Release 2 (9.2)
Part Number A96584-01
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| Oracle Call Interface Programmer's Guide Release 2 (9.2) Part Number A96584-01 |
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This chapter revisits the basic concepts of binding and defining that were introduced in Chapter 2, "OCI Programming Basics", and provides more detailed information about the different types of binds and defines you can use in OCI applications. Additionally, this chapter discusses the use of arrays of structures, as well as other issues involved in binding, defining, and character conversions.
This chapter includes the following sections:
Most DML statements, and some queries (such as those with a WHERE clause), require a program to pass data to Oracle as part of a SQL or PL/SQL statement. Such data can be constant or literal data, known when your program is compiled. For example, the following SQL statement, which adds an employee to a database contains several literals, such as 'BESTRY' and 2365:
INSERT INTO emp VALUES (2365, 'BESTRY', 'PROGRAMMER', 2000, 20)
Coding a statement like this into an application would severely limit its usefulness. You would need to change the statement and recompile the program each time you add a new employee to the database. To make the program more flexible, you can write the program so that a user can supply input data at runtime.
When you prepare a SQL statement or PL/SQL block that contains input data to be supplied at runtime, placeholders in the SQL statement or PL/SQL block mark where data must be supplied. For example, the following SQL statement contains five placeholders, indicated by the leading colons (for example, :ename), that show where input data must be supplied by the program.
INSERT INTO emp VALUES (:empno, :ename, :job, :sal, :deptno)
You can use placeholders for input variables in any DELETE, INSERT, SELECT, or UPDATE statement, or PL/SQL block, in any position in the statement where you can use an expression or a literal value. In PL/SQL, placeholders can also be used for output variables.
For each placeholder in the SQL statement or PL/SQL block, you must call an OCI routine that binds the address of a variable in your program to the placeholder. When the statement executes, Oracle gets the data that your program placed in the input, or bind, variables and passes it to the server with the SQL statement. Data does not have to be in a bind variable when you perform the bind step. At the bind step, you are only specifying the address, datatype, and length of the variable.
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Note: If program variables do not contain data at bind time, make sure they contain valid data when you execute the SQL statement or PL/SQL block using |
For example, given the INSERT statement
INSERT INTO emp VALUES (:empno, :ename, :job, :sal, :deptno)
and the following variable declarations
text *ename, *job; sword empno, sal, deptno;
the bind step makes an association between the placeholder name and the address of the program variables. The bind also indicates the datatype and length of the program variables, as illustrated in Figure 5-1.
| See Also:
The code that implements this example is found in the section "Steps Used in Binding". |
If you change only the value of a bind variable, it is not necessary to rebind in order to execute the statement again. The bind is a bind by reference, so as long as the address of the bind variable and bind handle remain valid, you can reexecute a statement that references the variable without rebinding.
In the Oracle server, new datatypes have been implemented for named datatypes, REFs and LOBs, and they may be bound as placeholders in a SQL statement.
The SQL statement in the previous section is an example of a named bind. Each placeholder in the statement has a name associated with it, such as 'ename' or 'sal'. When this statement is prepared and the placeholders are associated with values in the application, the association is made by the name of the placeholder using the OCIBindByName() call with the name of the placeholder passed in the placeholder parameter.
A second type of bind is known as a positional bind. In a positional bind, the placeholders are referred to by their position in the statement rather than their names. For binding purposes, an association is made between an input value and the position of the placeholder, using the OCIBindByPos() call.
The example from the previous section could also be used for a positional bind:
INSERT INTO emp VALUES (:empno, :ename, :job, :sal, :deptno)
The five placeholders would then each be bound by calling OCIBindByPos() and passing the position number of the placeholder in the position parameter. For example, the :empno placeholder would be bound by calling OCIBindByPos() with a position of 1, :ename with a position of 2, and so on.
