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Performance Tuning Basics 15 : AWR Report Analysis

The Oracle’s Automatic Workload Repository (AWR) collects, processes, and maintains performance statistics for problem detection and self-tuning purposes.  The report generated by AWR is a big report and it can take years of experience to actually understand all aspects of this report. In this post we will try to explain some important sections of AWR, significance of those sections and also some important tips. Please note that explaining all sections of AWR will not be possible so we will stick to some of the most frequently used sections.

Note that this is not comprehensive information and goal is to help in giving an overview of few key sections to Junior DBAs as a primer and to encourage them to build further the knowledge in related fields.

To start with let us mention some high level important tips regarding AWR:

1. Collect Multiple AWR Reports: It’s beneficial to have two AWR Reports, one for the good time and other when performance is poor or you can create three reports (Before/Meantime/After reports) during the time frame problem was experienced and compare it with the time frame before and after.

2. Stick to Particular Time: You must have a specific time when Database was slow so that you can choose a shorter timeframe to get a more precise report.

3. Split Large AWR Report into Smaller Reports: Instead of having one report for long time like one report for  3 hrs. it is better to have three reports each for one hour. This will help to isolate the problem

4. FOR RAC, take each instance’s individual report: For RAC environment, you need to do it separately of all the instances in the RAC to see if all the instances are balanced the way they should be.

5. Use ASH also : Use AWR to identify the troublesome areas and then  use ASH to confirm those areas.

6. Increase the retention period : Some instances where you get more performance issues you should increase the retention time so that you can have historical data to compare.


TIME UNITS USED IN VARIOUS SECTIONS OF AWR REPORTS

-> s – second
-> cs – centisecond – 100th of a second
-> ms – millisecond – 1000th of a second
-> us – microsecond – 1000000th of a second


Top Header

SIGNIFICANCE OF THIS SECTION:

This contains information about the Database and environment. Along with the snapshot Ids and times. Important thing to notice is that the configuration like CPU and Memory has not changed when the performance is degraded.

 PARAMETER      DESCRIPTION  ANALYSIS
 DB TIME Time spent in database during the Elapsed Time
OR
Sum of the time taken by all sessions in the database during the ‘Elapsed’ time.
DB Time= CPU Time + Non IDLE wait time.

Note: it does not include background processes
 DB TIME > Elapsed Time will mean that the sessions were active on database concurrently

You cam find the average active sessions during AWR Time:

DB TIME/ELAPSED  => 1964.97/899.99 = 2.18

So database load (average active sessions) = 2.18

(SAME IDEA AS CPU LOAD on UNIX)

It means that

May be ~2 users were active on database for ‘Elapsed’ Time.
or
May be 4 users were active for Elapsed Time/2 each
or
May be 8 users were active for Elapsed Time/4 each
or
May be 64 users were active for Elapsed Time/32 each

If DB Time has higher value means DB Activity/Sessions were High during the AWR Time.

AWR REPORT THAT WE WILL REVIEW BELOW IS BASED ON THIS DB TIME.

This means that for every minute of Elapsed time there is 2.2 minutes of work in done in the database

 

 ELAPSED TIME The time duration in which this AWR report has been generated.
Elapsed time should contain the issue duration.
Take manual snapshots if required
 CPUs Thread count per core. It is not “actual” CPU.  
 STARTUP TIME Database startup time  
 RAC   If you have more than one node then take AWR from all nodes if you don’t know issues are happening in which node.

Load Profile

SIGNIFICANCE OF THIS SECTION:
 

Here in load profile (average active sessions, DB CPU, logical and physical reads, user calls, executions, parses, hard parses, logons, rollbacks, transactions) —

check if the numbers are consistent with each other and with general database profile (OLTP/DWH/mixed)

  • Pay most attention to physical reads, physical writes, hard parse to parse ratio and executes to transaction ratio.
  • The ratio of hard parses to parses tells you how often SQL is being fully parsed. Full parsing of SQL statements has a negative effect on performance.
  • High hard parse ratios (>2 – 3 percent) indicate probable bind variable issues or maybe versioning problems.
  • Rows per sort can also be reviewed here to see if large sorts are occurring.
  • This section can help in the load testing for application releases. You can compare this section for the baseline as well as high load situation.
 PARAMETER  DESCRIPTION  ANALYSIS
 Redo Size (Bytes) The main sources of redo are (in roughly descending order): INSERT, UPDATE and DELETE. For INSERTs and UPDATE s Not very scary number in our report

High redo figures mean that either lots of new data is being saved into the database, or existing data is undergoing lots of changes.

What do you do if you find that redo generation is too high (and there is no business reason for that)? Not much really — since there is no “SQL ordered by redo” in the AWR report.

Just keep an eye open for any suspicious DML activity. Any unusual statements? Or usual statements processed more usual than often? Or produce more rows per execution than usual? Also, be sure to take a good look in the segments statistics section (segments by physical writes, segments by DB block changes etc.) to see if there are any clues there.

 DB CPU Its the amount of CPU time spent on user calls. Same as DB time it does not include background process. The value is in microseconds We have 8 CORES and so we can potentially use 8 seconds of CPU time per second. In this case DB CPU (s) : 1.9 (per second) is reporting that the system
is using 1.9 seconds of CPU of the potential 8 seconds/second that it can use.
We are not CPU Bound
 LOGICAL READS  Consistent gets + db block gets = Logical Reads

As a process, Oracle will try to see if the data is available in Buffer cache i.e. SGA?

If it does, then logical read increases to 1.

To explain a bit further, if Oracle gets the data in a block which is consistent with a given point in time, then a counter name “Consistent Gets” increases to 1.

But if the data is found in current mode, that is, the most up-to-date copy of the data in that block, as it is right now or currently then it increases a different counter name “db block Gets”.

Therefore, a Logical read is calculated as = Total number of “Consistent Gets” + Total number of “db block gets”.

These two specific values can be observed in ‘Instance Activity Stats’ section.

 Logical and physical reads combined shows measure of how many IO’s (Physical and logical) that the database is performing..

If this is high go to section “SQL by logical reads”. That may help in pointing which SQL is having more logical reads.

 USER QUERIES      Number of user queries generated  
 PARSES     The total of all parses, hard and soft  
 HARD PARSES The parses requiring a completely new parse of the SQL statement. These consume both latches and shared pool area. How much hard parsing is acceptable?

It depends on too many things, like number of CPUs, number of executions, how sensitive are plans to SQL parameters etc. But as a rule of a thumb, anything below 1 hard parse per second is probably okay, and everything above 100 per second suggests a problem (if the database has a large number of CPUs, say, above 100, those numbers should be scaled up accordingly). It also helps to look at the number of hard parses as % of executions (especially if you’re in the grey zone).

