Archive for May, 2011

As promise in Oracle 9i Architecture post here is new post for how SQL statement processe internaly. This is for them those whi are planning to start career as Oracle DBA. If they want to make clear picture of Oracle Architecture then first of all they need to understand how sql statement procees in oracle.These are the statement which we are going to run.

Sqlplus scott/tiger@prod

SQL>select * from emp;

SQL>update emp set sallary=30000 where empid=10;


So we will understand what is happening internaly

  1. Once we hit sqlplus statement as above client process(user) access sqlnet listener.
  2. Sql net listener confirms that DB is open for buisness & create server process.
  3. Server process allocates PGA.
  4. ‘Connected’ Message returned to user.
  5. SQL>select * from emp;
  6. Server process checks the SGA to see if data is already in buffer cache.
  7. If not then data is retrived from disk and copied into SGA (DB Cache).
  8. Data is returned to user via PGA & server process.
  9. Now another statement is   SQL>Update emp set sallary=30000 where empid=10;
  10. Server process (Via PGA) checks SGA to see if data is already there in buffer cache.
  11. In our situation chances are the data is still in the SGA (DB Cache).
  12. Data updated in DB cache and mark as ‘Dirty Buffer’.
  13. Update employee placed into redo buffer.
  14. Row updated message returned to user
  15. SQL>commit;
  16. Newest SCN obtained from control file.
  17. Data in DB cache is marked as ‘Updated and ready for saving’.
  18. commit palced into redo buffer.
  19. LGWR writes redo buffer contents to redo log files & remove from redo buffer.
  20. Control file is updated with new SCN.
  21. Commit complete message return to user.
  22. Update emp table in datafile & update header of datafile with latest SCN.
  23. SQL>exit;
  24. Unsaved changes are rolled back.
  25. Server process deallocates PGA.
  26. Server process terminates.
  27. After some period of time redo log are archived by ARCH process.    

Hope you guy’s enjoy this post.Very soon I wil come up with Parsing & execution as well.

Experts are always welcome for their valuable comment or suggestion for the above post.

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Checking PGA for each sessions

You can check session level PGA using V$SESSTAT and V$SESSION view and also you can check the username, who is using that memory.

name = ‘session pga memory’
AND s.sid=a.sid
ORDER BY s.value;

———- ———- ——————————
487276 1070 APPS
552812 1068 SYS
552812 1088
618348 1009 APPS_READ_ONLY
683884 1091
749420 846 MOBILEADMIN
749420 1090
749420 1051 APPLSYSPUB
749420 1000 APPLSYSPUB
749420 929 APPLSYSPUB
790412 1093

To check the total PGA in use and hit ratio for PGA


————————————————– ———- ————
aggregate PGA target parameter 4294967296 bytes
aggregate PGA auto target 3674290176 bytes
global memory bound 201252864 bytes
total PGA inuse 218925056 bytes
total PGA allocated 433349632 bytes
maximum PGA allocated 1526665216 bytes
total freeable PGA memory 86835200 bytes
process count 113
max processes count 250
PGA memory freed back to OS 8.3910E+10 bytes
total PGA used for auto workareas 6505472 bytes

————————————————– ———- ————
maximum PGA used for auto workareas 70296576 bytes
total PGA used for manual workareas 0 bytes
maximum PGA used for manual workareas 4292608 bytes
over allocation count 0
bytes processed 2.1553E+11 bytes
extra bytes read/written 10403840 bytes
cache hit percentage 99.99 percent
recompute count (total) 205474

19 rows selected.

The ideal way to perform sorts is by doing the entire job in memory. A sort job that Oracle performs entirely in memory is said to be an optimal sort. If you set the PGA_AGGREGATE_TARGET too low, some of the sort data is written out directly to disk (temporary tablespace) because the sorts are too large to fit in memory. If only part of a sort job spills over to disk, it’s called a 1-pass sort. If the instance performs most of the sort on disk instead of in memory, the response time will be high. This is called multi pass sort.

Another method of checking the efficiency of PGA memory is to check V$SQL_WORKAREA_HISTOGRAM.

