Restore table statistics using dbms_stats

Hi all, another quick as reference. Table statistics as everyone knows are  very important when the CBO is creating the execution plan. When you have new statistics Oracle will invalid the current execution plans affected by them and create new execution plans based on the new statistics. Most of the time it gets right  if that causes your execution plan to change for worse?

In this situations dba_tab_stats_history   view and dbms_stats.restore_table_stats procedures are your friends.

To validate set and check and set the stats history:

select dbms_stats.get_stats_history_retention from dual; 
exec dbms_stats.alter_stats_history_retention(30);

On dba_tab_stats_history you can view the last statistics available:

select table_name, stats_update_time, from dba_tab_stats_history where table_name='TABLE1' and owner='USER' order by 2 desc;

Use the dbms_stats.restore_table_stats to restore the statistics:

exec dbms_stats.restore_table_stats(ownname=>'USER',tabname=>'TABLE1',AS_OF_TIMESTAMP=>'DATE from column STATS_UPDATE_TIME on dba_tab_stats_history');

Hope it helps!

Elisson Almeida

Oracle: Explicitly Lock a Table

Hello all,
So I client had a process that kept need big chunks locks on a table. Problem is that due application use this lock occasionally failed (lock timeout) and the whole process fail. So how to guarantee this?

Using explicit locks for the table before starting the procedure, with command below:


This lock is released as soon as the session commit or rollback transaction in the current session. Killing session means an implicit rollback, as usual.

The SHARE permits concurrent queries but prohibits updates to the locked table.

Nice right?
Hope it helps you too!

OEM Report: Last 6 month Database Space Usage and Growth

Hello All!
So I had worked in some very useful reports to have in OEM. In next weeks I’ll share some code you may like… 🙂

To create it? Enterprise -> Reports -> Information Reports. There are several nice default reports there you me like.

Now, let’s go to the first Report, as per title:

1. First you Select the Database:


2. Then you see the report:


That’s nice, right?
Here is a report for another database with actual 640GB average growth per month (its expected, once DBSize is over 60TB):


So, Mat, can you share the queries? Of course:

More“OEM Report: Last 6 month Database Space Usage and Growth”

Oracle Trigger Follows Clause: Simultaneous Ordered Triggers

Hi All!
I was reviewing some features in Oracle and, basically, every single time I review them I find something new. Seems Oracle Databases’ features are near to infinite and we frequently find some that can really add value to our solutions.

So here is something you may not be aware and is really cool:

Follows Clause

Oracle allows more than one trigger to be created for the same timing point, but up to version 11g is not possible to establish the execution order of execution. The Oracle 11g trigger syntax now includes the FOLLOWS clause to guarantee execution order for triggers defined with the same timing point.

The example below establish that second_trigger is executed after first_trigger before inserting each row in table_example.

BEFORE INSERT ON table_example
FOLLOWS first_trigger

Nice, han?

Regular Expressions with SUBEXPR

Hi All!

Did you know since 11g the REGEXP_INSTR and REGEXP_SUBSTR functions include a new SUBEXPR parameter that limits the pattern match to a specific subexpression in the search pattern?

Also, a new function, REGEXP_COUNT, returns the number of times the search pattern appears in source string.

Below a simple example of each one:

SQL> SELECT REGEXP_INSTR('1234567890', '(123)(4(56)(78))', 1, 1, 0, 'i', 3) FROM dual;


SQL> SELECT REGEXP_SUBSTR('1234567890', '(123)(4(56)(78))', 1, 1, 'i', 3) FROM dual;


SQL> SELECT REGEXP_COUNT('123 123 123 123', '123', 1, 'i') FROM dual;



Oracle: Create Disabled Triggers

I believe you may know about this already, but in case its something new to you: Yes, this is possible since 11g!

Prior to 11g, it was only possible to create triggers in the enabled state, and after creating to disable them. Since 11g they can be explicitly enabled or disabled at creation time using clause ENABLE or DISABLE. As per example:

BEFORE INSERT ON table_example

If didn’t know it yet. Np! You know it now! 😀


Oracle SQL Join Factorization Transformation (JFT)

Hi All!
Following the line of last week post, let’s talk today about the JFT…

Oracle SQL Join Factorization Transformation (JFT)

The UNION operator is great for merging the results of multiple queries that return similar rowsets, essentially executing each query separately and merging the results together into a single result set. Oracle 11g release 2 come with improvements for SQL optimizer on how Oracle handles UNION ALL performance with the new join factorization transformation (JFT). The Join Factorization Transformation applies only to UNION ALL queries.

