3 Database Concepts

Connect to the Database

The complete syntax of the CONNECT statement will be discussed in the next few sections. Here it is:

EXEC SQL CONNECT { :user IDENTIFIED BY :oldpswd | :usr_psw }
   [[ AT { dbname | :host_variable }] USING :connect_string ]
      [ {ALTER AUTHORIZATION :newpswd  |  IN { SYSDBA | SYSOPER } MODE} ] ;

Your Pro*C/C++ program must connect to the database before querying or manipulating data. To log on, simply use the CONNECT statement

EXEC SQL CONNECT :username IDENTIFIED BY :password ; 

where username and password are char or VARCHAR host variables.

Or, you can use the statement

EXEC SQL CONNECT :usr_pwd; 

where the host variable usr_pwd contains your username and password separated by a slash character (/).

These are simplified subsets of the CONNECT statement.

The CONNECT statement must be the first SQL statement executed by the program. That is, other SQL statements can physically but not logically precede the CONNECT statement in the precompilation unit.

To supply the Oracle username and password separately, you define two host variables as character strings or VARCHARs. (If you supply a username containing both username and password, only one host variable is needed.)

Make sure to set the username and password variables before the CONNECT is executed, or it will fail. Your program can prompt for the values, or you can hard-code them as follows:

char *username = "SCOTT"; 
char *password = "TIGER"; 
... 
EXEC SQL WHENEVER SQLERROR ... 
EXEC SQL CONNECT :username IDENTIFIED BY :password; 

However, you cannot hard-code a username and password into the CONNECT statement. You also cannot use quoted literals. For example, both of the following statements are invalid:

EXEC SQL CONNECT SCOTT IDENTIFIED BY TIGER; 
EXEC SQL CONNECT 'SCOTT' IDENTIFIED BY 'TIGER'; 

Hard coding usernames and passwords is not recommended practise.

EXEC SQL CONNECT ..;   /* No ALTER AUTHORIZATION clause */

EXEC SQL CONNECT .. ALTER AUTHORIZATION :newpswd;

CLUSTER$username 

... 
char  *oracleid = "/"; 
... 
EXEC SQL CONNECT :oracleid; 

... 
char oracleid[10] = "/    ";
... 
EXEC SQL CONNECT :oracleid; 

CLUSTER$<username>

[IN { SYSDBA | SYSOPER } MODE]

EXEC SQL CONNECT ... IN SYSDBA MODE ;

Advanced Connection Options

This section describes the available options for advanced connections.

HR.US.ORACLE.COM

Concurrent Logons

Pro*C/C++ supports distributed processing through Oracle Net. Your application can concurrently access any combination of local and remote databases or make multiple connections to the same database. In Figure 3-1, an application program communicates with one local and three remote Oracle databases. ORA2, ORA3, and ORA4 are simply logical names used in CONNECT statements.

Figure 3-1 Connecting through Oracle Net

Description of "Figure 3-1 Connecting through Oracle Net"

By eliminating the boundaries in a network between different machines and operating systems, Oracle Net provides a distributed processing environment for Oracle tools. This section shows you how Pro*C/C++ supports distributed processing through Oracle Net. You learn how your application can

• Directly or indirectly access other databases

• Concurrently access any combination of local and remote databases

• Make multiple connections to the same database

For details on installing Oracle Net and identifying available databases, refer to the Oracle Database Net Services Administrator's Guide and your system-specific Oracle documentation.

EXEC SQL CONNECT :username IDENTIFIED BY :password; 

EXEC SQL CONNECT :usr_pwd; 

CLUSTER$username 

char oracleid = '/'; 
...
EXEC SQL CONNECT :oracleid; 

/* declare needed host variables */
char  username[10]  = "scott"; 
char  password[10]  = "tiger";
char  db_string[20] = "NYNON"; 

/* give the database connection a unique name */ 
EXEC SQL DECLARE DB_NAME DATABASE; 

/* connect to the nondefault database  */
EXEC SQL CONNECT :username IDENTIFIED BY :password 
   AT DB_NAME USING :db_string; 

/* declare needed host variables */ 
char  username[10]  = "scott"; 
char  password[10]  = "tiger";
char  db_name[10]   = "oracle1"; 
char  db_string[20] = "NYNON"; 

/* connect to the nondefault database using db_name */
EXEC SQL CONNECT :username IDENTIFIED BY :password 
   AT :db_name USING :db_string; 
... 