V$SQL_WORKAREA_HISTOGRAM displays the cumulative work area execution statistics (cumulated since instance startup) for different work area groups. The work areas are split into 33 groups based on their optimal memory requirements with the requirements increasing in powers of two. That is, work areas whose optimal requirement varies from 0 KB to 1 KB, 1 KB to 2 KB, 2 KB to 4 KB, … and 2 TB to 4 TB.

For each work area group, the V$SQL_WORKAREA_HISTOGRAM view shows how many work areas in that group were able to run in optimal mode, how many were able to run in one-pass mode, and finally how many ran in multi-pass mode. The DBA can take a snapshot at the beginning and the end of a desired time interval to derive the same statistics for that interval.

low_optimal_size/1024 “Low (K)”,
(high_optimal_size + 1)/1024 “High (K)”,
optimal_executions “Optimal”,
onepass_executions “1-Pass”,
multipasses_executions “>1 Pass”
FROM v$sql_workarea_histogram
WHERE total_executions <> 0;

Low (K) High (K) Optimal 1-Pass >1 Pass
———- ———- ———- ———- ———-
2 4 6254487 0 0
64 128 110568 0 0
128 256 20041 0 0
256 512 86399 0 0
512 1024 145082 0 0
1024 2048 31207 0 0
2048 4096 104 0 0
4096 8192 79 2 0
8192 16384 116 0 0
16384 32768 30 0 0
32768 65536 4 0 0

Low (K) High (K) Optimal 1-Pass >1 Pass
———- ———- ———- ———- ———-
65536 131072 2 0 0

12 rows selected.

You can check the proportion of work areas since you started the Oracle instance, using optimal, 1-pass, and multipass PGA memory sizes.

DECODE(total, 0, 0, ROUND(cnt*100/total)) PERCENTAGE
FROM (SELECT name, value cnt, (sum(value) over ()) total
WHERE name like ‘workarea exec%’);

————————————————– ———- ———-
workarea executions – optimal 6650608 100
workarea executions – onepass 2 0
workarea executions – multipass 0 0

Since almost all the sorting and temporary operation are carried out inder optimal catagory we can conclude that out PGA is sized correctly.

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When a SQL is submitted by a user to Oracle database, it never happens that Oracle will execute the SQL continuously at one go. Oracle process never get to work on the execution of statement without any interruptions. Often the process has to pause or wait for some event or some other resource to be released. Thus active Oracle process is doing one of the following thing at any point of time.

The process is executing the SQL statement.

The process is waiting for something (for example, a resource such as a database buffer or a latch). It could be waiting for an action such as a write to the buffer cache to complete.
That’s why the response time—the total time taken by Oracle to finish work—is correctly
defined as follows:

response time = service time + wait time

So only part of the time is spend by oracle process to actually “do” some thing. Rest of the time process just wait for some resource to get freed up. It can be waiting log writter process or database writter process or any other resources.

The wait event may also be due to unavailable buffers or latches.

Four dynamic performance views contain wait information: V$SESSION, V$SYSTEM_EVENT, V$SESSION_EVENT, and V$SESSION_WAIT. These four views list just about all the events the instance was waiting for and the duration of these waits. Understanding these wait events is essential for resolving performance issues.

There are different wait classes defined in database. Each class will contain different wait events. There are around 860 wait events defined in Oracle database 10g and are classified under different wait classes.

Some of the main wait classes includes:

  • Administrative: Waits caused by administrative commands, such as rebuilding an index, for example.
  • Application: Waits due to the application code.
  • Cluster: Waits related to Real Application Cluster management.
  • Commit: Consists of the single wait event log file sync, which is a wait caused by commits in the database.
  • Concurrency: Waits for database resources that are used for locking; for example, latches.
  • Configuration: Waits caused by database or instance configuration problems, including a low shared-pool memory size, for example.
  • Idle: Idle wait events indicate waits that occur when a session isn’t active; for example, the ‘SQL*Net message from client’ wait event.

You can see the complete list of wait classes using V$SESSION_WAIT_CLASS dynamic performance view.

