Weblogic JDBC Connection Pool misconfiguration and how it can cause serious performance problems

Part 1 of 2

Connection Pool (mis)configuration has long been a pet peeve of mine, and we have only recently changed our standards after a series of serious performance problems on our main production databases. In my opinion this is so important that I will post two blogs on this. This post will be about configuration parameters to minimize connections and disconnections. The second post will be about how high concurrency can lead to such severe degradation that the only way to recover is to shut down the database.

This post is going to focus on why a high frequency of connections and disconnections is bad for your Oracle database. There was a 9i Oracle documentation article called Top Ten Mistakes Found in Oracle Systems and the first and foremost point was “Bad Connection Management” where they stated:
“The application connects and disconnects for each database interaction. This problem is common with stateless middleware in application servers. It has over two orders of magnitude impact on performance, and it is totally unscalable.”

So over ten years ago, Oracle had the vision to alert its DBA’s about this, and people chose to ignore this prophetic advice at their own peril. The middleware in this case is Weblogic, and setting up JDBC connection pools in a dynamic manner leads to a high frequency of connections and disconnections. Two orders of magnitude means that performance could be affected by over a hundred-fold, and this is obviously unscalable. What this means is that if you expect your applications to scale with data growth over time, without performance degradation, they will not. Usually it does not matter how much hardware you throw at a legacy scalability problem – it will never be powerful enough because you will just reach your bottleneck more quickly. It’s not only the data that changes, it’s the number of users, the complexity and interaction of systems, and competition for resources on a server with finite resources.

Why should repetitive connections and connections be such a bad thing for the database? This is a question that has puzzled architects and developers for years, and some still don’t believe that it is bad. Let’s look at the amount of work the Oracle RDBMS and the server have to do for each connection. A new server process has to be created, which takes CPU and at least 20M of memory for 11g. The PGA has to be grown to possibly accommodate for future work areas. Oracle then has to check its dictionary and dependancy tree semantically and syntactically for user privileges, which has a large CPU cost. Just this check can comprise of scores of dictionary reads with its associated parses, executes and fetches. After all of this work, then there is the matter of executing logon triggers, if any, and then all the overhead of finding space for that new session’s SQL in the SGA, and all future DML of the session.

If you have the space for keeping a slightly higher number of session s connected, it is really far better to do so.  It is good practice to acquire a connection, do the maximum work needed for that connection, then return that connection to the pool as soon as possible. Having a connection pool create a new session, do some work, then disconnect (shrink the pool) is the worst practice.

So, for many years I had this argument with architects and developers, and it was eventually agreed to keep connection pools static. This link states that an easy way to boost performance is to keep INIT and MAX the same. In this way they could never grow or shrink right? Wrong. If you were in a situation where the application server thought a connection was down, but Oracle though it was up, the setting of MIN and MAX to the same value is not going to prevent a flood of connection attempts when oracle becomes available. Weblogic tests connections periodically and if the result does not return timeously, Weblogic will create a new connection regardless and remove the non-functioning one from the pool. So we decided it was better to set the Shrink Frequency (Harvest Interval) to a high value like 30 minutes, so that the frequency of disconnections will not be a problem.

So these are the values we proposed in our new standards, after many discussions and meetings with architects:

  • Select the MAX (Maximum Capacity)
  • Set the MIN (Minimum Capacity) to be at least half (50%) of MAX so that the connection pool cannot shrink below this value
  • Set the INIT (Initial Capacity) to be at least quarter (25%) of MAX. This is the initial number of connections created when the application server is started
  • Set the Shrink Frequency (Harvest Interval) to 1800 (30 minutes). This setting is critical in managing the frequency of disconnections.
  • Set Connection Creation Retry Frequency to 120 (2 minutes)
  • Set Test Frequency to 120 (2 minutes). This is to poll the database to test the connection. This defaults to “SELECT 1 FROM DUAL” which bypasses the buffer cache so it’s not bad.
  • Set Seconds to trust an Idle Pool Connection to 10.
  • Cache for prepared statements must be set to 20 or higher. Setting this higher should be discussed with your DBA. The total number of cached cursors would be the cache setting * number of connections in pool * number of servers in cluster. This should be set very carefully. That calulation equates to the number of open cursors allowed per session, so if it is set too low then cursors are repeatedly closed and opened. This leads to excessive hard parsing of SQL statements which can also lead to performance degradation. In fact “Bad Use of Cursors” is the number 2 point on the “Top Ten Mistakes” list, and it says that this has an order of magnitude impact in performance, and is totally unscalable.

