different, undesired execution plan. With CURSOR_SHARING=SIMILAR, Oracle determines which literals are “safe” for substitution with bind variables. This will result in some SQL not being shared in an attempt to provide a more efficient execution plan.

See Note 94036.1 for details of this parameter.

SESSION_CACHED_CURSORS parameter

<Parameter:SESSION_CACHED_CURSORS> is a numeric parameter which can be set at instance level or at session level using the command:

        ALTER SESSION SET session_cached_cursors = NNN;

The value NNN determines how many ‘cached’ cursors there can be in your session.

Whenever a statement is parsed Oracle first looks at the statements pointed to by your private session cache – if a sharable version of the statement exists it can be used. This provides a shortcut access to frequently parsed statements that uses less CPU and uses far fewer latch gets than a soft or hard parse.

To get placed in the session cache the same statement has to be parsed 3 times within the same cursor – a pointer to the shared cursor is then added to your session cache. If all session cache cursors are in use then the least recently used entry is discarded.

If you do not have this parameter set already then it is advisable to set it to a starting value of about 50. The statistics section of the bstat/estat report includes a value for ‘session cursor cache hits’ which shows if the cursor cache is giving any benefit. The size of the cursor cache can then be increased or decreased as necessary. SESSION_CACHED_CURSORS are particularly useful with Oracle Forms applications when forms are frequently opened and closed.

CURSOR_SPACE_FOR_TIME parameter

<Parameter:CURSOR_SPACE_FOR_TIME> controls whether parts of a cursor remain pinned between different executions of a statement. This may be worth setting if all else has failed as it can give some gains where there are sharable statements that are infrequently used, or where there is significant pinning / unpinning of cursors (see <View:V$LATCH_MISSES> – if most latch waits are due to “kglpnc: child” and “kglupc: child” this is due to pinning / unpinning of cursors) .

You must be sure that the shared pool is large enough for the work load otherwise performance will be badly affected and ORA-4031 eventually signalled.
If you do set this parameter to TRUE be aware that:

       If the SHARED_POOL is too small for the workload then an ORA-4031 is much more likely to be signalled.

       If your application has any cursor leak then the leaked cursors can waste large amounts of memory having an adverse effect on performance after a period of operation.

       There have historically been problems reported with this set to TRUE. The main known issues are:

   Bug 770924 (Fixed 8061 and 8160) ORA-600 [17302] may occur

   Bug 897615 (Fixed 8061 and 8160) Garbage Explain Plan over DBLINK

   Bug 1279398 (Fixed 8162 and 8170) ORA-600 [17182] from ALTER SESSION SET NLS…

CLOSE_CACHED_OPEN_CURSORS parameter

This parameter has been obsoleted in Oracle8i.
<Parameter:CLOSE_CACHED_OPEN_CURSORS> controls whether PL/SQL cursors are closed when a transaction COMMITs or not. The default value is FALSE which causes PL/SQL cursors to be kept open across commits which can help reduce the number of hard parses which occur. If this has been set to TRUE then there is an increased chance that the SQL will be flushed from the shared pool when not in use.

SHARED_POOL_RESERVED_SIZE parameter

There are quite a few notes explaining <Parameter:SHARED_POOL_RESERVED_SIZE> already in circulation. The parameter was introduced in Oracle 7.1.5 and provides a means of reserving a portion of the shared pool for large memory allocations. The reserved area comes out of the shared pool itself.

From a practical point of view one should set SHARED_POOL_RESERVED_SIZE to about 10% of SHARED_POOL_SIZE unless either the shared pool is very large OR SHARED_POOL_RESERVED_MIN_ALLOC has been set lower than the default value:

       If the shared pool is very large then 10% may waste a significant amount of memory when a few Mb will suffice.

       If SHARED_POOL_RESERVED_MIN_ALLOC has been lowered then many space requests may be eligible to be satisfied from this portion of the shared pool and so 10% may be too little.

It is easy to monitor the space usage of the reserved area using the <View:V$SHARED_POOL_RESERVED> which has a column FREE_SPACE.

SHARED_POOL_RESERVED_MIN_ALLOC parameter

In Oracle8i this parameter is hidden.
SHARED_POOL_RESERVED_MIN_ALLOC should generally be left at its default value, although in certain cases values of 4100 or 4200 may help relieve some contention on a heavily loaded shared pool.