In the case of a duplicate bind, only a single bind call may be necessary. Consider the following SQL statement, which queries the database for those employees whose commission and salary are both greater than a given amount:
SELECT empno FROM emp WHERE sal > :some_value AND comm > :some_value
An OCI application could complete the binds for this statement with a single call to OCIBindByName() to bind the :some_value placeholder by name. In this case, the second placeholder inherits the bind information from the first placeholder.
You can pass data to Oracle in various ways. You can execute a SQL statement repeatedly using the OCIStmtExecute() routine and supply different input values on each iteration. Alternatively, you can use the Oracle array interface and input many values with a single statement and a single call to OCIStmtExecute(). In this case you bind an array to an input placeholder, and the entire array can be passed at the same time, under the control of the iters parameter.
The array interface significantly reduces round trips to Oracle when you need to update or insert a large volume of data. This reduction can lead to considerable performance gains in a busy client/server environment. For example, consider an application that needs to insert 10 rows into the database. Calling OCIStmtExecute() ten times with single values results in ten network round trips to insert all the data. The same result is possible with a single call to OCIStmtExecute() using an input array, which involves only one network round trip.
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Note: When using the OCI array interface to perform inserts, row triggers in the database are fired as each row of the insert gets inserted. |
You process a PL/SQL block by placing the block in a string variable, binding any variables, and executing the statement containing the block, just as you would with a single SQL statement.
When you bind placeholders in a PL/SQL block to program variables, you must use OCIBindByName() or OCIBindByPos() to perform the basic bind binds. You can use OCIBindByName() or OCIBindByPos() to bind host variables that are either scalars or arrays.
The following short PL/SQL block contains two placeholders, which represent IN parameters to a procedure that updates an employee's salary, given the employee number and the new salary amount:
char plsql_statement[] = "BEGIN\ RAISE_SALARY(:emp_number, :new_sal);\ END;" ;
These placeholders can be bound to input variables in the same way as placeholders in a SQL statement.
When processing PL/SQL statements, output variables are also associated with program variables using bind calls.
For example, in a PL/SQL block such as
BEGIN SELECT ename,sal,comm INTO :emp_name, :salary, :commission FROM emp WHERE ename = :emp_number; END;
you would use OCIBindByName() to bind variables in place of the :emp_name, :salary, and :commission output placeholders, and in place of the input placeholder :emp_number.
| See Also:
For more information about binding PL/SQL placeholders see "Information for Named Datatype and REF Binds". |
Binding placeholders is done in one or more steps. For a simple scalar or array bind, it is only necessary to specify an association between the placeholder and the data. This is done by using OCI bind by name (OCIBindByName()) or OCI bind by position (OCIBindByPos()) call.
| See Also:
See the section "Named Binds and Positional Binds" for information about the difference between these types of binds. |
Once the bind is complete, the OCI library knows where to find the input data (or where to put PL/SQL output data) when the SQL statement is executed. As mentioned in the section "Binding", program input data does not need to be in the program variable when it is bound to the placeholder, but the data must be there when the statement is executed.
The following code example shows handle allocation and binding for each of five placeholders in a SQL statement.