EXEC SQL AT DB_NAME SELECT ... 
EXEC SQL AT DB_NAME INSERT ... 
EXEC SQL AT DB_NAME UPDATE ... 

EXEC SQL AT :db_name DELETE ... 

EXEC SQL AT :db_name EXECUTE
    begin
        /* PL/SQL block here */
    end;
END-EXEC;

EXEC SQL AT :db_name DECLARE emp_cursor CURSOR FOR ... 
EXEC SQL OPEN emp_cursor ... 
EXEC SQL FETCH emp_cursor ... 
EXEC SQL CLOSE emp_cursor; 

EXEC SQL AT :db_name DECLARE emp_cursor CURSOR FOR ... 
strcpy(db_name, "oracle1"); 
EXEC SQL OPEN emp_cursor; 
EXEC SQL FETCH emp_cursor INTO ... 
strcpy(db_name, "oracle2"); 
EXEC SQL OPEN emp_cursor;   /*  illegal, cursor still open */ 
EXEC SQL FETCH emp_cursor INTO ... 

... 
EXEC SQL CLOSE emp_cursor;  -- close cursor first 
strcpy(db_name, "oracle2"); 
EXEC SQL OPEN emp_cursor; 
EXEC SQL FETCH emp_cursor INTO ... 

EXEC SQL AT :db_name EXECUTE IMMEDIATE :sql_stmt; 

EXEC SQL AT :db_name DECLARE sql_stmt STATEMENT; 
EXEC SQL PREPARE sql_stmt FROM :sql_string; 
EXEC SQL EXECUTE sql_stmt; 

EXEC SQL AT :db_name DECLARE sql_stmt STATEMENT; 
EXEC SQL PREPARE sql_stmt FROM :sql_string; 
EXEC SQL DECLARE emp_cursor CURSOR FOR sql_stmt; 
EXEC SQL OPEN emp_cursor ... 
EXEC SQL FETCH emp_cursor INTO ... 
EXEC SQL CLOSE emp_cursor;

/* declare needed host variables */ 
char  username[10]   = "scott"; 
char  password[10]   = "tiger"; 
char  db_string1[20] = "NYNON1"; 
char  db_string2[20] = "CHINON"; 
... 
/* give each database connection a unique name */ 
EXEC SQL DECLARE DB_NAME1 DATABASE; 
EXEC SQL DECLARE DB_NAME2 DATABASE; 
/* connect to the two nondefault databases */ 
EXEC SQL CONNECT :username IDENTIFIED BY :password 
   AT DB_NAME1 USING :db_string1; 
EXEC SQL CONNECT :username IDENTIFIED BY :password 
   AT DB_NAME2 USING :db_string2; 

/* declare needed host variables */ 
char  username[10]   = "scott";
char  password[10]   = "tiger";
char  db_name[20];
char  db_string[20];
int   n_defs = 3;    /* number of connections to make */
...
for (i = 0; i < n_defs; i++)
{
    /* get next database name and OracleNet string */
    printf("Database name: ");
    gets(db_name);
    printf("OracleNet) string: ");
    gets(db_string);
    /* do the connect */
    EXEC SQL CONNECT :username IDENTIFIED BY :password
        AT :db_name USING :db_string;
}

strcpy(db_string, "NYNON");
for (i = 0; i < ndefs; i++)
{
    /* connect to the nondefault database */
    printf("Database name: ");
    gets(db_name);
    EXEC SQL CONNECT :username IDENTIFIED BY :password 
       AT :db_name USING :db_string;
}
... 

EXEC SQL CREATE DATABASE LINK db_link 
    CONNECT TO username IDENTIFIED BY password 
    USING 'NYNON'; 

EXEC SQL SELECT ENAME, JOB INTO :emp_name, :job_title 
    FROM emp@db_link 
    WHERE DEPTNO = :dept_number; 

EXEC SQL CREATE SYNONYM emp FOR emp@db_link; 

EXEC SQL SELECT ENAME, JOB INTO :emp_name, :job_title 
    FROM emp 
    WHERE DEPTNO = :dept_number; 

EXEC SQL CREATE DATABASE LINK db_link1 
    CONNECT TO username1 IDENTIFIED BY password1 
        USING 'NYNON'; 
EXEC SQL CREATE DATABASE LINK db_link2 
    CONNECT TO username2 IDENTIFIED BY password2 
        USING 'CHINON'; 
EXEC SQL CREATE SYNONYM emp FOR emp@db_link1; 
EXEC SQL CREATE SYNONYM dept FOR dept@db_link2; 

EXEC SQL SELECT ENAME, JOB, SAL, LOC 
    FROM emp, dept 
    WHERE emp.DEPTNO = dept.DEPTNO AND DEPTNO = :dept_number; 

Definitions of Transactions Terms

Before delving into the subject of transactions, you should know the terms defined in this section.