If you suspect that excessive parsing is hurting your database’s performance:

1) check “time model statistics” section (hard parse elapsed time, parse time elapsed etc.)

2) see if there are any signs of library cache contention in the top-5 events

3) see if CPU is an issue.

 Soft Parses: Soft parses are not listed but derived by subtracting the hard parses from parses. A soft parse reuses a previous hard parse; hence it consumes far fewer resources.  
 Physical Reads      But if it Oracle does not find the data in buffer cache, then it reads it from physical block and increases then Physical read count to 1. Clearly, buffer get is less expensive than physical read because database has to work harder (and more) to get the data. Basically time it would have taken if available in buffer cache + time actually taken to find out from physical block.  If this is high go to section “SQL by Physical reads”. That may help in pointing which SQL is having more Physical reads.
 Executes  (SQL)    If executes per second looks enormous then its a red flag.

Example: This numbers, combined with high CPU usage, are enough to suspect there MAY BE context switching as the primary suspect: a SQL statement containing a PL/SQL function, which executes a SQL statement hundreds of thousands of time per function call.

if it is high then it also suggests that most of the database load falls on SQL statements in PL/SQL routines.

User Calls number of calls from a user process into the database – things like “parse”, “fetch”, “execute”, “close”  This is an extremely useful piece of information, because  it sets the scale for other statistics (such as commits, hard parses etc.).

In particular, when the database is executing many times per a user call, this could be an indication of excessive context switching (e.g. a PL/SQL function in a SQL statement called too often because of a bad plan). In such cases looking into “SQL ordered by executions” will be the logical next step.

 Logons     logons – really means what it means. Number of logons Establishing a new database connection is also expensive (and even more expensive in case of audit or triggers). “Logon storms” are known to create very serious performance problems. If you suspect that high number of logons is degrading your performance, check “connection management elapsed time” in “Time model statistics”.
 

Sorts

 Sort operations consume resources. Also, expensive sorts may cause your SQL fail because of running out of TEMP space.  So obviously, the less you sort, the better (and when you do, you should sort in memory). However, I personally rarely find sort statistics particularly useful: normally, if expensive sorts are hurting your SQL’s performance, you’ll notice it elsewhere first.  
 DB Time        average number of active sessions is simply DB time per second.
 Block Changes   Number of blocks modified during the sample interval  

Instance Efficiency Percentage

SIGNIFICANCE OF THIS SECTION:
Rule of thumb: Always minimize the number of Hard parses. This reduction yields the benefits of minimizing CPU overhead spent performing costly parse work.
Every ratio here should reach 100%
 PARAMETER  DESCRIPTION  ANALYSIS
In memory sort % Shows %of times Sorting operations happened in memory than in the disk (temporary tablespace). In Memory Sort being low (in the high 90s or lower) indicates PGA_AGGREGATE_TARGET or SORT_AREA_SIZE issues
soft parse % Shows % of times the SQL in shared pool is used. Shows how often sessions issued a SQL statement that is already in the shared pool and how it can use an existing version of that statement. Soft Parsing being low indicates bind variable and versioning issues. With 99.25 % for the soft parse meaning that about 0.75 % (100 – soft parse) is happening for hard parsing. Low hard parse is good for us.
 % Non-Parse CPU  Oracle utilizes the CPU mostly for statement execution but not for parsing. If this value is near 100% means most of the CPU resources are used into operations other than parsing, which is good for database health.

Most of our statements were already parsed so we weren’t doing a lot of re parsing. Re parsing is high on CPU and should be avoided.

 Execute to Parse % Shows how often parsed SQL statements are reused without re-parsing.  The way this ratio is computed, it will be a number near 100 percent when the application executes a given SQL statement many times over but has parsed it only once.

If the number of parse calls is near the number of execute calls, this ratio trends towards zero.

If the number of executes increase while parse calls remain the same this ratio trends up.

When this number is low, parsing is consuming CPU and shared pool latching.

 Parse CPU to Parse Elapsed % Gives the ratio of CPU time spent to parse SQL statements. If the value are low then it means that there could be a parsing problem. You may need to look at bind variable issues or shared pool sizing issue. If low it also means some bottleneck is there related to parsing. We would start by reviewing library cache contention and contention in shared pool latches. You may need to increase the shared pool.
 Buffer Hit % Measures how many times a required block was found in memory rather than having to execute an expensive read operation on disk to get the block.  
 Buffer Nowait% Indicates % of times data buffers were accessed directly without any wait time. This ratio relates to requests that a server process makes for a specific buffer. This is the percentage of those requests in which the requested buffer is immediately available. All buffer types are included in this statistic. If the ratio is low, check the Buffer Wait Statistics section of the report for more detail on which type of block is being contended. Most likely, additional RAM will be required.
 Library Hit% Shows % of times SQL and PL/SQL found in shared pool.  Library hit % is great when it is near 100%. If this was under 95% we would investigate the size of the shared pool.

In this ration is low then we may need to:
• Increase the SHARED_POOL_SIZE init parameter.
• CURSOR_SHARING may need to be set to FORCE.
• SHARED_POOL_RESERVED_SIZE may be too small.
• Inefficient sharing of SQL, PLSQL or JAVA code.
• Insufficient use of bind variables

Latch Hit % Shows % of time latches are acquired without having to wait.  If Latch Hit % is <99%, you may have a latch problem. Tune latches to reduce cache contention
Redo NOWait% Shows whether the redo log buffer has sufficient size.

Top 10 Foreground Wait Events by Total Wait Time

SIGNIFICANCE OF THIS SECTION:
This section is critical because it shows those database events that might constitute the bottleneck for the system.
Here, first of all check for wait class if wait class is  User I/O , System I/O,  Others etc this could be fine but if wait class has value “Concurrency” then there could be some serious problem. Next to look at is Total Wait Time (sec) which show how many times DB was waiting in this class and then Wait Avg (ms). If Total Wait Time(sec) are high but Wait Avg(ms) is low then you can ignore this. If both are high or Wait Avg(ms) is high then this has to further investigate.

Note that there could be significant waits that are not listed here, so check the Foreground Wait Events (Wait Event Statistics) section for any other time consuming wait events.

For the largest waits look at the Wait Event Histogram to identify the distribution of waits.

 PARAMETER  DESCRIPTION  ANALYSIS
 DB CPU  Time running in CPU (waiting in run-queue not included)
Here 84.8% is the %DB Time for this Event which is really HIGH!