Analyzing Instance Performance

You can check the percentage of time spend by the database in waiting for resource and percentage of time spend by database in actual execution.

             FROM V$SYSMETRIC
            WHERE METRIC_NAME IN (‘Database CPU Time Ratio’,
           ‘Database Wait Time Ratio’) AND
           INTSIZE_CSEC =
           (select max(INTSIZE_CSEC) from V$SYSMETRIC);

METRIC_NAME                                                           VALUE
—————————————————————- ———-
Database Wait Time Ratio                                         15.6260647
Database CPU Time Ratio                                          84.3739353

If the database performance shows that ‘Database Wait Time Ratio’ is greater then ‘Database CPU Time Ratio’ or the value for ‘Database Wait Time Ratio’ is quite significant, then in that case you need to dig inside to get the information about where exactly oracle is waiting. You need to basically find the type of wait. This will give you root cause. Once you get the root cause you can work to fix the same.

you can determine the total waits and percentage of waits by wait class.

round(100 * (TIME_WAITED / SUM_TIME),2) PCT_TIME
(select WAIT_CLASS,
where WAIT_CLASS != ‘Idle’)

——————– ———– ———— ————— ———-
System I/O                180300        19.96          3008.8      49.53
Commit                     67855         7.51         1302.46      21.44
User I/O                  291565        32.28         1056.55      17.39
Application                 3637           .4          596.66       9.82
Other                      15388          1.7            67.4       1.11
Concurrency                 1264          .14           38.12        .63
Network                   343169        37.99            3.86        .06
Configuration                 22            0               1        .02

8 rows selected.

In the above output percentage of time waited (last column) is more important and gives the correct picture of the impact due to wait. Example if we see total number of Network waits are large but the actual percentage of time contributed to the wait is very less (0.06%).

The key dynamic performance tables for finding wait information are the V$SYSTEM_EVENT, V$SESSION_EVENT, V$SESSION_WAIT, and the V$SESSION views. The first two views show the waiting time for different events.


The V$SYSTEM_EVENT view shows the total time waited for all the events for the entire system since the instance started up. The view doesn’t focus on the individual sessions experiencing waits, and therefore it gives you a high-level view of waits in the system. You can use this view to find out that the top instance-wide wait events are. You can calculate the top n waits in the system by dividing the event’s wait time by the total wait time for all events.

where wait_class != ‘Idle’
order by time_waited desc;

—————————— ———– ———– ——————–
log file parallel write             128953      210308 System I/O
log file sync                        67904      130313 Commit
db file sequential read             259065       73686 User I/O
enq: TX – row lock contention          226       59080 Application
control file parallel write          28282       57929 System I/O
db file parallel write               19155       32924 System I/O
db file scattered read               31841       30925 User I/O
os thread startup                       95        3262 Concurrency
rdbms ipc reply                        485        2116 Other
PX Deq: Signal ACK                    1971        1103 Other
local write wait                       245         864 User I/O

we can get the session level waits for each event using V$SESSION_EVENT view. In this view the TIME_WAITED is the wait time per session.


where WAIT_CLASS != ‘Idle’
order by TIME_WAITED;

———- —————————— ———– ———– ————
390 os thread startup                       55      1918   Concurrency
393 db file sequential read              10334      4432   User I/O
396 db file parallel write                8637      14915  System I/O
397 db file parallel write               10535      18035  System I/O
394 control file parallel write          28294      57928  System I/O
395 log file parallel write             129020      210405 System I/O

As we can see from above output that session 395 is having maximum wait time because of system I/O. Here system I/O is the I/O because of background processes like DBWR and LGWR etc.

You can get all the database wait events from V$EVENT_NAME and the related meaning of all the wait events available in oracle 10g by checking this documentation link.


The third dynamic view is the V$SESSION_WAIT view, which shows the current waits or just completed waits for sessions. The information on waits in this view changes continuously based on the types of waits that are occurring in the system. The real-time information in this view provides you with tremendous insight into what’s holding up things in the database right now. The
V$SESSION_WAIT view provides detailed information on the wait event, including details such as file number, latch numbers, and block number. This detailed level of information provided by the V$SESSION_WAIT view enables you to probe into the exact bottleneck that’s slowing down the database. The low-level information helps you zoom in on the root cause of performance problems.