The jobs or tasks that Oracle manages are called sessions. A user session is invoked when you run an application program or a tool such as SQL*Forms, and connect to the database.

Oracle allows user sessions to work simultaneously and share computer resources. To do this, Oracle must control concurrency, the accessing of the same data by many users. Without adequate concurrency controls, there might be a loss of data integrity. That is, changes to data or structures might be made in the wrong order.

Oracle uses locks (sometimes called enqueues) to control concurrent access to data. A lock gives you temporary ownership of a database resource such as a table or row of data. Thus, data cannot be changed by other users until you finish with it.

You need never explicitly lock a resource, because default locking mechanisms protect Oracle data and structures. However, you can request data locks on tables or rows when it is to your advantage to override default locking. You can choose from several modes of locking such as row share and exclusive.

A deadlock can occur when two or more users try to access the same database object. For example, two users updating the same table might wait if each tries to update a row currently locked by the other. Because each user is waiting for resources held by another user, neither can continue until Oracle breaks the deadlock. Oracle signals an error to the participating transaction that had completed the least amount of work, and the "deadlock detected while waiting for resource" Oracle error code is returned to sqlcode in the SQLCA.

When a table is being queried by one user and updated by another at the same time, Oracle generates a read-consistent view of the table's data for the query. That is, once a query begins and as it proceeds, the data read by the query does not change. As update activity continues, Oracle takes snapshots of the table's data and records changes in a rollback segment. Oracle uses information in the rollback segment to build read-consistent query results and to undo changes if necessary.

How Transactions Guard Your Database

Oracle is transaction oriented. That is, Oracle uses transactions to ensure data integrity. A transaction is a series of one or more logically related SQL statements you define to accomplish some task. Oracle treats the series of SQL statements as a unit so that all the changes brought about by the statements are either committed (made permanent) or rolled back (undone) at the same time. If your application program fails in the middle of a transaction, the database is automatically restored to its former (pre-transaction) state.

The coming sections show you how to define and control transactions. Specifically, you learn how to:

• Connect to the database.

• Make concurrent connections.

• Begin and end transactions.

• Use the COMMIT statement to make transactions permanent.

• Use the SAVEPOINT statement with the ROLLBACK TO statement to undo parts of transactions.

• Use the ROLLBACK statement to undo whole transactions.

• Specify the RELEASE option to free resources and log off the database.

• Use the SET TRANSACTION statement to set read-only transactions.

• Use the FOR UPDATE clause or LOCK TABLE statement to override default locking.

For details about the SQL statements discussed in this chapter, see Oracle Database SQL Language Reference.

How to Begin and End Transactions

You begin a transaction with the first executable SQL statement (other than CONNECT) in your program. When one transaction ends, the next executable SQL statement automatically begins another transaction. Thus, every executable statement is part of a transaction. Because they cannot be rolled back and need not be committed, declarative SQL statements are not considered part of a transaction.

You end a transaction in one of the following ways:

• Code a COMMIT or ROLLBACK statement, with or without the RELEASE option. This explicitly makes permanent or undoes changes to the database.

• Code a data definition statement (ALTER, CREATE, or GRANT, for example), which issues an automatic COMMIT before and after executing. This implicitly makes permanent changes to the database.

A transaction also ends when there is a system failure or your user session stops unexpectedly because of software problems, hardware problems, or a forced interrupt. Oracle rolls back the transaction.

If your program fails in the middle of a transaction, Oracle detects the error and rolls back the transaction. If your operating system fails, Oracle restores the database to its former (pre-transaction) state.

Using the COMMIT Statement

If you do not subdivide your program with the COMMIT or ROLLBACK statement, Oracle treats the whole program as a single transaction (unless the program contains data definition statements, which issue automatic COMMITS).