DB Time was 1965 minutes and DB CPU is (100000/60= 1667 minutes)
1667/1965  is 84.8% which is shown in above table.

We can find

1) DB CPU LOAD

=Total wait time /AWR TIME
=100,000/(900*60)
=1.85

2) DB CPU UTLIZATION  %
=DB CPU LOAD/Number of Cores
=
=(1.85/8) X 100
= 23% of Host cores

IMPORTANT: Your server may have other database instances sharing the CPU resources so take into account those too.

Also this do not mean that server CPU is 84% Utilized!

 

 Sum of %DB Time     The sum should be approx 100%. If it is way below 100% then it may mean that wait events were irrelevant OR Server is overloaded.
 enq TX – row lock contention          waited for locked rows This parameter value currently is only 0.2% of total DB time so we don’t have to much worry about it.

Say that it was higher value, 10% then we will have to look into root cause.

You will have to go to “Segments by Row Lock Waits” and see what tables are getting locked and then you will have to see in which SQL_ID these are used.

 

 DB FILE SEQUENTIAL READ          single block i/o

Sequential read is an index read followed by table read because it is doing index lookups which tells exactly which block to go to

 

 Average I/O call is 2ms which is not very high. If you have say very high wait average example 100ms or 200ms, it means that your disks are slow

Are your SQLs returning too many rows, is the I/O response pretty bad on the server, is DB not sized to cache enough result sets

You need to see then the “File IO Stats” section in the AWR report.

The event indicates that index scan is happening while reading data from table. High no. of such event may be a cause of unselective indexes i.e. oracle optimizer is not selecting proper indexes from set of available indexes. This will result in extra IO activity and will contribute to delay in SQL execution. Generally high no. is possible for properly tuned application having high transaction activity.

•If Index Range scans are involved, more blocks than necessary could be being visited if the index is un-selective.By forcing or enabling the use of a more selective index, we can access the same table data by visiting fewer index blocks (and doing fewer physical I/Os).

•If indexes are fragmented, we have to visit more blocks because there is less index data per block. In this case, re-building the index will compact its contents into fewer blocks.

• If the index being used has a large Clustering Factor, then more table data blocks have to be visited in order to get the rows in each index block. By rebuilding the table with its rows sorted by the particular index columns we can reduce the Clustering Factor and hence the number of table data blocks that we have to visit for each index block.

 

 LOG FILE SYNC       Here Wait AVG (MS) is 6 which is not a cry number.
Above 20ms we don’t consider good number
Also go to “Instance Activity Stats” section and see how many commits actually happened and then see here that what % of COMMITS have to wait.Remember that short transactions, frequent commits is property of OLTP Application.
 DB FILE SCATTERED READ caused due to full table scans may be because of insufficient indexes or un-avilablity of updated statistics To avoid this event, identify all the tables on which FTS is happening and create proper indexes so that oracle will do Index scans instead of FTS. The index scan will help in reducing no. of IO operations.

To get an idea about tables on which FTS is happening please refer to “Segment Statistics” -> “Segments By Physical Read” section of AWR report. This section lists down both Tables and Indexes on which Physical Reads are happening. Please note that physical reads doesn’t necessarily means FTS but a possibility of FTS.

 Concurrency, wait class    Concurrency wait class is not good and if high then need to be analyzed.
 direct path read temp or direct path write temp  this wait event shows Temp file activity (sort,hashes,temp tables, bitmap)
check pga parameter or sort area or hash area parameters. You might want to increase them
 
 Wait Class, column  helps in classifying whether the issue is related to application or infrastructure. Wait events are broadly classified in to different WAIT CLASSES:

Administrative
Application
Concurrency
User IO
System IO
Cluster
Commit
Configuration
Idle
Network

 Buffer Busy Wait    Indicates that particular block is being used by more than one processes at the same. When first process is reading the block the other processes goes in a wait as the block is in unshared more. Typical scenario for this event to occur is, when we have batch process which is continuously polling database by executing particular SQL repeatedly and there are more than one parallel instances running for the process. All the instances of the process will try to access same memory blocks as the SQL they are executing is the same. This is one of the situation in which we experience this event.
 enq: TX – row lock contention:  Oracle maintenance data consistency with the help of locking mechanism. When a particular row is being modified by the process, either through Update/ Delete or Insert operation, oracle tries to acquire lock on that row. Only when the process has acquired lock the process can modify the row otherwise the process waits for the lock. This wait situation triggers this event. The lock is released whenever a COMMIT is issued by the process which has acquired lock for the row. Once the lock is released, processes waiting on this event can acquire lock on the row and perform DML operation.  
 enq: UL – contention:  This enq wait occurs when application explicitly locks by executing the lock table command.  
 enq: TM – contention  This usually happens due to a missing foreign key constraint on a table that’s part of a DML operation.  

Host CPU

SIGNIFICANCE OF THIS SECTION:
A high level of DB CPU usage in the Top N Foreground Events (or Instance CPU: %Busy CPU) does not necessarily mean that CPU is a bottleneck. In this example also we have DB CPU as the highest consuming category in the “Top 10 Foreground Events”
Look at the Host CPU and Instance CPU sections. The key things to look for are the values “%Idle” in the “Host CPU” section and “%Total CPU” in the “Instance CPU” section.
If the “%Idle” is low and “%Total CPU” is high then the instance could have a bottleneck in CPU (be CPU constrained).  Otherwise, the high DB CPU usage just means that the database is spending a lot of time in CPU (processing) compared to I/O and other events.  In either case (CPU is a bottleneck or not) there could be individual expensive SQLs with high CPU time, which could indicate suboptimal
execution plans, especially if accompanied with high (buffer) gets.
If you see in our case %idle is high 74% AND %Total CPU is just 7.45 so CPU is not a bottle neck in this example.
 PARAMETER  DESCRIPTION  ANALYSIS
 CPUs  are actually threads.
Here we have 8 Cores and 8 Threads per Core so
CPU = number of core X number of threads per core
= 8 X 8
= 64
 
 Load Average   Compare Load average with Cores.

Very Ideal thing is that Load Average  should be less than Cores although this may not be happening (any it may not be issue also!)

%Idle    Can be misleading as sometimes your %Idle can be 50% but your server is starving for CPU.

50% means that all your cores are BUSY. You may have free threads (CPU) but you can not run two processes CONCURRENTLY on same CORE.