I am not saying that this is necessarily the most optimum way of doing things. In your situtation, setting your pools to static might be the easiest and best implementation method. What I am saying is be very careful of repetitive Oracle session connections and disconnections and make sure you that use your existing CP sessions as efficiently as possible.

In Part 2 I will discuss the more complicated topic of high concurrency where I will dive into hairy internals like latches and mutexes. Happy computing and thanks for reading!

Huge performance gains from using arrays and bulk collects.

Consider a simple select which I ran in SQL*Plus connected as SYSTEM:

select * from dba_users;

I ran a trace on the session and did a tkprof on the result.

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.29       0.76        201        205          0           0
Execute      1      0.00       0.00          0          0          0           0
Fetch     1353      0.99       1.79       1303       4536          0       20276

What do we notice about these stats? Parse count is 1, which is expected, and it will still be 1 if I run that select again. Check my previous post where I’ve talked about bind variables and why it’s bad for performance to repeat parsing, especially if it is hard parsing. Also, execute count is 1, which is expected, and you want to keep that to 1 as far as possible per select. I’ve seen applications do exotic selects with rowid’s in cursors and the execute column has the same number as the fetch column, which is the most inefficient way to get your data back.

If you had done ‘set array 2‘ in SQLPlus, then the Fetch stats would have looked like this:

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Fetch    10134      1.35       1.88        676      12896          0       20266

Funny enough, when I then used ‘set array 1‘, Oracle ignored the setting and used a value of 2. Somewhere in the recesses of the Oracle core there must be an AI that is intelligent enough to question and compensate for the obvious stupidity of getting back one row per fetch!

By running a simple select at the SQL prompt in SQLPlus, Oracle created an implicit cursor which you can see if you look at the raw trace file before it was tkprof’d.

PARSING IN CURSOR #18446744071486994888 len=23 dep=0 uid=5 oct=3 lid=5 tim=11879507487109 hv=665675061 ad='47520d368' sqlid='ckyh1gcmuut9p'
select * from dba_users
END OF STMT

Now, why were there only 1353 fetches done for 20276 rows of data returned? Implicit cursors will use the default array size of 15 which is the SQLPlus (client tool) setting for array[size].

Let’s write an explicit cursor via an anonymous block to see what Oracle does.

SET SERVEROUTPUT ON
declare 
  cursor c1 is
    select * from v$session;
begin   
  for c in c1 loop 
    null;
  end loop;  
end;
/

Checking the tkprof result:

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.01       0.01          0          0          0           0
Execute      1      0.00       0.00          0          0          0           1
Fetch        0      0.00       0.00          0          0          0           0
------- ------  -------- ---------- ---------- ---------- ----------  ----------

That’s funny… why no result in the Fetch row? Going to Oracle’s documentation, it states that ‘Fetches are only performed for SELECT statements‘. Further more, as shown in a previous post, the anonymous block is declared and run in the session being traced, so it shows a value for Parse and Execute. The “select * from v$session” is run on the server as the cursor c1 that we had defined.