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Note: The |
... /* The SQL statement, associated with stmthp (the statement handle) by calling OCIStmtPrepare() */ text *insert = (text *) "INSERT INTO emp(empno, ename, job, sal, deptno)\ VALUES (:empno, :ename, :job, :sal, :deptno)"; ... /* Bind the placeholders in the SQL statement, one for each bind handle. */ checkerr(errhp, OCIBindByName(stmthp, &bnd1p, errhp, (text *) ":ENAME", strlen(":ENAME"), (ub1 *) ename, enamelen+1, STRING_TYPE, (dvoid *) 0, (ub2 *) 0, (ub2) 0, (ub4) 0, (ub4 *) 0, OCI_DEFAULT)) checkerr(errhp, OCIBindByName(stmthp, &bnd2p, errhp, (text *) ":JOB", strlen(":JOB"), (ub1 *) job, joblen+1, STRING_TYPE, (dvoid *) &job_ind, (ub2 *) 0, (ub2) 0, (ub4) 0, (ub4 *) 0, OCI_DEFAULT)) checkerr(errhp, OCIBindByName(stmthp, &bnd3p, errhp, (text *) ":SAL", strlen(":SAL"), (ub1 *) &sal, (sword) sizeof(sal), INT_TYPE, (dvoid *) &sal_ind, (ub2 *) 0, (ub2) 0, (ub4) 0, (ub4 *) 0, OCI_DEFAULT)) checkerr(errhp, OCIBindByName(stmthp, &bnd4p, errhp, (text *) ":DEPTNO", strlen(":DEPTNO"), (ub1 *) &deptno,(sword) sizeof(deptno), INT_TYPE, (dvoid *) 0, (ub2 *) 0, (ub2) 0, (ub4) 0, (ub4 *) 0, OCI_DEFAULT)) checkerr(errhp, OCIBindByName(stmthp, &bnd5p, errhp, (text *) ":EMPNO", strlen(":EMPNO"), (ub1 *) &empno, (sword) sizeof(empno), INT_TYPE, (dvoid *) 0, (ub2 *) 0, (ub2) 0, (ub4) 0, (ub4 *) 0,OCI_DEFAULT))
Perhaps the most common use for PL/SQL blocks in an OCI program is to call stored procedures or stored functions. For example, assume that there is a procedure called RAISE_SALARY stored in the database, and you want to call this procedure from an OCI program. You do this by embedding a call to that procedure in an anonymous PL/SQL block, then processing the PL/SQL block in the OCI program.
The following program fragment shows how to embed a stored procedure call in an OCI application. For the sake of brevity, only the relevant portions of the program are reproduced here.
The program passes an employee number and a salary increase as inputs to a stored procedure called raise_salary, which takes these parameters:
raise_salary (employee_num IN, sal_increase IN, new_salary OUT);
This procedure raises a given employee's salary by a given amount. The increased salary which results is returned in the stored procedure's OUT variable new_salary, and the program displays this value.
/* Define PL/SQL statement to be used in program. */ text *give_raise = (text *) "BEGIN\ RAISE_SALARY(:emp_number,:sal_increase, :new_salary);\ END;"; OCIBind *bnd1p = NULL; /* the first bind handle */ OCIBind *bnd2p = NULL; /* the second bind handle */ OCIBind *bnd3p = NULL; /* the third bind handle */ static void checkerr(); sb4 status; main() { sword empno, raise, new_sal; dvoid *tmp; OCISession *usrhp = (OCISession *)NULL; ... /* attach to database server, and perform necessary initializations and authorizations */ ... /* allocate a statement handle */ checkerr(errhp, OCIHandleAlloc( (dvoid *) envhp, (dvoid **) &stmthp, OCI_HTYPE_STMT, 100, (dvoid **) &tmp)); /* prepare the statement request, passing the PL/SQL text block as the statement to be prepared */ checkerr(errhp, OCIStmtPrepare(stmthp, errhp, (text *) give_raise, (ub4) strlen(give_raise), OCI_NTV_SYNTAX, OCI_DEFAULT)); /* bind each of the placeholders to a program variable */ checkerr( errhp, OCIBindByName(stmthp, &bnd1p, errhp, (text *) ":emp_number", -1, (ub1 *) &empno, (sword) sizeof(empno), SQLT_INT, (dvoid *) 0, (ub2 *) 0, (ub2) 0, (ub4) 0, (ub4 *) 0, OCI_DEFAULT)); checkerr( errhp, OCIBindByName(stmthp, &bnd2p, errhp, (text *) ":sal_increase", -1, (ub1 *) &raise, (sword) sizeof(raise), SQLT_INT, (dvoid *) 0, (ub2 *) 0, (ub2) 0, (ub4) 0, (ub4 *) 0, OCI_DEFAULT)); /* remember that PL/SQL OUT variable are bound, not defined */ checkerr( OCIBindByName(stmthp, &bnd3p, errhp, (text *) ":new_salary", -1, (ub1 *) &new_sal, (sword) sizeof(new_sal), SQLT_INT, (dvoid *) 0, (ub2 *) 0, (ub2) 0, (ub4) 0, (ub4 *) 0, OCI_DEFAULT)); /* prompt the user for input values */ printf("Enter the employee number: "); scanf("%d", &empno); /* flush the input buffer */ myfflush(); printf("Enter employee's raise: "); scanf("%d", &raise); /* flush the input buffer */ myfflush(); /* execute PL/SQL block*/ checkerr(errhp, OCIStmtExecute(svchp, stmthp, errhp, (ub4) 1, (ub4) 0, (OCISnapshot *) NULL, (OCISnapshot *) NULL, OCI_DEFAULT)); /* display the new salary, following the raise */ printf("The new salary is %d\n", new_sal); }
The following is one possible sample output from this program. Before execution, the salary of employee 7954 is 2000.