All % in this reports are calculated based on CPU ( which are actually threads)

 

 Cores   Here we have 8 core system
So we have 8 cores, meaning in a 60 min hour we have 60 X 8 = 480 CPU minsand total AWR duration is 8 hoursso 480X8 = 3840 CPU minutes in total

 


Instance CPU

SIGNIFICANCE OF THIS SECTION:

A high level of DB CPU usage in the Top N Foreground Events (or Instance CPU: %Busy CPU) does not necessarily mean that CPU is a bottleneck. In this example also we have DB CPU as the highest consuming category in the “Top 10 Foreground Events”

Look at the Host CPU and Instance CPU sections. The key things to look for are the values “%Idle” in the “Host CPU” section and “%Total CPU” in the “Instance CPU” section.

If the “%Idle” is low and “%Total CPU” is high then the instance could have a bottleneck in CPU (be CPU constrained). Otherwise, the high DB CPU usage just means that the database is spending a lot of time in CPU (processing) compared to I/O and other events. In either case (CPU is a bottleneck or not) there could be individual expensive SQLs with high CPU time, which could indicate suboptimal
execution plans, especially if accompanied with high (buffer) gets.

If you see in our case %idle is high 74% AND %Total CPU is just 7.45 so CPU is not a bottle neck in this example.


Cache Sizes

SIGNIFICANCE OF THIS SECTION:

From Oracle 10g onwards, database server does Automatic Memory Management for PGA and SGA components. Based on load, database server keeps on allocating or deallocating memory assigned to different components of SGA and PGA. Due to this reason, we can observe different sizes for Buffer Cache and Shared Pool, at the beginning or end of AWR snapshot period.


Shared Pool Statistics

SIGNIFICANCE OF THIS SECTION:
 
 PARAMETER  DESCRIPTION  ANALYSIS
 Memory Usage%  shared pool usage
 If your usage is low (<85 percent) then your shared pool is over sized. if Memory Usage % is too large like 90 % it could mean that your shared pool is tool small
     
 % SQL with executions >1   Shows % of SQLs executed more than 1 time. The % should be very near to value 100.  If your reuse is low (<60 – 70 percent) you may have bind variable or versioning issues. Ideally all the percentages in this area of the report should be as high (close to 100) as possible.
 memory for SQL w/exec>1  From the memory space allocated to cursors, shows which % has been used by cursors more than 1.  

Time Model Statistics

SIGNIFICANCE OF THIS SECTION:
Important statistics here is the DB Time. The statistic represents total time spent in database calls. It is calculated by aggregating the CPU time and wait time of all sessions not waiting on idle event (non-idle user sessions). Since this timing is cumulative time for all non-idle sessions, it is possible that the time will exceed the actual wall clock time.
 PARAMETER  DESCRIPTION  ANALYSIS
 SQL EXECUTE ELAPSED TIME  Time spent executing the SQL Statement
Out of all the DB Time which is 117,898.47 seconds , 92.32% of time, 108,843.51 seconds, database is executing the SQL query so our attention will be to find out what all SQLs took so much of DB Time

 

 DB CPU      DB CPU represents time spent on CPU resource by foreground user processes. This time doesn’t include waiting time for CPU.  DB time and DB CPU define two important timescales:

wait times should be measured against the DB TIME,
while CPU consumption during certain activity (e.g. CPU time parsing) should be measured against DB CPU.

Above % showing for DB CPU may not be the only % to focus on. You should find below number and then see what is DB CPU usage.

DB CPU usage (% of CPU power available) =
CPU time / NUM_CPUS / elapsed time

Where NUM_CPUS is found in the Operating System statistics section.
Of course, if there are other major CPU users in the system, the formula must be adjusted accordingly. To check that, look at OS CPU usage statistics either directly in the OS.

 Parse Time Elapsed  “Parse time elapsed” represents time spent for Syntax and Semantic checks.  
 Hard parse elapsed time  “Hard parse include time” represents time spent for Syntax and Semantic checks PLUS time spent for optimizing the SQL and generating optimizer plan.  
 % DB Time  In the time model statistics hierarchy, a child statistic may be counted under more than one parent and that is why the sum of the percentages equal more than 100  
 soft parse  can be get by subtracting parse time from hard parse

Foreground Wait Class

SIGNIFICANCE OF THIS SECTION:
This is of less use. A wait could have multiple possible causes (in different classes) depending on the context. There are normally only a handful of time consuming waits, which can be analyzed and investigated separately.
There are over 800 distinct wait events. Oracle has grouped these wait events in 12 wait classes. These wait classes are further divided in 2 categories, Administrative Wait Class and Application Wait Class.
These wait classes gives overall information about whether the waits happening for Application or for System events.
 PARAMETER  DESCRIPTION  ANALYSIS
 User I/O
High User IO means,

From the pool of available indexes proper indexes are not being used
OR
FTS is happening on big tables with millions of rows


Foreground Wait Events

SIGNIFICANCE OF THIS SECTION:
Mostly The idle events are listed down in the end which should not be focused much.
This is useful because there could be time consuming report wait events that do not appear in the “Top N Timed Foreground Events”.
For the larger waits look at the Wait Event Histogram to identify the distribution of waits. Are they closely clustered around an average value or are there a wide variance of values ? Are there a large number of smaller waits or a few larger waits ?
 PARAMETER  DESCRIPTION  ANALYSIS
SQL*Net Message from client      Idle wait event
We can find the number of average inactive sessions by this wait event

Number of inactive sessions
= Total Wait Time/ (AWR Time * 60)

= 12679570/ (900 * 60)

= 235 average inactive sessions

This doesn’t mean user sessions as such but the number of such connections from Application Server connection pool.

 

 Direct path read/write to temp  Shows excessive sorting/hashing/global temp table/bitmap activity going to your temporary tablespace. Review PGA_AGGREGATE_TARGET settings. Even if it looks like it is big enough, if you aregetting multiple small sorts to disk it could mean your user load is over-utilizing it.
 SQL*Net Message to client  SQL*Net message to client waits almost always indicates network contention.
 SQL*Net more data from client  If it is very low then it indicates that the Oracle Net session data unit size is likely set correctly.
 Db file sequential reads    Usually indicates memory starvation, look at the db cache analysis and for buffer busy waits along with cache latch issues.
 Db file scattered reads    Usually indicates excessive full table scans, look at the AWR segment statistics for tables that are fully scanned
 Log file Sync    Log file related waits: Look at excessive log switches, excessive commits or slow IO subsystems.

 


Wait Event Histogram

SIGNIFICANCE OF THIS SECTION:
This can be used to determine the distribution of wait times. These days less than 5ms is expected and more than 10ms is considered poor. An analysis of the histogram can indicate if a high average time is due to a few individual long waits.
 PARAMETER  DESCRIPTION  ANALYSIS
 DB FILE SEQUENTIAL READ
This parameter will have mostly higher number of wait events in the histogram.