So going back to our trusty V$SQL which shows all statements executed since startup:

select 
  SQL_TEXT, PARSE_CALLS, EXECUTIONS, FETCHES
from V$SQL
where upper(sql_text) like '%V$SESSION%';

SQL_TEXT                  PARSE_CALLS   EXECUTIONS   FETCHES
SELECT * FROM V$SESSION   1             1            7

That’s interesting… Fetches=7. The select returned 696 rows so this means that the array size has been set to 100. Either that, or Oracle has performed an implicit bulk collect as stated by PL/SQL guru Steven Feuerstein here :
Oracle Database automatically optimizes cursor FOR loops to perform similarly to BULK COLLECT queries… and fetch 100 rows at a time.

From the oracle documentation of SET ARRAY, it states that “Values over approximately 100 provide little added performance.” So how can we use much larger “arrays” and get the intuitive performance benefit of collecting a large number of rows into a memory structure? This is where we have to use BULK COLLECT and FORALL.

Consider this code which will show the usage of BULK COLLECT. I am not going to exhaustively list all the BULK usage options, such as SQL%BULK_EXCEPTIONS, as that is the purpose of the documentation, and I don’t want to sidetrack from the point I am making about huge performance gains. However it is worth mentioning the LIMIT clause which limits the number of rows returned by the BULK COLLECT.

declare 
  TYPE ARRAY IS TABLE OF dba_objects%ROWTYPE;
  obj_data ARRAY;
  CURSOR c1 IS 
    SELECT * FROM dba_objects;
begin
  OPEN c1;
    LOOP
      FETCH c1 BULK COLLECT INTO obj_data;     
      EXIT WHEN c1%NOTFOUND;
    END LOOP;
  CLOSE c1;
end;
/

The trace shows the code but we have to check the stats from V$SQL:

select 
  SQL_TEXT, PARSE_CALLS, EXECUTIONS, FETCHES, BUFFER_GETS
from V$SQL
where upper(sql_text) like '%DBA_OBJECTS%';

SQL_TEXT                    PARSE_CALLS   EXECUTIONS   FETCHES   BUFFER_GETS 
SELECT * FROM DBA_OBJECTS   1             1            1         3100

Lo and behold, we get the awesome result of Fetches=1 for a select of over 100,000 rows! Why is this so significant instead of multiple fetches? If the application is client/server, then every fetch is an extra network round trip between client (thin or fat) and the database server. If the SQL is on the server then every fetch is an extra context switch. In both cases, this can result in huge performance benefits on busy production servers that execute thousands or even millions of queries per hour. Have you ever wondered about innocuous “idle” SQL*Net waits events? The majority of these could be as a result of not using bulk collects and arrays efficiently! Every one of them means an extra network round trip and this means wastage of CPU and I/O resources that could have been used for the next process in the run queue.

Now could the bulk mechanism be used for DML (insert, update, delete)? Sure, this is where the FORALL statement is used. In the example above I could have inserted this code after the FETCH:

FORALL i IN 1..obj_data.COUNT
  INSERT INTO some_table VALUES obj_data(i);

This would load the entire array with one write and therefore one roundtrip or context switch, giving the same benefit the BULK COLLECT had done.

Note that BULK COLLECT can be used with your dynamic SQL in the EXECUTE IMMEDIATE statement, but as stated in a previous post it is usually better to limit the use of dynamic sql in busy OLTP production environments.

Array usage in some other development tools

We’ve already seen the use of array[size] in SQL*Plus. This can also be used in Forms.

Pro*C can use pre-fetching transparently with:

$ proc .... PREFETCH=200

This can also be set at compilation time with ANSI C and C++ and overrides the usage of CURSIZE within the C code.

Using the property weblogic.oci.cacheRows, Weblogic JDBC implements array fetching using its Row Caching feature.

Magic” is the RAD tool we’ve been using and arrays can be set at table level. Every development language worth its salt should be able to use arrays and if their oracle drivers are properly written then the relevant OCI calls can be made for arrays and bulk collects.

Conclusion
If you want to experience a massive performance gain in both your DSS and OLTP production environments, make complete and gratuitous use of arrays and bulk collect mechanisms in your code.