You use the COMMIT statement to make changes to the database permanent. Until changes are COMMITted, other users cannot access the changed data; they see it as it was before your transaction began. Specifically, the COMMIT statement

• Makes permanent all changes made to the database during the current transaction

• Makes these changes visible to other users

• Erases all savepoints (see the next section)

• Releases all row and table locks, but not parse locks

• Closes cursors referenced in a CURRENT OF clause or, when MODE=ANSI, closes all explicit cursors for the connection specified in the COMMIT statement

• Ends the transaction

The COMMIT statement has no effect on the values of host variables or on the flow of control in your program.

When MODE=ORACLE, explicit cursors that are not referenced in a CURRENT OF clause remain open across COMMITs. This can boost performance.

Because they are part of normal processing, COMMIT statements should be placed inline, on the main path through your program. Before your program terminates, it must explicitly COMMIT pending changes. Otherwise, Oracle rolls them back. In the following example, you commit your transaction and disconnect from Oracle:

EXEC SQL COMMIT WORK RELEASE; 

The optional keyword WORK provides ANSI compatibility. The RELEASE option frees all Oracle resources (locks and cursors) held by your program and logs off the database.

You need not follow a data definition statement with a COMMIT statement because data definition statements issue an automatic COMMIT before and after executing. So, whether they succeed or fail, the prior transaction is committed.

     EXEC SQL 
         DECLARE C1 CURSOR WITH HOLD
         FOR SELECT ENAME FROM EMP
         WHERE EMPNO BETWEEN 7600 AND 7700
     END-EXEC.

CLOSE_ON_COMMIT = {YES | NO} 

Using the SAVEPOINT Statement

You use the SAVEPOINT statement to mark and name the current point in the processing of a transaction. Each marked point is called a savepoint. For example, the following statement marks a savepoint named start_delete:

EXEC SQL SAVEPOINT start_delete; 

Savepoints let you divide long transactions, giving you more control over complex procedures. For example, if a transaction performs several functions, you can mark a savepoint before each function. Then, if a function fails, you can easily restore the Oracle data to its former state, recover, then reexecute the function.

To undo part of a transaction, you use savepoints with the ROLLBACK statement and its TO SAVEPOINT clause. In the following example, you access the table MAIL_LIST to insert new listings, update old listings, and delete (a few) inactive listings. After the delete, you check the third element of sqlerrd in the SQLCA for the number of rows deleted. If the number is unexpectedly large, you roll back to the savepoint start_delete, undoing just the delete.

... 
for (;;) 
{ 
   printf("Customer number? "); 
   gets(temp);
   cust_number = atoi(temp);
   printf("Customer name? "); 
   gets(cust_name); 
  EXEC SQL INSERT INTO mail_list (custno, cname, stat) 
     VALUES (:cust_number, :cust_name, 'ACTIVE'); 
... 
} 
 
for (;;) 
{ 
   printf("Customer number? "); 
   gets(temp);
   cust_number = atoi(temp);
   printf("New status? "); 
   gets(new_status); 
   EXEC SQL UPDATE mail_list 
     SET stat = :new_status 
     WHERE custno = :cust_number; 
} 
/* mark savepoint */ 
EXEC SQL SAVEPOINT start_delete; 
 
EXEC SQL DELETE FROM mail_list 
    WHERE stat = 'INACTIVE'; 
if (sqlca.sqlerrd[2] < 25)  /* check number of rows deleted */ 
    printf("Number of rows deleted is  %d\n", sqlca.sqlerrd[2]); 
else 
{ 
    printf("Undoing deletion of %d rows\n", sqlca.sqlerrd[2]); 
    EXEC SQL WHENEVER SQLERROR GOTO sql_error; 
    EXEC SQL ROLLBACK TO SAVEPOINT start_delete; 
} 
 
EXEC SQL WHENEVER SQLERROR CONTINUE; 
EXEC SQL COMMIT WORK RELEASE; 
exit(0); 
sql_error: 
EXEC SQL WHENEVER SQLERROR CONTINUE; 
EXEC SQL ROLLBACK WORK RELEASE; 
printf("Processing error\n"); 
exit(1); 

Rolling back to a savepoint erases any savepoints marked after that savepoint. The savepoint to which you roll back, however, is not erased. For example, if you mark five savepoints, then roll back to the third, only the fourth and fifth are erased.

If you give two savepoints the same name, the earlier savepoint is erased. A COMMIT or ROLLBACK statement erases all savepoints.

The ROLLBACK Statement

You use the ROLLBACK statement to undo pending changes made to the database. For example, if you make a mistake, such as deleting the wrong row from a table, you can use ROLLBACK to restore the original data. The TO SAVEPOINT clause lets you roll back to an intermediate statement in the current transaction, so you do not have to undo all your changes.

