About pessimistick lock in Oracle

Hello,
I have some questions about pessimistick lock in Oracle:
I know if i do a SELECT..FOR UPDATE statement, those records are locked, and just me can opperate on them.. but me - WHO? How i'm identified in Oracle? If i do a SELECT..FOR UPDATE select in an A procedure, then i call the update procedure B, how can i know that both calls belongs to the same user? If someone tries to update the same records, how the server knows if that person is allowed to make those updates or not? because everyone uses the same connection string.. isn't it?
Probably it works if i make a SELECT..FOR UPDATE and i make the effective update in that procedure, but this means that the Business Logic is in that procedure..
Please correct me if i'm wrong somewhere and clarify me.
Thanks!

By default the other session has to wait (no error message):
From SQL Reference SELECT section: http://download.oracle.com/docs/cd/B28359_01/server.111/b28286/statements_10002.htm#SQLRF01702
>
NOWAIT | WAIT
The NOWAIT and WAIT clauses let you tell the database how to proceed if the SELECT statement attempts to lock a row that is locked by another user.
Specify NOWAIT to return control to you immediately if a lock exists.
Specify WAIT to instruct the database to wait integer seconds for the row to become available and then return control to you.
If you specify neither WAIT nor NOWAIT, then the database waits until the row is available and then returns the results of the SELECT statement.
>
Edited by: P. Forstmann on 7 janv. 2010 09:00
Edited by: P. Forstmann on 7 janv. 2010 09:01

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76A65058 76A6516C 797 TX 327697 1053730 6 0 1053 0
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76930C7C 76930C94 797 TM 54595 0 3 0 1053 0
76930BE8 76930C00 797 TM 94880 0 3 0 1053 0
76930B54 76930B6C 797 TM 87295 0 2 0 1053 0
76930AC0 76930AD8 797 TM 86821 0 2 0 1053 0
76930A2C 76930A44 797 TM 82679 0 2 0 1053 0
76930998 769309B0 797 TM 82662 0 2 0 1053 0
76930904 7693091C 797 TM 81118 0 2 0 1053 0
76930870 76930888 797 TM 66195 0 2 0 1053 0
769307DC 769307F4 797 TM 64474 0 2 0 1053 0
76930748 76930760 797 TM 60364 0 3 0 1053 0
769306B4 769306CC 797 TM 58038 0 2 0 1053 0
76930620 76930638 797 TM 57939 0 2 0 1053 0
7693058C 769305A4 797 TM 54933 0 2 0 1053 0
769304F8 76930510 797 TM 54913 0 2 0 1053 0
76930464 7693047C 797 TM 54906 0 2 0 1053 0
769303D0 769303E8 797 TM 54898 0 2 0 1053 0
7693032C 76930344 797 TM 54890 0 2 0 1053 0
76930298 769302B0 797 TM 54877 0 2 0 1053 0
76930204 7693021C 797 TM 54869 0 2 0 1053 0
76930170 76930188 797 TM 54831 0 2 0 1053 0
769300DC 769300F4 797 TM 54822 0 2 0 1053 0
76930048 76930060 797 TM 54820 0 2 0 1053 0
7692FFB4 7692FFCC 797 TM 54818 0 2 0 1053 0
7692FF20 7692FF38 797 TM 54808 0 3 0 1053 0
7692FE8C 7692FEA4 797 TM 54799 0 2 0 1053 0
7692FDF8 7692FE10 797 TM 54787 0 2 0 1053 0
7692FD64 7692FD7C 797 TM 54783 0 2 0 1053 0
7692FCD0 7692FCE8 797 TM 54774 0 2 0 1053 0
7692FC3C 7692FC54 797 TM 54767 0 2 0 1053 0
7692FBA8 7692FBC0 797 TM 54757 0 2 0 1053 0
7692FB14 7692FB2C 797 TM 54751 0 2 0 1053 0
7692FA80 7692FA98 797 TM 54594 0 2 0 1053 0
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Dropping a column can take a while if the column has a large number of values. In this case it may be better to set it to be unused and drop it when the number of users on the system are fewer to avoid extended locks.
Questions:
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1. Can you explain this to me please?! fewer than before? fewer than? What if users kept increasing during the years! then this "fewer" may not happen until the company collapse!
2. Why do we need to use unused columns? When should we use unused columns?