Enter the employee number: 7954 Enter employee's raise: 1000 The new salary is 3000.
The previous section and example demonstrated how to perform a simple scalar bind. In that case, only a single bind call is necessary. In some cases, additional bind calls are necessary to define specific attributes for specific bind datatypes or execution modes. These more sophisticated bind operations are discussed in the following section.
Oracle also provides predefined C datatypes that map object attributes.
| See Also:
Information about binding these datatypes, such as OCIDate and OCINumber, can be found in Chapter 12, "Direct Path Loading". |
The section "What is Binding?" discussed how a basic bind operation is performed to create an association between a placeholder in a SQL statement and a program variable using OCIBindByName() or OCIBindByPos().
This section covers more advanced bind operations, including multi-step binds, and binds of named data types and REFs.
In certain cases, additional bind calls are necessary to define specific attributes for certain bind data types or certain execution modes.
The following sections describe these special cases, and the information about binding is summarized in Table 5-1, "Bind Information for Different Bind Types".
For information on binding named data types (objects),
For information on this topic,
There are two ways of binding LOBs:
Both of these ways are discussed next.
Either a single locator or an array of locators can be bound in a single bind call. In each case, the application must pass the address of a LOB locator and not the locator itself. For example, if an application has prepared a SQL statement like
INSERT INTO some_table VALUES (:one_lob)
where one_lob is a bind variable corresponding to a LOB column, and has made the following declaration:
OCILobLocator * one_lob;
Then the following sequence of steps would be used to bind the placeholder, and execute the statement
/* initialize single locator */ one_lob = OCIDescriptorAlloc(...OCI_DTYPE_LOB...); ... /* pass the address of the locator */ OCIBindByName(...,(dvoid *) &one_lob,... SQLT_CLOB, ...); OCIStmtExecute(...,1,...) /* 1 is the iters parameter */
You could also do an array insert using the same SQL INSERT statement. In this case, the application would include the following code:
OCILobLocator * lob_array[10]; ... for (i=0; i<10, i++) lob_array[i] = OCIDescriptorAlloc(...OCI_DTYPE_LOB...); /* initialize array of locators */ ... OCIBindByName(...,(dvoid *) lob_array,...); OCIBindArrayOfStruct(...); OCIStmtExecute(...,10,...); /* 10 is the iters parameter */
Note that you must allocate descriptors with the OCIDescriptorAlloc() routine before they can be used. In the case of an array of locators, you must initialize each array element using OCIDescriptorAlloc(). Use OCI_DTYPE_LOB as the type parameter when allocating BLOBs, CLOBs, and NCLOBs. Use OCI_DTYPE_FILE when allocating BFILEs
Oracle allows nonzero binds for INSERTs and UPDATEs of any size LOB. So you can bind up to 4 gigabytes of data into a LOB column using OCIBindByPos(), OCIBindByName(), and PL/SQL binds. Because you can have multiple LOBs in a row, you can bind up to 4 gigabytes of data for each one of those LOBs in the same INSERT or UPDATE statement.