SQL> select sid, event, WAIT_CLASS, WAIT_TIME, SECONDS_IN_WAIT, STATE from v$session_wait
2  where wait_class != ‘Idle’;

       SID EVENT                          WAIT_CLASS            WAIT_TIME
———- —————————— ——————– ———-
————— ——————-
337 SQL*Net message to client      Network                      -1

Here wait time -1 means that session has waited for less then 1/100th of a second.
You can get the complete wait information for a particular session using V$SESSION view. You can get SQLID of the sql, which is causing wait.


For getting the wait statistics you can go with the following methodology.

  • First, look at the V$SYSTEM_EVENT view and rank the top wait events by the total amount of time waited, as well as the average wait time for that event.
  • Next, find out more details about the specific wait event that’s at the top of the list. You can check V$WAITSTAT view for the same. Check the type of wait this view is showing. If the wait is due to undo header or undo block then wait is related to undo segment.
  • Finally, use the V$SESSION view to find out the exact objects that may be the source of a problem. For example, if you have a high amount of db file scattered read-type waits, the V$SESSION view will give you the file number and block number involved in the wait events.

In V$SESSION we have a column called BLOCKING_SESSION_STATUS. IF this column value is ‘valid’, then we can presume that corresponding session is getting blocked.


The V$SESSION_WAIT_HISTORY view holds information about the last ten wait events for each active session. The other wait-related views, such as the V$SESSION and the V$SESSION_WAIT, show you only the wait information for the most recent wait. This may be a short wait, thus escaping your scrutiny.

SQL> select sid from v$session_wait_history
2  where wait_time = (select max(wait_time) from v$session_wait_history);

      SEQ# EVENT                            P1         P2         P3  WAIT_TIME
———- ———————— ———- ———- ———- ———-
1 rdbms ipc message            180000          0          0     175787
2 rdbms ipc message            180000          0          0     175787
3 rdbms ipc message            180000          0          0      60782
4 rdbms ipc message            180000          0          0     175787
5 rdbms ipc message            180000          0          0     138705
6 db file sequential read           1        368          1          0
7 rdbms ipc message            180000          0          0     158646
8 db file sequential read           1        368          1          0
9 db file sequential read           1         73          1          0
10 db file sequential read           1         30          1          0

Note that a zero value under the WAIT_TIME column means that the session is waiting for a specific wait event. A nonzero value represents the time waited for the last event.


The V$SESSION_WAIT view tells you what resource a session is waiting for. The V$SESSION view also provides significant wait information for active sessions. However, neither of these views provides you with historical information about the waits in your instance. Once the wait is over, you can no longer view the wait information using the V$SESSION_WAIT view. The waits are so fleeting that by the time you query the views, the wait in most times is over. The new Active Session History (ASH) feature, by recording session information, enables you to go back in time and review the history of a performance bottleneck in your database. Although the AWR provides hourly snapshots
of the instance by default, you won’t be able to analyze events that occurred five or ten minutes ago, based on AWR data. This is where the ASH information comes in handy. ASH samples the V$SESSION view every second and collects the wait information for all active sessions. An active session is defined as a session that’s on the CPU or waiting for a resource. You can view the ASH session statistics through the view  V$ACTIVE_SESSION_HISTORY, which contains a single row for each active session in your instance. ASH is a rolling buffer in memory, with older information being overwritten by new session data. Every 60 minutes,the MMON background process flushes filtered ASH data to disk, as part of the hourly AWR snapshots. If the ASH buffer is full, the MMNL background process performs the flushing of data. Once the ASH data is flushed to disk, you won’t be able to see it in the V$ACTIVE_SESSION_HISTORY view. You’ll now have to use the DBA_HIST_ACTIVE_SESS_HISTORY view to look at the historical data.