SHARED_POOL_SIZE parameter

<Parameter:SHARED_POOL_SIZE> controls the size of the shared pool itself. The size of the shared pool can impact performance. If it is too small then it is likely that sharable information will be flushed from the pool and then later need to be reloaded (rebuilt). If there is heavy use of literal SQL and the shared pool is too large then over time a lot of small chunks of memory can build up on the internal memory freelists causing the shared pool latch to be held for longer which in-turn can impact performance. In this situation a smaller shared pool may perform better than a larger one. This problem is greatly reduced in 8.0.6 and in 8.1.6 onwards due to the enhancement in Bug 986149 .

NB: The shared pool itself should never be made so large that paging or swapping occur as performance can then decrease by many orders of magnitude.

See Note 1012046.6 to calculate the SHARED_POOL_SIZE requirements based on your current workload.

_SQLEXEC_PROGRESSION_COST parameter (8.1.5 onwards)

This is a hidden parameter which was introduced in Oracle 8.1.5. The parameter is included here as the default setting has caused some problems with SQL sharability. Setting this parameter to 0 can avoid these issues which result in multiple versions statements in the shared pool.
Eg: Add the following to the init.ora file

        # _SQLEXEC_PROGRESSION_COST is set to ZERO to avoid SQL sharing issues

        # See Note:62143.1 for details

        _sqlexec_progression_cost=0

Note that a side effect of setting this to ‘0’ is that the V$SESSION_LONGOPS view is not populated by long running queries.
See Note 68955.1 for more details of this parameter.

Precompiler HOLD_CURSOR and RELEASE_CURSOR Options

When using Oracle Precompiler the behavior of the shared pool can be modified by using parameters RELEASE_CURSOR and HOLD_CURSOR when precompiling the program. These parameters will determine the status of a cursor in the library cache and the session cache once the execution of the cursor ends.

For further information on these parameters, please refers to Note 73922.1

DBMS_SHARED_POOL.KEEP

This procedure (defined in the DBMSPOOL.SQL script in the RDBMS/ADMIN directory) can be used to KEEP objects in the shared pool. DBMS_SHARED_POOL.KEEP allows one to ‘KEEP’ packages, procedures, functions, triggers (7.3+) and sequences (7.3.3.1+) and is fully described in Note 61760.1

It is generally desirable to mark frequently used packages such that they are always KEPT in the shared pool. Objects should be KEPT shortly after instance startup since the database does not do it automatically after a shutdown was issued.

NB: Prior to Oracle 7.2 DBMS_SHARED_POOL.KEEP does not actually load all of the object to be KEPT into the shared pool. It is advisable to include a dummy procedure in each package to be KEPT. This dummy procedure can then be called after calling DBMS_SHARED_POOL.KEEP to ensure the object is fully loaded. This is not a problem from 7.2 onwards.

Flushing the SHARED POOL

On systems which use a lot of literal SQL the shared pool is likely to fragment over time such that the degree of concurrency which can be achieved diminishes.Flushingthe shared pool will often restore performance for a while as it can cause many small chunks of memory to be coalesced. After the flush there is likely to be an interim spike in performance as the act of flushing may remove sharable SQL from the shared pool but does nothing to improve shared pool fragmentation. The command to flush the shared pool is:

        ALTER SYSTEM FLUSH SHARED_POOL;

Contrary to reports elsewhere items kept in the shared pool using DBMS_SHARED_POOL.KEEP will NOT be flushed by this command. Any items (objects or SQL) actually pinned by sessions at the time of the flush will also be left in place.

NB:Flushingthe shared pool will flush any cached sequences potentially leaving gaps in the sequence range. DBMS_SHARED_POOL.KEEP(‘sequence_name’,’Q’) can be used to KEEP sequences preventing such gaps.

Using V$ Views (V$SQL and V$SQLAREA)

Note that some of the V$ views have to take out relevant latches to obtain the data to reply to queries. This is notably so for views against the library cache and SQL area. It is generally advisable to be selective about what SQL is issued against these views. In particular use of V$SQLAREA can place a great load on the library cache latches. Note that V$SQL can often be used in place of V$SQLAREA and can have less impact on the latch gets – this is because V$SQLAREA is a GROUP BY of statements in the shared pool while V$SQL does not GROUP the statements.

MTS, Shared Server and XA

The multi-threaded server (MTS) adds to the load on the shared pool and can contribute to any problems as the User Global Area (UGA) resides in the shared pool. This is also true of XA sessions in Oracle7 as their UGA is located in the shared pool. (In Oracle8/8i XA sessions do NOT put their UGA in the shared pool). In Oracle8 the Large Pool can be used for MTS reducing its impact on shared pool activity – However memory allocations in the Large Pool still make use of the “shared pool latch”. See Note 62140.1 for a description of the Large Pool.