Now if you see approx 50% wait events  have less than 1 ms of wait and another 30% has less than 2 ms. It means that our disks are working good. Wait is low for most of the sessions going to database.

We simply don’t want that high% (and high wait events numbers) are above 8ms of wait.

Now if you see that the DB FILE SEQUENTIAL READ is the key wait event then next thing will be to find

a) which segment is the bottleneck (go to “Segments by Physical Reads” section
b) which sql query has that segment used.

 

 


SQL ordered by Elapsed Time

SIGNIFICANCE OF THIS SECTION:
This can be used to identify the long running SQLs that could be responsible for a performance issue. It can give useful information about the CPU time, the number of executions
and the (SQL) Module. The Top SQLs can be matched to long running or slow Processes in the application.

In this report, look for query has low executions and high Elapsed time per Exec (s) and this query could be a candidate for troubleshooting or optimizations. In above report, you can see first query has maximum Elapsed time but only 2 execution. So you have to investigate this.

NOTE 1: The Elapsed time can indicate if a SQL is multithreaded (either Parallel DML/SQL or multiple workers). In this case the elapsed time will be multiple times the AWR duration (or the
observed clock time of the process/SQL). The elapsed time for multithreaded SQL will be the total of elapsed time for all workers or parallel slaves.
NOTE 2:  The “SQL Ordered” sections can often contain the PL/SQL call that contains SQLs. So in this case the procedure WF_ENGINE (via procedures) ultimately calls the SQL b6mcn03jvfg41. Also if you see the first line here that is also a package BEGIN XXINV7566…. and inside this package it is running the SQL query running in the line 2 which is insert into XXINV7566_IQR…..
 PARAMETER  DESCRIPTION  ANALYSIS
 Elapse per Exec (s)  Elapse time in seconds for per execution of the SQL.  
 Captured SQL Account for 79.1% of
total DB Time
 Shows that how many % of SQL this AWR report was able to capture and show us  Remember that AWR reports shows those SQL which were in shared pool at the end of the AWR Time.

This number should be high value which will mean that we were able to capture all those SQLs which consumed the DB Time.

If this is low number than try to generate AWR for lower snap duration so that we are able to capture the required SQLs which are consuming DB Time

 

 Executions Total no. of executions for the SQL during the two snapshot period.
An important point, if executions is 0 also sometimes, it doesn’t means query is not executing, this might be the case when query was still executing and you took AWR report. That’s why query completion was not covered in Report.
 
 SQL Module   Provides module detail which is executing the SQL. Process name at the OS level is displayed as SQL Module name.

If the module name starts with any of the names given below, then don’t consider these SQLs for tuning purpose as these SQLs are oracle internal SQLs,

DBMS,
sqlplusw,
TOAD,
rman,
SQL,
Enterprise Manager,
ORACLE,
MMON_SLAVE,
emagent etc…

In the list XXIN1768 has two SQLIDs. The SQL id #1 is PL/SQL code as a wrapper and it took around 53k seconds. The sql #2 took 51k seconds and seems to be called in sql ID# 1, as their module names are same. Since the SQL#2 insert statement took almost all of the time so we sill focus on this query for tuning.

 Elasped Time  The Elapsed Time is the sum of all individual execution time for the sql_id. So if multiple sessions execute the same SQLs, the elapsed time can be greater than the period of two snap_ids.

 


SQL ordered by CPU Time

SIGNIFICANCE OF THIS SECTION:
The most useful sections are SQL ordered by Elapsed Time, CPU Time, Gets and Reads. All the sections can be useful in identifying if a particular SQL from a particular module was
running during the AWR report period
However, In most cases this section does not reveal much more information than the “SQL Ordered by Elapsed Time” section. However, it does sort by CPU and can output SQLs that are not
in the previous section.
 PARAMETER  DESCRIPTION  ANALYSIS
 The top record in this table
The first and second report are part of same transaction.
Second SQL is the inside part of first PLSQL.
It is accounting huge % of the DB CPU and remember that DB CPU was the top event in our AWR.

SQL ordered by Gets

SIGNIFICANCE OF THIS SECTION:
The most useful sections are SQL ordered by Elapsed Time, CPU Time, Gets and Reads. All the sections can be useful in identifying if a particular SQL from a particular module was
running during the AWR report period
This  is the logical Reads from Buffer Cache
When “CPU Other” is a significant component of total Response Time, then it is likely that the time is being spent retrieving and manipulating Blocks and Buffers (Block accesses are also known as Buffer Gets and Logical I/Os). Then as a next step we will come to this section to find the SQL statements that access the most blocks because these are likely to be responsible for the majority of this time.
This section will tell you HOW MANY BLOCKS WERE READ and “SQL Ordered by Executions” section will tell you how many rows were fetched. Many a time this and that section will have same SQL_ID.
Now if more BLOCKS are read here and that section tells that number of rows fetched are real low means something is not right with the query. Why it has to read so many blocks to get less rows.May be bad execution plan.
A high number of buffer gets is one of the main indicators of SQLs with suboptimal execution plans Example if you see in this example, insert statement in row number 4 (which is related to PLSQL in row number 3) is doing very high number of buffer gets for single execution..
BUT Bear in mind that the SQL could have a good execution plan and just be doing a lot of work. So we need to bring into account the data volume (and parameters that are getting passed to the query).
You can easily see execution plans by running @awrsqrpt.sql and passing offending SQL_ID as parameter.
You can refer to the “SQL Ordered by Executions” after reading this section.
 PARAMETER  DESCRIPTION  ANALYSIS
  Gets per Exec  HOW MANY BLOCKS WERE READ  insert statement in XXINV1738 module has Gets per Exec that is too high, you need to analyze the SQL with some additional output such as sqlt and sqlhc.

 


SQL ordered by Reads

SIGNIFICANCE OF THIS SECTION:
This section reports the contents of the SQL area ordered by the number of reads from the data files and can be used to identify SQL causing IO bottlenecks which consume the following resources.
• CPU time needed to fetch unnecessary data.
• File IO resources to fetch unnecessary data.
• Buffer resources to hold unnecessary data.
• Additional CPU time to process the query once the data is retrieved into the buffer.
• % Total can be used to evaluate the impact of each statement.
If we talk about wait time then “Wait Time” is the sum of time waited for non-idle Wait Events. These include I/O waits for reading blocks from disk as measured by the Wait Events ‘db file sequential read’ for single-block reads and ‘db file scattered read’ for multi-block reads.
When such Wait Events are found to be significant components of Response Time, the next step is to find the SQL statements that read the most blocks from disk. We will refer to this section then.
This  is the Physical Reads from Disk
If the physical I/O waits (e.g. db file sequential read, db file scattered read, direct path read) are relatively high then this section can indicate which SQLs are responsible.
This section can help identify the SQLs that are responsible for high physical I/O and may indicate suboptimal execution plans, particularly if the execution plan contains full table scans or large index range scans (where more selective index scans are preferable).
 PARAMETER  DESCRIPTION  ANALYSIS
 Captured SQL account for  76% of total
 Our goal is to have this % as high as possible. Probably breaking down this AWR into smaller interval will increase this %.