obtaining the objects with highest waits

SELECT a.current_obj#, o.object_name, o.object_type, a.event,
SUM(a.wait_time +
a.time_waited) total_wait_time
FROM v$active_session_history a,
dba_objects o
WHERE a.sample_time between sysdate – 30/2880 and sysdate
AND a.current_obj# = o.object_id
GROUP BY a.current_obj#, o.object_name, o.object_type, a.event
ORDER BY total_wait_time;

 OBJECT_NAME                OBJECT_TYPE         EVENT                  TOTAL_WAIT_TIME
——————–       ——————- ———————– —————-
FND_LOGINS                 TABLE               db file sequential read   47480
KOTTB$                     TABLE               db file sequential read   48077
SCHEDULER$_WINDOW          TABLE               db file sequential read   49205
ENG_CHANGE_ROUTE_STEPS_TL  TABLE               db file sequential read   52534
JDR_PATHS_N1               INDEX               db file sequential read   58888
MTL_ITEM_REVISIONS_B       TABLE               SQL*Net more data to client

select p1text, p1, p2text, p2, p3text, p3, a.event
from v$active_session_history a
WHERE a.sample_time between sysdate – 30/2880 and sysdate
AND a.current_obj# = 1938000

P1TEXT     P1          P2TEXT     P2   P3TEXT          P3  EVENT
——–   —         ——- ——- ———— —— ————————
file#      71          block#    4389  blocks           1  db file sequential read
file#      187         block#   89977  blocks           1  db file sequential read
file#      80          block#   79301  blocks           1  db file sequential read
driver id  675562835   #bytes       1  0
file#      11          block#     831  blocks           1  db file sequential read
driver id  675562835   #bytes       1  0

so we can see few history wait events for a particular object in database. We can get the segment stats for this object. Finally we can got to some conclusing and implementaing the solution to reduce the wait. For example if it is a ‘db file sequential read’ wait then

  • Increase buffer cache size, but this wont help much. For this to do, you need to check cache miss percentages.
  • Check the query and optimize it, so that it can read less number of blocks
  • Increase freelists for that segment

some important wait events

The following query lists the most important wait events in your database in the last 15 minutes:

SELECT a.event,
SUM(a.wait_time +
a.time_waited) total_wait_time
FROM v$active_session_history a
WHERE a.sample_time between
sysdate – 30/2880 and sysdate
GROUP BY a.event
ORDER BY total_wait_time DESC;

EVENT                                                            TOTAL_WAIT_TIME
—————————————————————- —————
enq: TX – row lock contention                                          877360289
TCP Socket (KGAS)                                                       13787430
SQL*Net break/reset to client                                            6675324
db file sequential read                                                  2318850
control file parallel write                                              1790011
log file parallel write                                                  1411201
db file scattered read                                                     62132
os thread startup                                                          39640
null event                                                                     0

Users with the Most Waits

The following query lists the users with the highest wait times within the last 15 minutes:

SELECT s.sid, s.username,
SUM(a.wait_time +
a.time_waited) total_wait_time
FROM v$active_session_history a,
v$session s
WHERE a.sample_time between sysdate – 30/2880 and sysdate
AND a.session_id=s.sid
GROUP BY s.sid, s.username
ORDER BY total_wait_time DESC;

       SID USERNAME                       TOTAL_WAIT_TIME
———- —————————— —————
773 APPS                                 877360543
670 APPS                                 374767126
797                                       98408003
713 APPS                                  97655307
638 APPS                                  53719218
726 APPS                                  39072236
673 APPS                                  29353667
762 APPS                                  29307261
746 APPS                                  29307183
653 APPS                                  14677170
675 APPS                                  14676426

Identifying SQL with the Highest Waits

Using the following query, you can identify the SQL that’s waiting the most in your instance with in last 15 mins

SELECT a.user_id,d.username,s.sql_text,
SUM(a.wait_time + a.time_waited) total_wait_time
FROM v$active_session_history a,
v$sqlarea s,
dba_users d
WHERE a.sample_time between sysdate – 30/2880 and sysdate
AND a.sql_id = s.sql_id
AND a.user_id = d.user_id
GROUP BY a.user_id,s.sql_text, d.username;

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