Using dedicated connections rather than MTS causes the UGA to be allocated out of process private memory rather than the shared pool. Private memory allocations do not use the “shared pool latch” and so a switch from MTS to dedicated connections can help reduce contention in some cases.

In Oracle9i, MTS was renamed to “Shared Server”. For the purposes of the shared pool, the behaviour is essentially the same.

  1. 7.            Useful SQL for looking at Shared Pool problems

This section shows some example SQL that can be used to help find potential issues in the shared pool. The output of these statements should be spooled to a file.
Note: These statements may add to any latch contention as described in “Using V$ Views (V$SQL and V$SQLAREA)” above.

       Finding literal SQL

                 SELECT substr(sql_text,1,40) “SQL”,

                        count(*) ,

                        sum(executions) “TotExecs”

                   FROM v$sqlarea

                  WHERE executions < 5

                  GROUP BY substr(sql_text,1,40)

                 HAVING count(*) > 30

                  ORDER BY 2

          ;

This helps find commonly used literal SQL – See “Eliminating Literal SQL” above.

       Finding the Library Cache hit ratio

               SELECT SUM(PINS) “EXECUTIONS”,

               SUM(RELOADS) “CACHE MISSES WHILE EXECUTING”

        FROM V$LIBRARYCACHE;

If the ratio of misses to executions is more than 1%, then try to reduce the library cache misses

       Checking hash chain lengths:

               SELECT hash_value, count(*)

                 FROM v$sqlarea

                GROUP BY hash_value

               HAVING count(*) > 5

        ;

This should usually return no rows. If there are any HASH_VALUES with high counts (double figures) then you may be seeing the effects of a bug, or an unusual form of literal SQL statement. It is advisable to drill down and list out all the statements mapping to the same HASH_VALUE. Eg:

 SELECT sql_text FROM v$sqlarea WHERE hash_value= <XXX>;

and if these look the same get the full statements from V$SQLTEXT. It is possible for many literals to map to the same hash value. Eg: In 7.3 two statements may have the same hash value if a literal value occurs twice in the statement and there are exactly 32 characters between the occurrences.

       Checking for high version counts:

               SELECT address, hash_value,

                       version_count ,

                       users_opening ,

                       users_executing,

                       substr(sql_text,1,40) “SQL”

                 FROM v$sqlarea

                WHERE version_count > 10

        ;

“Versions” of a statement occur where the SQL is character for character identical but the underlying objects or binds etc.. are different as described in “Sharable SQL” above. High version counts can occur in various Oracle8i releases due to problems with progression monitoring. This can be disabled by setting _SQLEXEC_PROGRESSION_COST to ‘0’ as described earlier in this note.

       Finding statement/s which use lots of shared pool memory:

               SELECT substr(sql_text,1,40) “Stmt”, count(*),

                       sum(sharable_mem)    “Mem”,

                       sum(users_opening)   “Open”,

                       sum(executions)      “Exec”

                 FROM v$sql

                GROUP BY substr(sql_text,1,40)

               HAVING sum(sharable_mem) > <MEMSIZE>

        

        ;

where MEMSIZE is about 10% of the shared pool size in bytes. This should show if there are similar literal statements, or multiple versions of a statements which account for a large portion of the memory in the shared pool.

       Allocations causing shared pool memory to be ‘aged’ out

               SELECT *

                 FROM x$ksmlru

               WHERE ksmlrnum>0

        ;

Note: This select returns no more than 10 rows and then erases the contents of the X$KSMLRU table so be sure to SPOOL the output. The X$KSMLRU table shows which memory allocations have caused the MOST memory chunks to be thrown out of the shared pool since it was last queried. This is sometimes useful to help identify sessions or statements which are continually causing space to be requested. If a system is well behaved and uses well shared SQL, but occasionally slows down this select can help identify the cause. Refer to Note 43600.1 for more information on X$KSMLRU.

  1. 8.            Issues in various Oracle Releases

These are some important issues which affect performance of the shared pool in various releases:

       Increasing the CPU processing power of each CPU can help reduce shared pool contention problems in all Oracle releases by decreasing the amount of time each latch is held. A faster CPU is generally better than a second CPU.

       If you have an EVENT parameter set for any reason check with Oracle support that this is not an event that will impact shared pool performance.

  1. 9.            Bug fixes and Enhancements

This is a summary listing of the main bugs and enhancements affecting the shared pool. The ‘Fixed’ column lists the 4 digit server releases where the problem / enhancement is fixed – eg: 8062 means fixed in 8.0.6.2, 9000 means the issue has been fixed for Oracle9i.
Note 190077.1.