The following example show how the optimizer improves the performance of UNION ALL by dynamically re-writing a UNION ALL query into a more efficient form using an in-line view:

Original Query:

select t1.c1, t2.c2     
    from t1, t2, t3     
    where t1.c1 = t2.c1 and t1.c1 > 1 and t2.c2 = 2 and t2.c2 = t3.c2    
union all     
    select t1.c1, t2.c2     
    from t1, t2, t4     
    where t1.c1 = t2.c1 and t1.c1 > 1 and t2.c3 = t4.c3;

Re-Written Query:

select t1.c1, VW_JF_1.item_2
     from t1, (select t2.c1 item_1, t2.c2 item_2
                    from t2, t3
                    where t2.c2 = t3.c2 and t2.c2 = 2
               union all
                    select t2.c1 item_1, t2.c2 item_2
                    from t2, t4
                    where t2.c3 = t4.c3) VW_JF_1
     where t1.c1 = VW_JF_1.item_1 and t1.c1 > 1;

Nice, right?

Oracle: “Invisible” Locks!

Have you ever heard:
“My session was killed. I’m trying again. There is no locks. But My SQL doesn’t run!”

By default, when a session is disconnected from Oracle, the SQLs uncommitted are undone (rollback). So, in case of a long routing there is a probably a rollback in place.

How Oracle rollback take that long? Well, it’s Oracle mechanism. The RDBMS basically assume you know what your are doing and start writing the new blocks so the commit will be very quick. Which, however, is bad in case of rollbacks.

How to check if this is your case?

select ktuxeusn, to_char(sysdate,'DD-MON-YYYY HH24:MI:SS') "Time", ktuxesiz, ktuxesta
from x$ktuxe
where ktuxecfl = 'DEAD'
4 /

---------- -------------------- ---------- ----------------
31 07-APR-2018 16:48:53 17705 ACTIVE
48 07-APR-2018 16:48:53 108856 ACTIVE

Hm.. What can I do? Not much really, we need to wait the rollback to complete.
One thing that can be done to increase the rollback is changing the fast_start_parallel_rollback parameter, as per:

SQL> show parameters roll

------------------------------------ ----------- ------------------------------
fast_start_parallel_rollback string LOW
rollback_segments string
transactions_per_rollback_segment integer 5
SQL> alter system set fast_start_parallel_rollback=HIGH;

System altered.

Hope it helps!

Oracle memory usage on Linux / Unix

Hi all,

So one of the most important things that we need to do when setting up a new server or checking the capacity of the server is to see how much memory Oracle is using.

When checking the capacity there are some practical things that always help me to get a fast glimpse of the system:

  • When opening topas and hitting M you will see this below
Topas Monitor for host: SERVER1 Interval: 2 Sat Dec 8 03:39:59 2019
0 0 59.8G 59.6G 212.3 16.3G 528 0-15
1 1 61.4G 61.2G 188.8 15.7G 536 16-31

On the memory session you will see 3 categories, INUSE, FREE and FILECACHE. There you may see what is being using for what but there is not much granularity there.

  • When using top you have this summary below
top - 11:48:08 up 119 days, 10:18, 1 user, load average: 26.76, 26.16, 25.95
Tasks: 1936 total, 38 running, 1898 sleeping, 0 stopped, 0 zombie
Cpu(s): 79.3%us, 1.1%sy, 0.0%ni, 15.1%id, 4.3%wa, 0.0%hi, 0.1%si, 0.0%st
Mem: 263750172k total, 219075656k used, 44674516k free, 797476k buffers
Swap: 16773116k total, 505760k used, 16267356k free, 88055108k cached

Same you have a high level usage. So here comes the question:

How are you to prove that you have a memory shortage?

I often use vmstat on Linux looking on the columns si and so equals to 0 (swap in and swap out) and when the free command, the free column you will also have no or very low swap being used