 

 The top record in this table   We have seen in the “Segments by Physical Reads” section that
MTL_ITEM_CATEGORIES account for 51.32% and looking here the SQL_ID which is a top and having 40.18% TOTAL is using this table.Yousee here that although it has executed 73 times but reads per executions is high making it top query consuming physical i/o.In contrast query at number 4 in this table has been executed around 40k times but since reads per execution is low so number 4th query is not the top query to worry about.
 Reads per Exec    Possible reasons for high Reads per Exec are use of unselective indexes require large numbers of blocks to be fetched where such blocks are not cached well in the buffer cache, index fragmentation, large Clustering Factor in index etc.

SQL ordered by Physical Reads (UnOptimized)

SIGNIFICANCE OF THIS SECTION:
This section is of concern when you have exadata machine in use
Read requests that are satisfied from the Smart Flash Cache in Exadata are termed ‘optimized’ since they are returned much faster than requests from disk (the implemention uses solid state device (SSD) technology). Additionally, read requests accessing Storage Indexes using smart scans in Oracle Exadata V2 (and significantly reducing  I/O operations) also fall under the category ‘optimized read requests’ since they avoid reading blocks that do not contain relevant data.
IMPORTANT: In database systems where ‘Optimized Read Requests’ are not present (which are not using EXA) , UnOptimized Read Reqs will be equal to Physical Read Reqs (I/O requests satisfied from disk).  In this case columns ‘UnOptimized Read Reqs’ and ‘Physical Read Reqs’ will display the same values and column ‘%Opt’ will display zero (as seen in extract from AWR report above).
 
 PARAMETER  DESCRIPTION  ANALYSIS
 Physica Read Reqs  Note that the ‘Physical Read Reqs’ column in the ‘SQL ordered by Physical Reads (UnOptimized)’ section is the number of I/O requests and not the number of blocks returned. Be careful not to confuse these with the Physical Reads statistics from the AWR section ‘SQL ordered by Reads’, which counts database blocks read from the disk not actual I/Os (a single I/O operation  may return many blocks from disk).  

 


SQL ordered by Parse Calls

SIGNIFICANCE OF THIS SECTION:
This section shows the number of times a statement was parsed as compared to the number of times it was executed. One to one parse/executions may indicate that:
• Bind variables are not being used.
• On RDBMS version 8172 and higher the init.ora parameter session_cached_cursors was not set in the init.ora (100 is usually the suggested starting value).
• The shared pool may be too small and the parse is not being retained long enough for multiple executions.
• init.ora cursor_sharing should be set to FORCE
When “CPU Parse” is a significant component of total Response Time the next step is to find the SQL statements that have the most parses.
AWR and Statspack list such SQL statements in sections such as “SQL ordered by Parse Calls”.

Tablespace IO Stats

SIGNIFICANCE OF THIS SECTION:
These are useful to see what your hot tablespaces are. For example, having the SYSTEM tablespace as the number one source of IO could indicate you have improper temporary tablespace assignments as these used to default to SYSTEM. Having the TEMP or UNDO tablespaces in the top position has already been discussed. Usually in an OLTP system one of your index tablespaces should be at the top. In a DWH or OLAP a data tablespace should be at the top. Also look at the latency values. For disk based systems 5.0 ms is considered good performance.
 PARAMETER  DESCRIPTION  ANALYSIS
 Av Rd(ms)
 Av Rd(ms) on the tablespace IO stats should be controlled under 10, which is ideal. But Avg read (ms) of up to 20 is acceptable for IO performance.

NOTE: When the figure in Reads column is too low, you can ignore the Av Rd(ms).

 

  Av Buf Wt(ms   Av Buf Wt(ms) on the tablespace IO stats should be controlled under 10, which is ideal. 

 


Buffer Pool Advisory

SIGNIFICANCE OF THIS SECTION:
The buffer pool advisory report answers the question, how big should you make your database buffer cache. It provides an extrapolation of the benefit or detriment that would result if you added or removed memory from the database buffer cache. These estimates are based on the current size of the buffer cache and the number of logical and physical IO’s encountered during the reporting point. This report can be very helpful in “rightsizing” your buffer cache.
See what the effect of doubling the cache size would be. If it is significant (>20 percent reduction in physical IO) you can suggest increasing the pool.
Example here if you see when size factor goes from 1 to 1.96 , the estimated physical read factor is going down from 1 to 0.26 which is significant drop and is good for database.
The two  ‘Size Factor’  and  ‘Estimated Phys Read Factor’  columns show how quickly or slowly number of  ‘Estimated Physical Reads’ goes down or up if the size of the buffer cache would go up or down. They greatly simplify evaluation of the buffer cache situation, as you do not need  to compare big figures  in columns ‘Size for Estimate’  and especially ‘Estimated Physical Reads’.
We will also generate the AWR at good time for comparison purpose. Our intention is to not add more buffer pool just because one bad query sucked it up ( in which case we will focus on query and not the buffer pool).
When you see huge Physical I/O happening in other section, you may be tempted to increase the database buffer cache in order to lessen the amount of physical I/O. That however does not take into account the overhead of serving the bigger buffer cache (at Oracle level, longer search for a required buffer in a much bigger space, in particular),  which may outweigh benefits of  further reduction in the amount of physical reads, especially if such a reduction is small compared with the portion of the increased buffer cache.
 PARAMETER  DESCRIPTION  ANALYSIS
 First Parameter “P”  Apart from default buffer cache – pool (or subpool) which is always present, buffer cache may have other subpools.