If you start a transaction that does not complete (a SQL statement might not execute successfully, for example), ROLLBACK lets you return to the starting point, so that the database is not left in an inconsistent state. Specifically, the ROLLBACK statement

• Undoes all changes made to the database during the current transaction

• Erases all savepoints

• Ends the transaction

• Releases all row and table locks, but not parse locks

• Closes cursors referenced in a CURRENT OF clause or, when MODE=ANSI, closes all explicit cursors

The ROLLBACK statement has no effect on the values of host variables or on the flow of control in your program.

When MODE=ORACLE, explicit cursors not referenced in a CURRENT OF clause remain open across ROLLBACKs.

Specifically, the ROLLBACK TO SAVEPOINT statement

• Undoes changes made to the database since the specified savepoint was marked

• Erases all savepoints marked after the specified savepoint

• Releases all row and table locks acquired since the specified savepoint was marked

  Note:

  You cannot specify the RELEASE option in a ROLLBACK TO SAVEPOINT statement.

Because they are part of exception processing, ROLLBACK statements should be placed in error handling routines, off the main path through your program. In the following example, you roll back your transaction and disconnect from Oracle:

EXEC SQL ROLLBACK WORK RELEASE; 

The optional keyword WORK provides ANSI compatibility. The RELEASE option frees all resources held by your program and disconnects from the database.

If a WHENEVER SQLERROR GOTO statement branches to an error handling routine that includes a ROLLBACK statement, your program might enter an infinite loop if the ROLLBACK fails with an error. You can avoid this by coding WHENEVER SQLERROR CONTINUE before the ROLLBACK statement, as shown in the following example:

EXEC SQL WHENEVER SQLERROR GOTO sql_error; 
 
for (;;) 
{ 
   printf("Employee number? "); 
    gets(temp);
    emp_number = atoi(temp);
   printf("Employee name? "); 
   gets(emp_name); 
   EXEC SQL INSERT INTO emp (empno, ename) 
       VALUES (:emp_number, :emp_name); 
... 
} 
... 
sql_error: 
EXEC SQL WHENEVER SQLERROR CONTINUE; 
EXEC SQL ROLLBACK WORK RELEASE; 
printf("Processing error\n"); 
exit(1); 

Oracle automatically rolls back transactions if your program terminates abnormally.

The RELEASE Option

Oracle automatically rolls back changes if your program terminates abnormally. Abnormal termination occurs when your program does not explicitly commit or roll back work and disconnect from Oracle using the RELEASE option. Normal termination occurs when your program runs its course, closes open cursors, explicitly commits or rolls back work, disconnects from Oracle, and returns control to the user.

Your program will exit gracefully if the last SQL statement it executes is either

EXEC SQL COMMIT WORK RELEASE; 

or

EXEC SQL ROLLBACK WORK RELEASE;
 

where the token WORK is optional. Otherwise, locks and cursors acquired by your user session are held after program termination until Oracle recognizes that the user session is no longer active. This might cause other users in a multiuser environment to wait longer than necessary for the locked resources.

The SET TRANSACTION Statement

You use the SET TRANSACTION statement to begin a read-only transaction. Because they allow "repeatable reads," read-only transactions are useful for running multiple queries against one or more tables while other users update the same tables. An example of the SET TRANSACTION statement follows:

EXEC SQL SET TRANSACTION READ ONLY;
 

The SET TRANSACTION statement must be the first SQL statement in a read-only transaction and can appear only once in a transaction. The READ ONLY parameter is required. Its use does not affect other transactions.

Only the SELECT, COMMIT, and ROLLBACK statements are allowed in a read-only transaction. For example, including an INSERT, DELETE, or SELECT FOR UPDATE OF statement causes an error.

During a read-only transaction, all queries refer to the same snapshot of the database, providing a multitable, multiquery, read-consistent view. Other users can continue to query or update data as usual.

A COMMIT, ROLLBACK, or data definition statement ends a read-only transaction. (Recall that data definition statements issue an implicit COMMIT.)

In the following example, as a store manager, you check sales activity for the day, the past week, and the past month by using a read-only transaction to generate a summary report. The report is unaffected by other users updating the database during the transaction.