Great! .... I got more questions, i just do not want to open a new same thread. Thus, i will just post the questions in here and i hope i will get help from experts...Please bare with me guys...The questions are numbered, unnumbered parts are information that helps you understand my question.
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Thanks for understanding
Page 2-7:
Certain columns can never be dropped such as columns that form part of the partitioning
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*2. Can you clarify the above sentence please*
*3. If i have set of columns that has large amount of data, i rather set them unused then*
drop them because the response time is going to be faster! I do not get it, can you
explain please? What i know is drop drops the column and release the disk space whilst
unused column make the columns useless and does not release disk space yet until we drop them, so
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Page 2-12
4. ALTER TABLE emp2 ADD CONSTRAINT emp_dt_fk
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Page 2-18
create table test1 (
pk NUMBER PRIMARY KEY,
fk NUMBER,
col1 NUMBER,
col2 NUMBER,
CONSTRAINT fk_constraint FOREIGN KEY (fk) REFERENCES test1,
CONSTRAINT ck1 CHECK (pk > 0 and col1 > 0),
CONSTRAINT ck2 CHECK (col2 > 0) );
-- "CONSTRAINT fk_constraint FOREIGN KEY (fk) REFERENCES test1"
*8. This is wrong isn't it? It references to test1 but no column specified.*
An error is returned for the following statements:
ALTER TABLE test1 DROP (pk); -- pk is a parent key.
*9. We can not drop it because we did not mention ON DELETE CASCADE. Am i right?*
ALTER TABLE test1 DROP (col1) -- col1 is referenced by multicolumn constraint ck1.
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but no references made. Secondly, is ck1 considered multicolumn because it check two columns?
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ALTER TABLE emp2
DROP COLUMN employee_id CASCADE CONSTRAINTS;
*11. This drop employee_id column and all its child. Correct?*
ALTER TABLE test1
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*13. Then what's the difference between ON DELETE CASCADE and CASCADE CONSTRAINTS?*
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Page 2-22
When you are expecting a large data load and want to speed the operation. You may want
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having a unique index on the primary key will still cause the data to be verified during
the load. So you can first create a nonunique index on the column designated as PRIMARY
KEY, and then create the PRIMARY KEY column and specify that it should use the existing
index.
Example:
1. create the table
create table new_emp
(employee_id number(6),
first_name varchar2(10)
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create index emp_id_idx2 on new_emp(employee_id);
"You may want to disable the constraints while performing the load and then enable them"
so i suggest to load all data i want into new_emp.
3. create the primary key
alter table new_emp ADD primary key (employee_id) USING index emp_id_idx2;
What i understand is the following:
If we want to load large data into the new_emp, its better to create the table without any
constraints - in our case the constraint is primary key. After that, we create nonunique
index points to employee_id and then load data into new_emp. Finally, specify employee_id
as primary key using the nonunique index.
*14. Is my explanation correct?*
"in which case having a unique index on the primary key will still cause the data to be
verified during the load."
*15. Data to be verified against what? Is it to be verified whether its NULL or NOT NULL? I*
know primary key does not take NULL and every value must be unique.
After loading all data we want, what if i did
"alter table new_emp ADD primary key (employee_id);"
*16. Will i face any problems or inefficient process?*
I do not think we need step two, we could do the following:
1. create the table
create table new_emp
(employee_id number(6),
first_name varchar2(10)
"You may want to disable the constraints while performing the load and then enable them"
so i suggest to load all data i want itno new_emp.
2. create the primary key
alter table new_emp ADD primary key (employee_id);
*17. The above steps are as efficient as the three steps i mentioned above. The only difference*
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Page 2-23
CREATE INDEX upper_dept_name_idx ON dept2(UPPER(department_name));
The following statement may use the index, but without the WHERE clause the
Oracle server may perform a full table scan:
select *
from employees
where UPPER(last_name) IS NOT NULL
ORDER BY UPPER (last_name);
"but without the WHERE clause the Oracle server may perform a full table scan"
*18. The above query let oracle server perform full table scan anyway! Right? It has to go*
through every field and check is it not null or not. I know we are using function-based
index but there are alot of not null last_name! so oracle server must scan one by one. If
we only had one not null field, then i would say Oracle server can point to that field
immediately by the aid of function-based index we created above. Can you clarify please...
Another related topic statement that i do not get it yet:
"The oracle server treats indexes with columns marked DESC as function-based indexes."
*19. The bove statements is so general. What if we have a column ordered by DESC order and we*
did not create any function-based indexes, will statement be true?!
Lets go back the above query:
ORDER BY UPPER (last_name);
*20. Its not DESC. To me, the above query does not flow with this statement "The oracle server treats*
*indexes with columns marked DESC as function-based indexes."?*
Page 2-27
Regarding FLASHBACK TABLE, you can invoke a flashback table operation on one or more
tables, even on tables in different schema. You specify the point in time to which you
want to revert by providing a valid timestamp. By default, database triggers are disabled
for all tables involved. You can override this default behavior by specifying the ENABLE
TRIGGERS clause.
"By default, database triggers are disabled for all tables involved. You can override this
default behavior by specifying the ENABLE TRIGGERS clause."
*21. What are database triggers?*
*22. About External Tables. What are external tables? When is it used in real-life? Why do*
we want External Tables?
Page 2-30
Oracle server provides two major access drivers for external tables. They are
ORACLE_LOADER access driver and ORACLE_DATAPUMP access driver. ORACLE_DATAPUMP used to
both import and export data using a platform-independent format.
"platform-independent format."
*23. What is the format? Is it .dat?*
Page 2-35
CREATE TABLE oldemp ( fname char(25), lname char(25) )
ORGANIZATION EXTERNAL
(TYPE ORACLE_LOADER
DEFAULT DIRECTORY emp_dir
ACCESS PARAMETERS
(RECORDS DELIMINATED BT NEWLINE
NOBADFILE
NOLOGFILE
FIELDS TERMINATED BY ',' (fname POSITION (1:20) CHAR, lname POSITION (22:41) CHAR)
LOCATION ('emp.dat') )
PARALLEL 5
REJECT LIMIT 200;
*24. Can you please explain the below part:*
ACCESS PARAMETERS
(RECORDS DELIMINATED BT NEWLINE
NOBADFILE
NOLOGFILE
FIELDS TERMINATED BY ',' (fname POSITION (1:20) CHAR, lname POSITION (22:41) CHAR)
*25. Can you please explain what is PARALLEL 5? and Why do we need it?*
Again, any help is appreciated...
Edited by: user11164565 on Jul 21, 2009 4:41 AM