Buffer Cache Advisory section  will then have separate subsection for each of those subpools distinguished from others by a letter in the very left column  of the section as follows:
‘D’ – Default buffer cache (always present),
‘K’ – Keep buffer cache (if db_keep_cache_size parameter is defined),
‘R’ – Recycle buffer cache (if db_recycle_cache_size parameter is defined),
<N> – Caches for non-default block sizes (if defined with parameters db_<N>k_cache_size)

 
 Size Factor  Changing ‘Size Factor’ shows ratio of the proposed size of the buffer cache (increased or decreased)  to the approximate actual size currently in use found in the row with ‘Size Factor’ = 1.0.  
 Estimated Phys Read Factor  Changing ‘Estimated Phys Read Factor’ shows ratio of the estimated number of Physical Reads for the proposed (increased or decreased) size of the buffer cache to the number of Physical Reads calculated for the current size of buffer cache found in the row with ‘Estimated Phys Read Factor’ = 1.0.  HERE WE CAN SEE THAT IF WE INCREASE OUR BUFFER CACHE FROM 6 GB TO 10 GB IT IS HELPING US SIGNIFICANTLY

THIS FACTOR WILL COME DOWN FROM 1 TO 0.3.

 


PGA Memory Advisory

SIGNIFICANCE OF THIS SECTION:
Similar to Buffer Pool Advisory, the statistic provides information on how the increase or decrease in PGA memory will cause increase or decrease in Estd PGA Cahce Hit %.
Starting point here is “Size Factor” = 1.0. This gives current memory allocation for PGA. In this example 12 GB is being allocated to PGA. With this allocation the Estd PGA Cahce Hit % is 100, which is good. Hence even if we increase PGA to any value Estd PGA Cahce Hit % won’t change. Hence it won’t be advisable to increase PGA further. However decreasing can save memory without hitting performance
In this section, you first need to find the row with the ‘Size Factr’ column value of 1.0. This column indicates the size factor of the PGA estimates; a value of 1 indicates the current PGA size. The ‘PGA Target Est(MB)’ value of this row will show your current PGA size: 12 GB in this example. Other columns you will be interested in are ‘Estd Extra W/A MB Read/Written to Disk ‘ and ‘Estd PGA Overalloc Count’.
When you go down or up the advisory section from the row with ‘Size Factr’ = 1.0, you get estimates for Disk usage – column ‘Estd Extra W/A MB Read/Written to Disk ‘ – for bigger or smaller settings of PGA_AGGREGATE_TARGET. The less Disk usage figure in this column, usually the better. A lower value means less work areas have to be spilled to disk, enhancing performance of the Oracle instance.
The question on whether to increase or decrease the PGA_AGGREGATE_TARGET from the current value should be always investigated. The answer depends on how much of total memory (SGA+PGA) can be allocated for this database instance on the machine, taking into account memory needs of other database instances on the same machine, non-Oracle software and the OS itself. Too much memory allocated wastes memory, and too less memory allocated causes possible performance issues in the Oracle environment.
 PARAMETER  DESCRIPTION  ANALYSIS
Estd PGA Overalloc Count Shows how many times the database instance processes would need to request more PGA memory at the OS level than the amount shown in the ‘PGA Target Est (MB)’ value of the respective row. Ideally this field should be 0 (indicating that the PGA is correctly sized, and no overallocations should take place), and that is your equally important second goal. In the given example this goal is achieved with PGA_AGGREGATE_TARGET of even 1,536MB.

So our PGA allocation is way too high.

 

 


Shared Pool Advisory

SIGNIFICANCE OF THIS SECTION:
SHARED POOL IS QUALITATIVE KIND OF POOL WHILE OTHER POOLS ARE QUANTITATIVE POOL. This means that it greatly depends “what” you are keeping in shared pool and not just “how much” you are keeping.
Similar to Buffer Pool Advisory and PGA, the statistic provides information on how the increase or decrease in Shared pool memory will cause increase or decrease in Estd LC Load Time (s).
Starting point here is “SP Size Factor” = 1.0. This gives current memory allocation for shared pool.
In this example approx 2 GB  is being allocated to shared pool. With this allocation the Estd LC Load Time (s) is 58,972. If we increase the shared pool size to 4 GB then Estd LC Load Time (s) will come down to value 39301. Which is not a huge benefit but still the shared pool can be increased if possible.
Also you can analyze by the “Est LC Time Saved Factr”  column . if “Est LC Time Saved Factr”  increases as the “Size Factor” increases then increasing the shared pool will improve performance.
 PARAMETER  DESCRIPTION  ANALYSIS
 EST LC TIME SAVED  LC means LIBRARY CACHE
 you have to see that if increase your shared pool then what is the amount of this time that you can save

 


SGA Target Advisory

SIGNIFICANCE OF THIS SECTION:

The SGA target advisory report is somewhat of a summation of all the advisory reports previously presented in the AWR report. It helps you determine the impact of changing the settings of the SGA target size in terms of overall database performance. The report uses a value called DB Time as a measure of the increase or decrease in performance relative to the memory change made. Also the report will summarize an estimate of physical reads associated with the listed setting for the SGA.

Starting at a “Size Factor” of 1 (this indicates the current size of the SGA). If the “Est DB Time (s)” decreases significantly as the “Size Factor” increases then increasing the SGA will significantly reduce the physical reads and improve performance. but here in our example the Est DB Time is not reducing as much with increase in SGA so increasing SGA in our case will not be beneficial.

When the SQL requires a large volume of data access, increasing the SGA_TARGET size can reduce the amount of disk I/O and improve the SQL performance.


Buffer Wait Statistics

SIGNIFICANCE OF THIS SECTION:

The buffer wait statistics report helps you drill down on specific buffer wait events, and where the waits are occurring

We focus on Total wait time(s) and in this example this value is only 702 seconds

Avg time(ms) is also only 1 ms


Enqueue Activities

The Enqueue activity report provides information on enqueues (higher level Oracle locking) that occur. As with other reports, if you see high levels of wait times in these reports, you might dig further into the nature of the enqueue and determine the cause of the delays.

This can give some more information for enqueue waits (e.g. Requests, Successful gets, Failed gets), which can give an indication of the percentage of times that an enqueue has to wait and the number of failed gets.

In our example the top row do have failed gets but the number of waits is only 55 and wait time (s) is also not high number. So Enqueue is not our major issue in this AWR.


Undo Segment Summary

SIGNIFICANCE OF THIS SECTION:
 PARAMETER  DESCRIPTION  ANALYSIS
 Min/MAX TR (mins) Represents Minimum and Maximum Tuned Retention Minutes for Undo data. This data will help to set the UNDO_RETENTION database parameter.  In this example this parameter can be set to 868.4 min
     
 Max Qry Len(s) Represents Maximum query length in seconds.   In this example the max query length is 51,263 seconds.
 STO/ OOS Represents count for Sanpshot Too Old and Out Of Space errors, occurred during the snapshot period.  In this example, we can see 0 errors occurred during this period.