EXEC SQL SET TRANSACTION READ ONLY; 
EXEC SQL SELECT sum(saleamt) INTO :daily FROM sales 
    WHERE saledate = SYSDATE; 
EXEC SQL SELECT sum(saleamt) INTO :weekly FROM sales 
    WHERE saledate > SYSDATE - 7; 
EXEC SQL SELECT sum(saleamt) INTO :monthly FROM sales 
    WHERE saledate > SYSDATE - 30; 
EXEC SQL COMMIT WORK; 
    /* simply ends the transaction since there are no changes 
       to make permanent */ 
/* format and print report */ 

Override Default Locking

By default, Oracle automatically locks many data structures for you. However, you can request specific data locks on rows or tables when it is to your advantage to override default locking. Explicit locking lets you share or deny access to a table for the duration of a transaction or ensure multitable and multiquery read consistency.

With the SELECT FOR UPDATE OF statement, you can explicitly lock specific rows of a table to make sure they do not change before an UPDATE or DELETE is executed. However, Oracle automatically obtains row-level locks at UPDATE or DELETE time. So, use the FOR UPDATE OF clause only if you want to lock the rows before the UPDATE or DELETE.

You can explicitly lock entire tables using the LOCK TABLE statement.

EXEC SQL DECLARE emp_cursor CURSOR FOR 
    SELECT ename, job, sal FROM emp WHERE deptno = 20 
        FOR UPDATE OF sal; 

EXEC SQL DECLARE emp_cursor CURSOR FOR 
    SELECT ename, job, sal FROM emp WHERE deptno = 20; 

EXEC SQL LOCK TABLE EMP IN ROW SHARE MODE NOWAIT; 

Fetch Across COMMITs

If you want to intermix COMMITs and FETCHes, do not use the CURRENT OF clause. Instead, SELECT the ROWID of each row, then use that value to identify the current row during the update or delete. An example follows:

... 
EXEC SQL DECLARE emp_cursor CURSOR FOR 
    SELECT ename, sal, ROWID FROM emp WHERE job = 'CLERK'; 
... 
EXEC SQL OPEN emp_cursor; 
EXEC SQL WHENEVER NOT FOUND GOTO ... 
for (;;) 
{ 
    EXEC SQL FETCH emp_cursor INTO :emp_name, :salary, :row_id; 
    ... 
    EXEC SQL UPDATE emp SET sal = :new_salary 
        WHERE ROWID = :row_id; 
    EXEC SQL COMMIT; 
... 
} 

Note, however, that the FETCHed rows are not locked. So, you might get inconsistent results if another user modifies a row after you read it but before you update or delete it.

Distributed Transactions Handling

A distributed database is a single logical database comprising multiple physical databases at different nodes. A distributed statement is any SQL statement that accesses a remote node using a database link. A distributed transaction includes at least one distributed statement that updates data at multiple nodes of a distributed database. If the update affects only one node, the transaction is non-distributed.

When you issue a COMMIT, changes to each database affected by the distributed transaction are made permanent. If instead you issue a ROLLBACK, all the changes are undone. However, if a network or machine fails during the commit or rollback, the state of the distributed transaction might be unknown or in doubt. In such cases, if you have FORCE TRANSACTION system privileges, you can manually commit or roll back the transaction at your local database by using the FORCE clause. The transaction must be identified by a quoted literal containing the transaction ID, which can be found in the data dictionary view DBA_2PC_PENDING. Some examples follow:

EXEC SQL COMMIT FORCE '22.31.83'; 
... 
EXEC SQL ROLLBACK FORCE '25.33.86'; 

FORCE commits or rolls back only the specified transaction and does not affect your current transaction. You cannot manually roll back in-doubt transactions to a savepoint.

The COMMENT clause in the COMMIT statement lets you specify a Comment to be associated with a distributed transaction. If ever the transaction is in doubt, Oracle stores the text specified by COMMENT in the data dictionary view DBA_2PC_PENDING along with the transaction ID. The text must be a quoted literal 50 characters in length. An example follows:

EXEC SQL COMMIT COMMENT 'In-doubt trans; notify Order Entry'; 

For more information about distributed transactions, see Oracle Database Concepts.

Guidelines

The following guidelines will help you avoid some common problems.

EXEC SQL INSERT INTO EMP ... 
EXEC SQL EXECUTE 
    BEGIN 
        UPDATE emp ... 
        ... 
    EXCEPTION 
        WHEN DUP_VAL_ON_INDEX THEN 
            ROLLBACK; 
        ... 
    END; 
END-EXEC; 
...