The bind of more than 4 kilobytes of data to a LOB column uses space from the temporary tablespace. Users of this features should make sure that their temporary tablespace is big enough to hold at least the amount of data equal to the sum of all the bind lengths for LOBs. If your temporary tablespace is extendable, it will be extended automatically after the existing space is fully consumed. Use the command:
"CREATE TABLESPACE ... AUTOEXTENT ON ... TEMPORARY ...;"
to create an extendable temporary tablespace.
LONG and LOB columns, then you can have binds of greater than 4 kilobytes for either the LONG column or the LOB columns, but not both in the same statement.OCILob*() functions.INSERT AS SELECT operation, Oracle does not allow binding of any length data to LOB columns.HEX to RAW or RAW to HEX for data of size more than 4000 bytes. The following PL/SQL code illustrates this:
create table t (c1 clob, c2 blob); declare text varchar(32767); binbuf raw(32767); begin text := lpad ('a', 12000, 'a'); binbuf := utl_raw.cast_to_raw(text); -- The following works ... insert into t values (text, binbuf); -- The following won't work because Oracle won't do implicit -- hex to raw conversion. insert into t (c2) values (text); -- The following won't work because Oracle won't do implicit -- raw to hex conversion. insert into t (c1) values (binbuf); -- The following won't work because we can't combine the -- utl_raw.cast_to_raw() operator with the >4k bind. insert into t (c2) values (utl_raw.cast_to_raw(text)); end; /
BLOB or a CLOB, and the data is filtered by a SQL operator, then Oracle will limit the size of the result to at most 4000 bytes.
For example:
create table t (c1 clob, c2 blob); -- The following command inserts only 4000 bytes because the result of -- LPAD is limited to 4000 bytes insert into t(c1) values (lpad('a', 5000, 'a')); -- The following command inserts only 2000 bytes because the result of -- LPAD is limited to 4000 bytes, and the implicit hex to raw conversion -- converts it to 2000 bytes of RAW data. insert into t(c2) values (lpad('a', 5000, 'a'));
Consider the following SQL statements which will be used in the examples that follow:
CREATE TABLE foo( a INTEGER ); CREATE TYPE lob_typ( A1 CLOB ); CREATE TABLE lob_long_tab (C1 CLOB, C2 CLOB, CT3 lob_typ, L LONG);
void insert() /* A function in an OCI program */ { /* The following is allowed */ ub1 buffer[8000]; text *insert_sql = "INSERT INTO lob_long_tab (C1, C2, L) VALUES (:1, :2, :3)"; OCIStmtPrepare(stmthp, errhp, insert_sql, strlen((char*)insert_sql), (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[0], errhp, 1, (dvoid *)buffer, 8000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[1], errhp, 2, (dvoid *)buffer, 8000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[2], errhp, 3, (dvoid *)buffer, 2000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIStmtExecute(svchp, stmthp, errhp, 1, 0, OCI_DEFAULT); }
void insert() { /* The following is allowed */ ub1 buffer[8000]; text *insert_sql = "INSERT INTO lob_long_tab (C1, L) VALUES (:1, :2)"; OCIStmtPrepare(stmthp, errhp, insert_sql, strlen((char*)insert_sql), (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[0], errhp, 1, (dvoid *)buffer, 2000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[1], errhp, 2, (dvoid *)buffer, 8000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIStmtExecute(svchp, stmthp, errhp, 1, 0, OCI_DEFAULT); }
void insert() { /* The following is allowed, no matter how many rows it updates */ ub1 buffer[8000]; text *insert_sql = (text *)"UPDATE lob_long_tab SET C1 = :1, C2=:2, L=:3"; OCIStmtPrepare(stmthp, errhp, insert_sql, strlen((char*)insert_sql), (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[0], errhp, 1, (dvoid *)buffer, 8000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[1], errhp, 2, (dvoid *)buffer, 8000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[2], errhp, 3, (dvoid *)buffer, 2000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIStmtExecute(svchp, stmthp, errhp, 1, 0, OCI_DEFAULT); }