/home/oracle> vmstat 1
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
15 3 505760 44896608 797480 88062288 0 0 7037 1304 0 0 29 2 61 8 0
16 1 505760 44922964 797480 88062320 0 0 432272 144314 38784 31348 41 2 52 5 0
14 2 505760 44943072 797480 88062320 0 0 468904 155424 32676 27522 34 1 60 5 0
15 2 505760 44943032 797480 88062328 0 0 431032 144275 32596 27469 34 1 60 5 0
15 2 505760 44920136 797480 88062352 0 0 396232 145052 30772 26657 32 1 62 6 0
19 1 505760 44928576 797480 88062360 0 0 429360 160158 33640 28012 36 1 58 5 0
15 3 505760 44935340 797480 88062368 0 0 477232 161849 28393 21423 41 1 53 5 0
17 1 505760 44924744 797480 88062368 0 0 515265 160212 27478 20578 40 1 54 5 0
16 1 505760 44921596 797480 88062368 0 0 495408 159304 25458 19548 37 1 58 5 0
18 1 505760 44918144 797480 88062384 0 0 552880 168895 28203 22774 38 1 56 5 0
15 2 505760 44922344 797480 88062392 0 0 546920 160463 25321 19151 37 1 58 5 0
16 4 505760 44921544 797480 88062400 0 0 571544 153810 25429 20011 36 1 58 5 0
16 1 505760 44919620 797480 88062400 0 0 577552 160004 27132 20111 40 1 54 5 0
19 2 505760 44360240 797480 88062400 0 0 584969 155553 29467 22145 41 2 52 5 0
/home/oracle> free
total used free shared buffers cached
Mem: 263750172 219060896 44689276 91608 797480 88062464
-/+ buffers/cache: 130200952 133549220
Swap: 16773116 505760 16267356

To check a process specific memory usage (RSS) I often use ps along with other commands to calculate the process memory for a specific process id as below:

/home/oracle> ps -eo rss,pid,euser,lstart,args:100 --sort %mem | grep -v grep | grep 35796 | awk '{printf $1/1024 "MB"; $1=""; print }'| sort
19.6016MB 35796 oracle Sat Sep 8 02:43:54 2018 ora_lg00_ORC1
34.957MB 32340 oracle Sat Jan 5 11:50:09 2019 oracleORC1 (LOCAL=NO)

RSS is resident memory, but when comes to shared memory like the Oracle SGA the methods above could be miss leading – not to say wrong – but as Oracle memory is shared we may see double counting on the results. I sometimes use pmap to check a process memory as well when available

/home/oracle> pmap 35796
35796: ora_lg00_ORC1
total 0K

But, still when checking a server wide scope, do you want to keep doing manual work and lots of math? I don’t think so. 🙂

That’s why when I came across SMEM made my life a lot easier. It is a python script which gives you a nice breakdown of the memory usage and without the miss leading double counting.

You can see the commands and processes and their memory:

[root@srv01 smem-1.4]# ./smem -trk | head
PID User Command Swap USS PSS RSS
4829 root /opt/stackdriver/collectd/s 444.0K 4.0G 4.0G 4.0G
5647 oracle asm_gen0_+ASM 50.1M 424.4M 425.0M 437.8M
16512 oracle rman software/product/11.2. 0 172.9M 173.7M 177.8M
85107 oracle ora_n001_db01 42.3M 147.8M 147.8M 185.8M
85103 oracle ora_n000_db01 42.4M 146.5M 146.6M 184.6M
85109 oracle ora_n002_db01 42.2M 145.6M 145.6M 183.5M
85111 oracle ora_n003_db01 42.1M 145.1M 145.2M 183.1M
7287 oracle ora_dia0_db01 1.6M 68.6M 68.8M 107.8M

As well the overall server per user:

root@srv01 smem-1.4]# ./smem -turk 
User Count Swap USS PSS RSS oracle 1358 4.8G 7.8G 8.0G 76.6G 
root 43 12.0M 4.1G 4.1G 4.2G user1 10 0 321.0M 328.0M 369.2M 
nobody 2 96.0K 2.1M 2.3M 6.0M user2 2 0 684.0K 1.7M 7.7M 
user4 2 0 632.0K 1.7M 7.9M user4 1 72.0K 536.0K 540.0K 2.1M 
ntp 1 424.0K 332.0K 368.0K 2.4M 
smmsp 1 1.3M 160.0K 298.0K 1.9M 
rpc 1 336.0K 68.0K 73.0K 1.7M 
rpcuser 1 808.0K 4.0K 16.0K 1.9M 
1422 4.8G 12.2G 12.5G 81.3G

Hope it helps, see you next time!

Oracle 11g / 12c identified by values – set an Oracle password to its current value without knowing the password

This post will be a quick one. I needed to set a Oracle password as it was about to expire but the client did not want to change the password itself.

The simplest way to get the encrypted password was:

select name, password from sys.user$ where name=‘USER’;

But after Oracle 11g/12c seems a better option for security ends to use DBMS_METADATA.get_ddl or select the spare4 column on sys.user$

select name, spare4 from sys.user$ where name=‘USER’;


set long 9999999
set longc 9999999
select dbms_metadata.get_ddl('USER','DBSNMP') from dual;

And how to set this password back to the user?

alter user [USER] identified by values '[encrypted password]';


Hope it helps!