Bug

Versions

Fixed

Description

Bug 1623256

8-90

9000

Identical SQL referencing SCHEMA.SEQUENCE.NEXTVAL not shared by different users

Bug 1366837

-90

8063 8171 9000

Cursors referencing a fully qualified FUNCTION are not shared

Bug 1484634

-90

8063 8171 9000

Large row cache can cause long shared pool latch waits (OPS only)

Bug 1318267

815-90

9000

INSERT AS SELECT may not share SQL when it should

Bug 1149820

-817

8062 8162 8170

ENHANCEMENT: Reduced latch gets purging from shared pool

Bug 1150143

-817

8062 8162 8170

ENHANCEMENT: Delay purge when bind mismatch

Bug 1258708

-817

8170

ENHANCEMENT: Reduce need to get PARENT library cache latch

Bug 1348501

8-817

8163 8170

MVIEW refresh unnecessarily invalidates shared cursors

Bug 1357233

815-817

8163 8170

ALTER SESSION FORCE PARALLEL PQ/DML/DDL does not share recursive SQL

Bug 1193003

815-817

8162 8170

Cursors may not be shared in 8.1 when they should be

Bug 1210242

815-817

8162 8170

Cursors not shared if both TIMED_STATISTICS and SQL_TRACE are enabled

Bug 986149

7-816

8060 8160

ENHANCEMENT: More freelists for shared pool memory chunks (reduced latch contention)

Bug 858015

815-816

8160

Shared pool memory for views higher if QUERY_REWRITE_ENABLED set

Bug 918002

815-816

8151 8160

Cursors are not shared if SQL_TRACE or TIMED_STATISTICS is TRUE

Bug 888551

815-816

8151 8160

TIMED_STATISTICS can affect cursor sharing / Dump from EXPLAIN or enable/disable SQL_TRACE

Bug 1065010

8-817

8062 8162 8170

Access to DC_HISTOGRAM_DEFS from Remote Queries can impact shared pool performan ce.

Bug 1115424

-817

8062 8162 8170

Cursor authorization and dependency lists too long – can impact shared pool

Bug 1131711

803-8062

8062 8150

SQL from PLSQL using NUMERIC binds may not be shared when it should

Bug 1397603

817

82

ORA-4031 / SGA memory leak of PERMANENT memory for buffer handles

Bug 1640583

816

8171 8200

ORA-4031 due to leak / cache buffer chain contention from AND-EQUAL access

Historic Notes

The notes here relate to pre-Oracle7.3 releases of Oracle and are included for completeness only:

       In 7.3 the PLSQL was enhanced to used paged executable code reducing the number of large allocations in the shared pool and reducing the need for KEEPing.

       Oracle 7.1.6 to 7.2.3 there are several known problems. See Note 32871.1

       Between Oracle 7.1 and 7.2 the latching mechanism over the library cache changed.

       Some historic bugs:

Bug

Versions

Fixed

Description

Bug 596953

80-81

8044 8052 8060 8150

Excessive shared pool fragmentation due to 2K context area chunk size.

Bug 724620

700-814

7344 8043 8052 8060

Select from VIEW now uses less shared memory (less latch gets)

Bug 633498

7-815

7343 8043 8050 8150

Selecting from some V$ views can make statements unsharable

Bug 625806

7X-806

7343 8042 8051 8060 8150

Cursor not shared for a VIEW using FUNCTION / with DBMS_SQL

Bug 520708

7X-804

7336 7342 8040

Better handling of small memory chunks in the SGA

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Sobre Alexandre Pires

ORACLE OCS Goldengate Specialist, OCE RAC 10g R2, OCP 12C, 11g, 10g , 9i e 8i - Mais de 25 anos de experiência na área de TI. Participei de projetos na G&P alocado na TOK STOK, EDINFOR alocado na TV CIDADE "NET", 3CON Alocado no PÃO DE AÇUCAR, DISCOVER alocado na VIVO, BANCO IBI e TIVIT, SPC BRASIL, UOLDIVEO alocado no CARREFOUR e atualmente na ORACLE ACS atendendo os seguintes projetos: VIVO, CLARO, TIM, CIELO, CAIXA SEGUROS, MAPFRE, PORTO SEGURO, SULAMERICA, BRADESCO SEGUROS, BANCO BRADESCO, BASA, SANTANDER, CNJ, TSE, ELETROPAULO, EDP, SKY, NATURA, ODEBRESHT, NISSEI, SICREDI, CELEPAR, TAM, TIVIT, IBM, SMILES, CELEPAR, SERPRO,OKI,BANCO PAN, etc
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