Overview

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$ ./cursor1
<CURSOR 1>
[Success declare cursor]

[Success open cursor]

[Fetch Cursor]                                                    
------------------------------------------------------------------
DNO      DNAME                          DEP_LOCATION       MGR_NO 
------------------------------------------------------------------
1     BUSINESS DEPT                  Seoul        100
2     BUSINESS DEPT                  Seoul        100
...Omitted...
583     BUSINESS DEPT                  Seoul        100
584     BUSINESS DEPT                  Seoul        100

Error : [-266450][HY000] Fetch out of sequence.                     /* Error occurred while performing fetch */
------------------------------------------------------------------
[Close Cursor]                                                    
------------------------------------------------------------------
Success close cursor

Cause

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When COMMIT/ROLLBACK is performed in the OPEN CURSOR state

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Altibase complies with the ANSI standard and is configured not to support the fetch across commit method by default. Therefore, if COMMIT or ROLLBACK is performed after opening the cursor, the cursor is forcibly closed according to the ANSI standard.

The reason an error occurs while performing FETCH to some extent is that the first large amount of records is stored in the communication buffer during FETCH. An error occurs when fetching all the records in the communication buffer and fetching the next certain amount of records into the communication buffer.

The following is an example of creating an application that may cause an error by performing COMMIT or ROLLBACK in the cursor OPEN state.

non-autocommit mode 

DECLARE CURSOR

OPEN CURSOR

while(1)
{
    커서(CURSOR) FETCH ;
    
    if (sqlca.sqlcode == SQL_SUCCESS) {
    
       /* Execute change DML */                     
       
       /* Perform COMMIT or ROLLBACK */ 
       
    }
    else if (sqlca.sqlcode == SQL_NO_DATA) {
       ...
    }
    else {              
      /* An error occurs at this stage when all the records in the first communication buffer are processed and then placed in the second communication buffer. */
       ...
    }   
}

Solution

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When COMMIT/ROLLBACK is performed in the OPEN CURSOR state

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Here are three solutions to deal with this error.

1. Separation of fetch session and modified DML session

By using multiple connections within one application, COMMIT or ROLLBACK does not affect the cursor.

• Create a session that uses a cursor and a session that executes modified DML statements. It is described by defining it as CONN1 and CONN2, respectively.

• Whenever cursor FETCH is executed in the session using the cursor (CONN1), change DML is executed in CONN2 and COMMIT or ROLLBACK is executed.

• The session (CONN2) that executes the modified DML statement is set to non-autocommit mode.

The following is an example of creating an application that reflects this action.

/* Session for FETCH */ 
EXEC SQL AT conn1 CONNECT; 
 
/* Session for performing change DML */
EXEC SQL AT conn2 CONNECT; 

EXEC SQL AT conn2 AUTOCOMMIT OFF; 
 
/* Declare cursor in CONN1, perform OPEN, FETCH */ 
EXEC SQL AT conn1 DECLARE cursor; 
EXEC SQL AT conn1 OPEN cursor; 
while (1)
{
    /* Perform FETCH on conn1 */ 
	EXEC SQL AT conn1 FETCH cursor 
		
    if (sqlca.sqlcode == SQL_SUCCESS) {  
      		
          /* Change DML and COMMIT or ROLLBACK in conn2 */ 
          EXEC SQL AT conn2 INSERT or UPDATE or DELETE ;
          
          /* check sqlca.sqlcode */
   		  if (sqlca.sqlcode == SQL_SUCCESS) {
    	     ...
    	  }
	      else {
	         ...
	      }
	      
          /* The commit or rollback performed in conn2 does not affect the cursor use of conn1. */  
	      EXEC SQL AT conn2 commit or rollback; 
	        
          /* check sqlca.sqlcode */
   		  if (sqlca.sqlcode == SQL_SUCCESS) {
    	  	  ...
    	  }
	      else {
	       	  ...
	      }			            		           		
  	}  
    else if (sqlca.sqlcode == SQL_NO_DATA)	{
        ...
    }
    else	{
        ...
    }          	  
}

2. Declare a cursor only enough to be stored in the communication buffer, and then repetitively open the cursor

After calculating the number of records enough to put in the communication buffer with one FETCH, declare the cursor using the LIMIT clause.

The communication buffer size of Altibase 5 and above is 32K. Since the number of records in the communication buffer depends on the record size, the last value of the LIMIT clause varies depending on the operating environment.

Specify the number of records to be stored in the communication buffer in the LIMIT clause, and open the cursor again and use it while changing the start value of the LIMIT clause before opening the cursor.

 
/* The cursor is declared using the LIMIT clause. n is the last record value to be returned in the LIMIT clause and must be defined according to the operating environment. The number of records in the communication buffer depends on the record size. */
DECLARE 커서
         SELECT ~
           FROM ~
          WHERE ~
          LIMIT :s_start, n;
              

/* Declare the starting value used in the LIMIT clause. */
s_start = 1;
   
while(1)
{ 
    /* Cursor open is repeated until all records meeting the conditions are fetched. */                   
    OPEN CURSOR
    
		while(1)
		{
		    FETCH CURSOR ;
		    
		    if (sqlca.sqlcode == SQL_SUCCESS) {
		    
		       /* Execute change DML */   		     		       
		    }
		    else if (sqlca.sqlcode == SQL_NO_DATA) {
		       ...
		    }
		    else {              
		       ...
		    }   
		}
		
		/* Perform COMMIT or ROLLBACK */    
 
       /* Specify the starting value of the LIMIT clause. n is an example. */ 
       s_start = s_start + n ; 
        
}

CLOSE CURSOR 또는 CLOSE RELEASE CURSOR
 

3. fetch across commit 

In the case of Precompiler (APRE), when declaring a cursor, declare and use a WITH HOLD cursor.

EXEC SQL DECLARE cursor_name 
     CURSOR WITH HOLD FOR SELECT statement

Reference

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Differences by version

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Depending on the Altibase version, the error messages that occur in the same situation may be different.

The difference in error messages that occurs when COMMIT/ROLLBACK is executed among FETCHs in a non-autocommit environment is as follows.

Reference manual

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This section introduces the manuals related to fetch across commit provided from Altibase 6.3.1.

• Precompiler User's Manual: 8. Cursor processing SQL statement -> Cursor-related embedded SQL statement -> DECLARE CURSOR

• CLI User's Manual: 2. ALTIBASE HDB CLI Function -> SQLSetStmtAttr

• JDBC User's Manual: 3. Advanced Features -> Using ResultSet -> Holdability

The manual can be downloaded from the pages below.

• http://support.altibase.com/en/manual/
• https://github.com/ALTIBASE/Documents/tree/master/Manuals