Optimizing Performance with Cursor Sharing and Shared Pool behaviour in Oracle 11g R2 (11.2.0.3)

Hi

With this post I’ve decided to create an introductory guide to how Oracle 11g R2 handles literals and binds in the shared pool, how cursor sharing works, and how to write your cursors to maximize performance.

The first parameter to look at is cursor_sharing. Trying to set it to SIMILAR is now deprecated and you will get a message in your alert log saying “WARNING: The parameter cursor_sharing was found to be set to the value SIMILAR. This setting will be ignored and cursor sharing will operate as though the value was set to FORCE instead.”

So let us set it to EXACT and see the behaviour. This means that statements with identical text share the same cursor, and in the documentation Oracle recommends that this is used in DSS (decision support systems / reporting / warehouses).

I created a table called TEST (xnum number) with values from 1 to 10, with an index on xnum.

begin
  for x in 1..10 loop
    insert into test values (x);
  end loop;
  commit;
end;
/

Let’s analyze:

exec DBMS_STATS.GATHER_TABLE_STATS(ownname=>'TEST', tabname=>'TEST', cascade=> TRUE);

Let’s try this statement to see how the optimizer responds:

select * from test where xnum=1;

Now selecting from V$SQL which conveniently holds statistics since instance startup:

select
  SQL_TEXT, SQL_ID, OPEN_VERSIONS, FETCHES, EXECUTIONS, PARSE_CALLS
from
  v$sql
where
  upper(sql_text) like '%TEST WHERE%';

the statement appears exactly like it is, with the literal value of 1, as expected.

I ran a few more literal selects and here are some selected rows:

SQL_TEXT                         SQL_ID        OPEN_VERSIONS FETCHES EXECUTIONS PARSE_CALLS
select * from test where xnum=2  a4hrnbrz05uwy             0       2          1           1
select * from test where xnum=1  cgsb5m608anzk             0       4          2           2
select * from test where  xnum=2 a91a8av266tb9             1       2          1           1

Notice from this that adding a single space in that last statement created a new entry in the shared pool with a new SQL_ID. The point I am really trying to make here is that if you write your applications to generate literal SQL like I’ve done, and you use cursor_sharing=exact, then this is the least optimal way to write sql. Imagine if these statements run thousands or even millions of times in an hour in a large environment… in addition to Oracle having to “hard parse” each new statement as it comes in, it will also have to find free chunks in which to place these statements in the shared pool, while ageing older statements out of the shared pool. So the CPU works extra hard in parsing and managing the shared pool. In time this creates shared pool fragmentation which can lead to many horrors such as library cache pin waits in a busy environment, which can lead to shutdowns and no way of fixing it unless you rewrite the application or change the cursor_sharing to force. However, we will get to that later.

Let’s do the same via an anonymous block and run it twice:

SET SERVEROUTPUT ON
declare
  x number;
  v_xnum number;
begin
  x:=3;
  select xnum into v_xnum from test where xnum=x;
  DBMS_OUTPUT.PUT_LINE('Answer: ' || TO_CHAR (v_xnum));
end;
/

SQL_TEXT        SQL_ID        OPEN_VERSIONS FETCHES EXECUTIONS PARSE_CALLS
declare x number; v_xnum number; begin x:=3; select xnum into v_xnum from test where xnum=x; DBMS_OUTPUT.PUT_LINE('Answer: ' || TO_CHAR (v_xnum)); end; 
                db6b351m50nkq             1       0          2           2
SELECT XNUM FROM TEST WHERE XNUM=:B1 
                7y76r2vywmrv5             1       2          2           1

This is a little better than literal SQL but not much. Oracle gives the block its own SQL_ID which uses a great deal more shareable memory than simple statements, but at least there is only one copy in the shared pool. However the problem arises when these blocks become much larger in an OLTP environment with complex systems with thousands of anonymous blocks especially if they make calls to packages. All underlying code gets loaded into free shareable memory and Oracle has to work to find space (chunks) to load them in. If they are not pinned (using DBMS_SHARED_POOL.KEEP) then you will find the loaded version (from v$sql) count increasing which results in fragmentation in a busy OLTP environment. The second row with the :B1 will be explained with the cursor example below.