void insert() { /* The following is allowed, no matter how many rows it updates */ ub1 buffer[8000]; text *insert_sql = (text *)"UPDATE lob_long_tab SET C1 = :1, C2=:2, L=:3"; OCIStmtPrepare(stmthp, errhp, insert_sql, strlen((char*)insert_sql), (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[0], errhp, 1, (dvoid *)buffer, 2000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[1], errhp, 2, (dvoid *)buffer, 2000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[2], errhp, 3, (dvoid *)buffer, 8000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIStmtExecute(svchp, stmthp, errhp, 1, 0, OCI_DEFAULT); }
void insert() { /* Piecewise, callback and array insert/update operations similar to * the allowed regular insert/update operations are also allowed */ }
void insert() { /* The following is NOT allowed because we try to insert >4000 bytes * to both LOB and LONG columns */ ub1 buffer[8000]; text *insert_sql = (text *)"INSERT INTO lob_long_tab (C1, L) VALUES (:1, :2)"; OCIStmtPrepare(stmthp, errhp, insert_sql, strlen((char*)insert_sql), (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[0], errhp, 1, (dvoid *)buffer, 8000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[1], errhp, 2, (dvoid *)buffer, 8000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIStmtExecute(svchp, stmthp, errhp, 1, 0, OCI_DEFAULT); }
void insert() { /* The following is NOT allowed because we try to insert data into * LOB attributes */ ub1 buffer[8000]; text *insert_sql = (text *)"INSERT INTO lob_long_tab (CT3) VALUES (lob_typ(:1))"; OCIStmtPrepare(stmthp, errhp, insert_sql, strlen((char*)insert_sql), (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[0], errhp, 1, (dvoid *)buffer, 2000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIStmtExecute(svchp, stmthp, errhp, 1, 0, OCI_DEFAULT); }
void insert() { /* The following is NOT allowed because we try to do insert as * select character data into LOB column */ ub1 buffer[8000]; text *insert_sql = (text *)"INSERT INTO lob_long_tab (C1) SELECT :1 from FOO"; OCIStmtPrepare(stmthp, errhp, insert_sql, strlen((char*)insert_sql), (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT); OCIBindByPos(stmthp, &bindhp[0], errhp, 1, (dvoid *)buffer, 8000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIStmtExecute(svchp, stmthp, errhp, 1, 0, OCI_DEFAULT); }
Other update operations similar to the disallowed insert operations are also not allowed. Piecewise and callback INSERT or UPDATE operations similar to the disallowed regular INSERT or UPDATE operations are also not allowed.
| See Also:
For more information about the OCILob functions, refer to Chapter 7, "LOB and FILE Operations". |
When using a FILE locator as a bind variable for an INSERT or UPDATE statement, you must first initialize the locator with a directory alias and filename (using OCILobFileSetName()) before issuing the INSERT or UPDATE statement.
If the mode parameter in a call to OCIBindByName() or OCIBindByPos() is set to OCI_DATA_AT_EXEC, an additional call to OCIBindDynamic() is necessary if the application will use the callback method for providing data at runtime. The call to OCIBindDynamic() sets up the callback routines, if necessary, for indicating the data or piece that is being provided.
If the OCI_DATA_AT_EXEC mode is chosen, but the standard OCI piecewise polling method will be used instead of callbacks, the call to OCIBindDynamic() is not necessary.
When binding RETURN clause variables, an application must use OCI_DATA_AT_EXEC mode, and it must provide callbacks.
| See Also:
For more information about piecewise operations, please refer to the section "Runtime Data Allocation and Piecewise Operations". |
Ref Cursors are bound to a statement handle with a bind datatype of SQLT_RSET.
The following table summarizes the bind calls necessary for different types of binds. For each type, the table lists the bind datatype (passed in the dty parameter of OCIBindByName() or OCIBindByPos()), and notes about the bind
| See Also:
For more information about datatypes and datatype codes, see Chapter 3, "Datatypes". |
Query statements return data from the database to your application. When processing a query, you must define an output variable or an array of output variables for each item in the select-list from which you want to retrieve data. The define step creates an association that determines where returned results are stored, and in what format.