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// Create the insert command
string insertCommandText =
    "INSERT /*+ append */ INTO PS_table " +
    "(SEQ_NBR, BUSINESS_UNIT, INVOICE, ASSIGNMENT_ID, END_DT, RI_TIMECARD_ID, IMAGE_ID, FILENAME, BARCODE_LABEL_ID, DIRECT_INVOICING, " +
    "EXCLUDE_FLG, DTTM_CREATED, DTTM_MODIFIED, IMAGE_DATA, PROCESS_INSTANCE) " +
    "VALUES (INV.nextval, :BUSINESS_UNIT, :INVOICE, :ASSIGNMENT_ID, :END_DT, :RI_TIMECARD_ID, :IMAGE_ID, :FILENAME, " +
    ":BARCODE_LABEL_ID, :DIRECT_INVOICING, :EXCLUDE_FLG, :DTTM_CREATED, :DTTM_MODIFIED, :IMAGE_DATA, :PROCESS_INSTANCE)";
// Add the insert command to the data adapter
da.InsertCommand = new OleDbCommand(insertCommandText);
da.InsertCommand.Connection = oc;
// Add the params to the insert
da.InsertCommand.Parameters.Add(":BUSINESS_UNIT", OleDbType.VarChar, 5, "BUSINESS_UNIT");
da.InsertCommand.Parameters.Add(":INVOICE", OleDbType.VarChar, 22, "INVOICE");
da.InsertCommand.Parameters.Add(":ASSIGNMENT_ID", OleDbType.VarChar, 15, "ASSIGNMENT_ID");
da.InsertCommand.Parameters.Add(":END_DT", OleDbType.Date, 0, "END_DT");
da.InsertCommand.Parameters.Add(":RI_TIMECARD_ID", OleDbType.VarChar, 10, "RI_TIMECARD_ID");
da.InsertCommand.Parameters.Add(":IMAGE_ID", OleDbType.VarChar, 8, "IMAGE_ID");
da.InsertCommand.Parameters.Add(":FILENAME", OleDbType.VarChar, 80, "FILENAME");
da.InsertCommand.Parameters.Add(":BARCODE_LABEL_ID", OleDbType.VarChar, 18, "BARCODE_LABEL_ID");
da.InsertCommand.Parameters.Add(":DIRECT_INVOICING", OleDbType.VarChar, 1, "DIRECT_INVOICING");
da.InsertCommand.Parameters.Add(":EXCLUDE_FLG", OleDbType.VarChar, 1, "EXCLUDE_FLG");
da.InsertCommand.Parameters.Add(":DTTM_CREATED", OleDbType.Date, 0, "DTTM_CREATED");
da.InsertCommand.Parameters.Add(":DTTM_MODIFIED", OleDbType.Date, 0, "DTTM_MODIFIED");
da.InsertCommand.Parameters.Add(":IMAGE_DATA", OleDbType.Binary, System.Convert.ToInt32(filedata.Length), "IMAGE_DATA");
da.InsertCommand.Parameters.Add(":PROCESS_INSTANCE", OleDbType.VarChar, 10, "PROCESS_INSTANCE");
// Update the table
da.Update(ds, "documents");