Note that the same applies to dynamic SQL (typically generated in packages then executed with “execute immediate”). Badly written dynamic SQL can also cause all kinds of fragmentation problems in your shared pool. “Execute immediate” with the “using” clause does make use of bind variables and is acceptable in some cases, but in my opinion is a lazy developer’s alternative to making proper use of cursors. Cursors are natural to Oracle. Oracle loves well written cursors, and your users will love you and Oracle in return. When performance is bad, they always come to you, the omniscient DBA right? And when you fix it, it was your fault in the first place. The manager or helpdesk person or end user never knew the pain you went through to troubleshoot and find that BAD statement or application server with the memory leak or that storage layer bottleneck…

Aah. Breathe.
Let’s write simple PL/SQL using a cursor in a procedure to see what happens.

create or replace
PROCEDURE test_num (p_num IN NUMBER, p_num_out IN OUT NUMBER) IS
  CURSOR c_num IS
    select xnum from test where xnum=p_num;
BEGIN
  for c in c_num loop
    p_num_out := c.xnum;
  end loop;
END;
/

SET SERVEROUTPUT ON
DECLARE v_pnum number;
BEGIN
  test_num(5,v_pnum);
  DBMS_OUTPUT.PUT_LINE('Answer: '||TO_CHAR(v_pnum));
END;
/

I ran the procedure 5 times with values of 5 and 6.

SQL_TEXT                             SQL_ID        OPEN_VERSIONS FETCHES EXECUTIONS PARSE_CALLS
SELECT XNUM FROM TEST WHERE XNUM=:B1 7y76r2vywmrv5             1       5          5           2

Notice firstly that Oracle transforms the statement (which is the cursor in the procedure) into upper case. Then it assigns a bind variable “:B1” to the statement. When I ran the procedure with value 5, parse calls was 1. When I changed the value to 6, parse calls became 2. This is what is known as a “soft parse”. Oracle peeked, detected a different value and checked statistics, instead of doing a hard parse which has the overhead of syntax and permissions checking. Soft parses are better for performance than hard parses. The advantage of using bind variables is that there is only one version of the statement in the shared pool, which leads to huge gains in performance because of reduced parsing, fragmentation and ageing.

HOWEVER, and this is a very important caveat: using cursor_sharing=exact in busy environments will ONLY work if you have perfectly written applications using cursors as in the above example. Now, which of us can say that we support perfect applications? Invariably, applications throw in anonymous blocks and dynamic SQL. They throw the odd literal statement in. The untrained business analyst connects and runs something naughty. The DBA adds to the fun with exotic scripts. Oracle itself adds dictionary statements that are far from optimal. Monitoring tools can throw in some horrors like segment checking. Woe betide you if the CIO connects. So you get the picture… you will never work in a perfect environment.

The corollary to this is that cursor_sharing=exact is fine for DSS environments, as Oracle states in their 11g R2 documentation:
“If you set CURSOR_SHARING, then Oracle recommends the FORCE setting unless you are in a DSS environment”.
The reason for this is that you rarely get shared pool fragmentation in a reporting or warehouse environment. Users there typically run long-running reports and statements which usually RELY on an expected explain plan that worked the same way in the dev and test environment (there are exceptions to this but I don’t want to go too far off topic. In IT, for everything that is said, there is usually an exception. This is not an exact science). Note that the default value is EXACT so it’s up to you, the conscientious DBA, to use the correct setting for OLTP production.