Latch Activity

SIGNIFICANCE OF THIS SECTION:
The latch activity report provides information on Oracle’s low level locking mechanism called a latch. From this report you can determine if Oracle is suffering from latching problems, and if so, which latches are causing the greatest amount of contention on the system.
  •  There are a plethora of latch statistics
  •  Misses, unless they cause significant amount of sleeps aren’t of concern
  •  Sleeps can be a problem
  •  May need to look at spin count if you have excessive sleeps
  •  Spin count (undocumented (_SPIN_COUNT) was based on CPU speed and 2000 setting was several years ago
  •  If latch waits or other latch related events aren’t showing up, then latches probably aren’t an issue
  •  Usually cache buffer and shared pool related latches are the major latches.
 PARAMETER  DESCRIPTION  ANALYSIS
 WAIT TIME (S)  

 should be 0
(Pct Get Miss     should be 0 or near 0 

Segments by Logical Reads

SIGNIFICANCE OF THIS SECTION:

The statistic displays segment details based on logical reads happened. Data displayed is sorted on “Logical Reads” column in descending order. It provides information about segments for which more logical reads are happening. Most of these SQLs can be found under section SQL Statistics -> SQL ordered by Gets.

These reports can help you find objects that are “hot” objects in the database. You may want to review the objects and determine why they are hot, and if there are any tuning opportunities available on those objects (e.g. partitioning), or on SQL accessing those objects.

When the segments are suffering from high logical I/O, those segments are listed here. When the table has high logical reads and its index has relatively small logical reads, there is a high possibility some SQL is using the index inefficiently, which is making a throw-away issue in the table. Find out the columns of the condition evaluated in the table side and move them into the index. When the index has high logical reads, the index is used excessively with wide range. You need to reduce the range with an additional filtering condition whose columns are in the same index.

If a SQL is suboptimal then this can indicate the tables and indexes where the workload or throwaway occurs and where the performance issue lies. It can be particularly useful if there are no actual statistics elsewhere (e.g. Row Source Operation Counts (STAT lines) in the SQL Trace or no actuals in the SQLT/Display Cursor report).


Segments by Physical Reads

SIGNIFICANCE OF THIS SECTION:
 
If there are a high number of physical read waits (db file scattered read, db file sequential read and direct path read) then this section can indicate on which segments (tables or
indexes) the issue occurs.
This can help identify suboptimal execution plan lines. It can also help identify changes to tablespace and storage management that will improve performance.
 
When the SQLs need excessive physical reads on the particular segments, this section lists them. You need to check if some of SQLs are using unnecessary full scan and wide range scan.
 
The statistic displays segment details based on physical reads happened. Data displayed is sorted on “Physical Reads” column in descending order. It provides information about segments for which more physical reads are happening.
Queries using these segments should be analysed to check whether any FTS is happening on these segments. In case FTS is happening then proper indexes should be created to eliminate FTS. Most of these SQLs can be found under section SQL Statistics -> SQL ordered by Reads.
These reports can help you find objects that are “hot” objects in the database. You may want to review the objects and determine why they are hot, and if there are any tuning opportunities available on those objects (e.g. partitioning), or on SQL accessing those objects.
For example, if an object is showing up on the physical reads report, it may be that an index is needed on that object.
 PARAMETER  DESCRIPTION  ANALYSIS
 Captured Segments account for 90.8% 
This % number is important.
It should be high value which shows that we are looking at correct data.
 The top segment record  MTL_ITEM_CATEGORIES MTL_ITEM_CATEGORIES account for 51.32% of the total physical reads which is a big number. we need to see which SQL statement is using this segment and probably tune that SQL.

You will have to go to “SQL Ordered by Reads” section of AWR to see which SQL Statement is using this segment.

 


Segments by Row Lock Waits

SIGNIFICANCE OF THIS SECTION:
If there is a high level of “enq: TX row lock contention” waits then this section can identify the segments (tables/indexes) on which they occur.
The statistic displays segment details based on total “Row lock waits” which happened during snapshot period. Data displayed is sorted on “Row Lock Waits” column in descending order. It provides information about segments for which more database locking is happening.
DML statements using these segments should be analysed further to check the possibility of reducing concurrency due to row locking.
When the segments are suffering from Row Lock, those segments are listed in this section. The general solution is to provide more selective condition for the SQL to lock only rows that are restricted. Or, after DML execution, commit or rollback as soon as possible. Or so on.

Segments by ITL Waits

SIGNIFICANCE OF THIS SECTION:

If there is a high level of “enq: TX allocate ITL entry” waits then this section can identify the segments (tables/indexes) on which they occur.

Whenver a transaction modifies segment block, it first add transaction id in the Internal Transaction List table of the block. Size of this table is a block level configurable parameter. Based on the value of this parameter those many ITL slots are created in each block.
ITL wait happens in case total trasactions trying to update same block at the same time are greater than the ITL parameter value.

Total waits happening in the example are very less, 34 is the Max one. Hence it is not recommended to increase the ITL parameter value.

Usually when the segments are suffering from Row Lock, those segments are listed in this section. The general solution is to provide more selective condition for the SQL to lock only rows that are restricted. Or, after DML execution, commit or rollback as soon as possible. Or so on.


Segments by Buffer Busy Waits

SIGNIFICANCE OF THIS SECTION:

If there is a high level of “Buffer Busy Waits” waits then this section can identify the segments (tables/indexes) on which they occur.

The section lists segments that are suffering from buffer busy waits. Based on the reason code or class#, the treatment of each is different. The physical segment’s attributes such as freelist, freelist groups, pctfree, pctused and so on are handled by rebuilding the object. But before this treatment, you need to check if your SQLs can visit different blocks at the same time if possible to avoid the contention.

Buffer busy waits happen when more than one transaction tries to access same block at the same time. In this scenario, the first transaction which acquires lock on the block will able to proceed further whereas other transaction waits for the first transaction to finish.
If there are more than one instances of a process continuously polling database by executing same SQL (to check if there are any records available for processing), same block is read concurrently by all the instances of a process and this result in Buffer Busy wait event.


 

This is one of the post in Performance Tuning Fundamentals Series. Click on below links to read more posts from the series:

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Brijesh Gogia

I’m an experienced Oracle Applications DBA with more than a decade of full-time DBA experience. I have gained a wide knowledge of the Oracle software stack and have worked on several big projects for multi-national companies. I enjoy working with the leading-edge technology and have passion for database performance and stability. Thankfully my work allows me time for researching new technologies (and to write about them).
  
You can connect with me on LinkedIn.

One Comment

  1. Bharat Kaushik Bharat Kaushik

    Excellent stuff Brijesh.. 🙂 Really enjoyed reading it…

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