Here is what Oracle is showing as blocking locks and the SQL that has been identified with each of the SIDS. Not sure why there is contention. There are no triggers or joined tables in this piece of code.
Here is the SQL all of the SIDs below are running:
INSERT INTO sysadm.PS_RI_INV_PDF_MERG (SEQ_NBR, BUSINESS_UNIT, INVOICE, ASSIGNMENT_ID, END_DT, RI_TIMECARD_ID, IMAGE_ID, FILENAME, BARCODE_LABEL_ID, DIRECT_INVOICING, EXCLUDE_FLG, DTTM_CREATED, DTTM_MODIFIED, IMAGE_DATA, PROCESS_INSTANCE) VALUES (SYSADM.INV_PDF_MERG.nextval,
:BUSINESS_UNIT, :INVOICE, :ASSIGNMENT_ID, :END_DT, :RI_TIMECARD_ID, :IMAGE_ID, :FILENAME, :BARCODE_LABEL_ID, :DIRECT_INVOICING, :EXCLUDE_FLG, :DTTM_CREATED, :DTTM_MODIFIED, :IMAGE_DATA, :PROCESS_INSTANCE)
SID 1452 (BTSUSER,BIZTPRDI,BTSNTSvc64.exe) in instance FSLX1 is blocking SID 1150 (BTSUSER,BIZTPRDI,BTSNTSvc64.exe) in instance FSLX1
SID 1452 (BTSUSER,BIZTPRDI,BTSNTSvc64.exe) in instance FSLX1 is blocking SID 1452 (BTSUSER,BIZTPRDI,BTSNTSvc64.exe) in instance FSLX1
SID 1452 (BTSUSER,BIZTPRDI,BTSNTSvc64.exe) in instance FSLX1 is blocking SID 1156 (BTSUSER,biztprdi,BTSNTSvc64.exe) in instance FSLX3
SID 1452 (BTSUSER,BIZTPRDI,BTSNTSvc64.exe) in instance FSLX1 is blocking SID 6 (BTSUSER,BIZTPRDI,BTSNTSvc64.exe) in instance FSLX2
SID 1452 (BTSUSER,BIZTPRDI,BTSNTSvc64.exe) in instance FSLX1 is blocking SID 1726 (BTSUSER,BIZTPRDI,BTSNTSvc64.exe) in instance FSLX2
SID 1452 (BTSUSER,BIZTPRDI,BTSNTSvc64.exe) in instance FSLX1 is blocking SID 2016 (BTSUSER,biztprdi,BTSNTSvc64.exe) in instance FSLX2

About pessimistick lock in Oracle

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