Just a final note about cursors before we move on. We’ve shown how to reduce parsing which gives huge CPU benefits. Now every statement has to go through a parse/execute cycle, and fetch if it returns a result set. The ideal statement, in a perfect world, is 1 parse, execute many times, and fetch many rows per execute. If you find that the number of executes is too close to the number of parses, then it usually means that you’re opening and closing your cursor too often. If you are returning one row per execute, then you need to investigate the use of Oracle’s array mechanism via BULK COLLECT and FORALL. This has huge I/O benefit which I will discuss in another post.

Before I go on to talk about cursor_sharing=force, it’s worthwhile to talk a little about how oracle treated binds in the past compared to how it does now. This will be the subject of another long post when I have the energy to do so. I’ve been using Oracle since Oracle 7 and even in those days, statements were sent to the shared pool, parsed, then executed based an explain plan derived from complicated rule-based or cost-based optimizer statistics and histograms based on whether the objects were analyzed or not. As rule-based moved towards obsoletion, cost-based became the norm and in Oracle 9i, the CBO (cost based optimizer) generated explain plans based on its histogram knowledge of the data distribution of a table. All was well in DBA world until 10g came along and the CBO decided to default to bind peeking which was the bane of many a DBA and some even chose to turn it off with _optim_peek_user_binds. Bind peeking worked like this: if you had a table with skewed data (for example a table of car sales) and if someone queried a common car (e.g. Beetle) from that table, Oracle would peek at this bind at the first hard parse, then generate an explain plan with full table scan based on this peek. Worse, it would lock the explain plan. So, if someone wanted to query sales statistics of an uncommon car (e.g. Rolls Royce) then the same full table scan would be done in future. This led people to either removing statistics for certain tables, turning off bind peeking, or being really clever and writing queries with literals in them.

So then came 11g with adaptive cursor sharing, and this is Oracle’s summary from metalink 1169017.1:
“Our recommendation is to set cursor_sharing = FORCE. This setting maximizes cursor sharing while leveraging the Adaptive Cursor Sharing framework to generate multiple execution plans based on different literal value ranges if necessary.”
What this means is that instead of peeking at the first value at hard parse time, Oracle now observes ranges of values passed to the bind variables. Based on this “bind sensitivity”, Oracle decides whether to recalculate the plan and add it to the profile of the SQL ID in question. So a statement can now have multiple explain plans, which in itself can be problematic which is the topic of another discussion where I will discuss Baselines and SQL Plan Management.

OK so let’s change cursor_sharing:
alter system set cursor_sharing=force scope=both;
and bounce the database to clear the shared pool.
Let’s do this again to see what happens:

select * from test where xnum=1;

select
  SQL_TEXT, SQL_ID, OPEN_VERSIONS, FETCHES, EXECUTIONS, PARSE_CALLS, IS_BIND_SENSITIVE, IS_BIND_AWARE, IS_SHAREABLE, CHILD_NUMBER
from v$sql 
where 
  upper(sql_text) like '%TEST WHERE XNUM%';

SQL_TEXT                                 SQL_ID        OPEN_VERSIONS FETCHES EXECUTIONS PARSE_CALLS IS_BIND_SENSITIVE IS_BIND_AWARE CHILD_NUMBER
select * from test where xnum=:"SYS_B_0" 50vnp9xux8us2             1       2          1           1                 N             N            0

So instead of a literal in the shared pool, oracle now FORCEs the bind variable. This is system generated and will always have the SYS_ prefix so that you can find them in your shared pool.

Now let’s run that select again with a value of 8 to see the result:

SQL_TEXT                                 SQL_ID        OPEN_VERSIONS FETCHES EXECUTIONS PARSE_CALLS IS_BIND_SENSITIVE IS_BIND_AWARE CHILD_NUMBER
select * from test where xnum=:"SYS_B_0" 50vnp9xux8us2             1       4          2           2                 N             N            0

As expected, it was executed and parsed again. Please note that this is a hard parse. This is the problem with system generated binds which are converted from statements with literals or from dynamic sql. In the long run, one parse per execute is very heavy on CPU especially if the statements run millions of times in a busy production environment.

Let’s see what it does with our anonymous block with xnum=3:

SQL_TEXT SQL_ID        OPEN_VERSIONS FETCHES EXECUTIONS PARSE_CALLS IS_BIND_SENSITIVE IS_BIND_AWARE IS_SHAREABLE CHILD_NUMBER
declare x number; v_xnum number; begin x:=3; select xnum into v_xnum from test where xnum=x; DBMS_OUTPUT.PUT_LINE('Answer: ' || TO_CHAR (v_xnum)); end;                       
         db6b351m50nkq             1       0          1           1                 N             N            Y            0
SELECT XNUM FROM TEST WHERE XNUM=:B1  
         7y76r2vywmrv5             1       1          1           1                 Y             N            Y            0

It created the block with its own ID and then converted the select to use a bind variable. This is not a system bind because the select was done with a variable in a code block (no literal value was used in the code block). Note that IS_BIND_SENSITIVE is marked Y (yes) which means that the select can potentially get a new plan in future. If IS_BIND_AWARE=Y then it means that the plan has changed. At this point, the table has uniformly distributed data. It has 10 rows, with numbers from 1 to 10 in an index, so Oracle has no need to change the plan based on the values it has seen.

Running the select TWICE with x=5 results in TWO parses of the anonymous block but just one parse of the select, so Oracle was happy to continue with the original explain plan.

SQL_TEXT SQL_ID        OPEN_VERSIONS FETCHES EXECUTIONS PARSE_CALLS IS_BIND_SENSITIVE IS_BIND_AWARE IS_SHAREABLE CHILD_NUMBER
declare x number; v_xnum number; begin x:=3; select xnum into v_xnum from test where xnum=x; DBMS_OUTPUT.PUT_LINE('Answer: ' || TO_CHAR (v_xnum)); end; 
         db6b351m50nkq             0       0          1           1                 N             N            Y            0
declare x number; v_xnum number; begin x:=5; select xnum into v_xnum from test where xnum=x; DBMS_OUTPUT.PUT_LINE('Answer: ' || TO_CHAR (v_xnum)); end; 
         c5khy7t3xkpnk             1       0          2           2                 N             N            Y            0
SELECT XNUM FROM TEST WHERE XNUM=:B1 
         7y76r2vywmrv5             1       3          3           1                 Y             N            Y            0

Now let’s skew the data a bit…

begin
  for x in 1..10000
  loop
    insert into test values (11);
  end loop;
  commit;
end;
/

At this stage we will not analyze, just to see what Oracle does with peeking and explain plans.

Run the same select in a cursor, fetching those 10000 rows we inserted.

declare
  type t_nums is varray(10000) of number;
  a_nums t_nums;
  x number;
cursor c1 is
  select xnum from test where xnum=x;
begin
  x:=11;
  open c1;
    fetch c1 bulk collect into a_nums;
  close c1;
end;
/

SQL_TEXT SQL_ID        OPEN_VERSIONS FETCHES EXECUTIONS PARSE_CALLS IS_BIND_SENSITIVE IS_BIND_AWARE IS_SHAREABLE CHILD_NUMBER
declare type t_nums is varray(10000) of number; a_nums t_nums; x number; cursor c1 is select xnum from test where xnum=x; begin x:=11; open c1; fetch c1 bulk collect into a_nums; close c1; end; 
         fcrk1v7mrcnkr             1       0          1           1                 N             N            Y            0
SELECT XNUM FROM TEST WHERE XNUM=:B1 
         7y76r2vywmrv5             1       3          3           1                 Y             N            Y            0
SELECT XNUM FROM TEST WHERE XNUM=:B1 
         7y76r2vywmrv5             1       1          1           1                 Y             N            Y            1

Interesting… it created a new version of the statement with the same SQL_ID.  Notice that CHILD_NUMBER is now 1… which means it created a child of the parent cursor. There is a lot to be said about multiple versions and child cursors, but that is the subject of another post. This is however VERY relevant to shared pool management and behaviour.

More info in this view:

select
  child_number, bucket_id, count from v$sql_cs_histogram
where 
  sql_id = '7y76r2vywmrv5';

CHILD_NUMBER BUCKET_ID COUNT
           1         0     0
           1         1     1
           1         2     0
           0         0     3
           0         1     0
           0         2     0

and this:

select 
  child_number, bind_set_hash_value, peeked, executions, rows_processed, buffer_gets, cpu_time
from 
  v$sql_cs_statistics
where 
  sql_id = '7y76r2vywmrv5';

CHILD_NUMBER BIND_SET_HASH_VALUE PEEKED EXECUTIONS ROWS_PROCESSED BUFFER_GETS CPU_TIME
           1          2671099528      Y          1 10000                   21        0
           0          2706503459      Y          1     1                    1        0

The PEEKED column shows that Oracle did peek at the bind values passed into the SQL.

Now how to get the adaptive cursor sharing framework to change plan? I ran the xnum=11 select three times and it still does a range scan of the index:

INDEX (RANGE SCAN) of TEST.TEST_PK (INDEX)

Let’s analyze to see if it makes any difference. No, it continues with the range scan. Grr!
Let’s drop the index! Then run the select where xnum=11, three times.

SQL_TEXT SQL_ID        OPEN_VERSIONS FETCHES EXECUTIONS PARSE_CALLS IS_BIND_SENSITIVE IS_BIND_AWARE IS_SHAREABLE CHILD_NUMBER PLAN_HASH_VALUE
declare type t_nums is varray(10000) of number; a_nums t_nums; x number; cursor c1 is select xnum from test where xnum=x; begin x:=11; open c1; fetch c1 bulk collect into a_nums; close c1; end; 
         fcrk1v7mrcnkr             1       0          3           3                 N             N            Y            0               0
SELECT XNUM FROM TEST WHERE XNUM=:B1 
         7y76r2vywmrv5             1       3          3           1                 Y             N            Y            0      1873712855
SELECT XNUM FROM TEST WHERE XNUM=:B1 
         7y76r2vywmrv5             1       3          3           1                 Y             N            Y            2       217508114

That’s expected… it picks up the dropped index, change in statistics, and creates child 2. Now that the index is gone, child 1 has magically aged out of the shared pool. Child 2 now has a new PLAN_HASH_VALUE=217508114 which shows TABLE ACCESS FULL. Child 0 which is the parent cursor still remains in the shared pool with PLAN_HASH_VALUE=1873712855 which is the INDEX RANGE SCAN. These can also be seen in OEM under the SQL Details screen where Plan Hash Value can be selected. So has adaptive cursor sharing worked? It’s hard to say, as this behaviour would have happened in 10g as well.

I would still like to create an example which changes the plan based on statistics and data, and will update this post.

Mysteries with current testing:

  • IS_BIND_AWARE is supposed to change to Y with the new plan (is this the definite way to check if adaptive cursor sharing has worked?).
  • v$sql_cs_selectivity shows zero rows for that SQL ID.

Conclusion
So, what have we learned? Literals are generally bad with cursor_sharing=similar unless you have full control of your application and explain plans are better for each statement. Literals are bad for CPU with cursor_sharing=force. It is better to use anonymous blocks or dynamic SQL, but not by much. Shared memory usage is a lot higher and there is more parsing to be done by the block as well. Efficient writing of cursors are the best for any cursor_sharing setting, as the code variables are always treated as bind variables and the statements are soft-parsed at worst.

The main point of this post is to show you how to write better SQL so that it can run as efficiently as possible on Oracle. So whether you are a DBA or a developer, I hope you can take something positive away from this.

Thanks for reading!
Shanil