For example, if your OCI statement processes the following statement:
SELECT name, ssn FROM employees WHERE empno = :empnum
you would normally need to define two output variables, one to receive the value returned from the name column, and one to receive the value returned from the ssn column.
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Note: If you were only interested in retrieving values from the |
If the SELECT statement being processed might return more than a single value for a query, the output variables you define may be arrays instead of scalar values.
Depending on the application, the define step can take place before or after the execute. If the datatypes of select-list items are known when the application is coded, the define can take place before the statement is executed. If your application is processing dynamic SQL statements--statements entered by you at runtime-- or statements that do not have a clearly defined select-list, such as
SELECT * FROM employees
the application must execute the statement and retrieve describe information before defining output variables.
| See Also:
See the section "Describing Select-List Items" for more information. |
The OCI processes the define call locally, on the client side. In addition to indicating the location of buffers where results should be stored, the define step also determines what type of data conversions, if any, will take place when data is returned to the application.
The dty parameter of the OCIDefineByPos() call specifies the datatype of the output variable. The OCI is capable of a wide range of data conversions when data is fetched into the output variable. For example, internal data in Oracle DATE format can be automatically converted to a string datatype on output.
| See Also:
For more information about datatypes and conversions, refer to Chapter 3, "Datatypes". |
Defining output variables is done in one or more steps. A basic define is accomplished with the OCI define by position call, OCIDefineByPos(). This step creates an association between a select-list item and an output variable. Additional define calls may be necessary for certain datatypes or fetch modes.
Once the define step is complete, the OCI library knows where to put retrieved data after fetching it from the database.
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Note: You can make your define calls again to redefine the output variables without having to reprepare or reexecute the SQL statement. |
The following example code shows a scalar output variable being defined following an execute and a describe.
/* The following statement was prepared, and associated with statement handle stmthp1. SELECT dname FROM dept WHERE deptno = :dept_input The input placeholder was bound earlier, and the data comes from the user input below */ printf("Enter employee dept: "); scanf("%d", &deptno); myfflush(); /* Execute the statement. If OCIStmtExecute() returns OCI_NO_DATA, meaning that no data matches the query, then the department number is invalid. */ if ((status = OCIStmtExecute(svchp, stmthp1, errhp, 0, 0, 0, 0, OCI_DEFAULT)) && (status != OCI_NO_DATA)) { checkerr(errhp, status); do_exit(EXIT_FAILURE); } if (status == OCI_NO_DATA) { printf("The dept you entered doesn't exist.\n"); return 0; } /* The next two statements describe the select-list item, dname, and return its length */ checkerr(errhp, OCIParamGet(stmthp1, errhp, &parmdp, (ub4) 1)); checkerr(errhp, OCIAttrGet((dvoid*) parmdp, (ub4) OCI_DTYPE_PARAM, (dvoid*) &deptlen, (ub4 *) 0, (ub4) OCI_ATTR_DATA_SIZE, (OCIError *) errhp )); /* Use the retrieved length of dname to allocate an output buffer, and then define the output variable. If the define call returns an error, exit the application */ dept = (text *) malloc((int) deptlen + 1); if (status = OCIDefineByPos(stmthp1, &defnp, errhp, 1, (ub1 *) dept, deptlen+1, SQLT_STRING, (dvoid *) 0, (ub2 *) 0, OCI_DEFAULT)) { checkerr(errhp, status); do_exit(EXIT_FAILURE); }
| See Also:
For an explanation of the describe step, see the section "Describing Select-List Items". |
In some cases the define step requires more than just a call to OCIDefineByPos(). There are additional calls that define the attributes of an array fetch (OCIDefineArrayOfStruct()) or a named data type fetch (OCIDefineObject()). For example, to fetch multiple rows with a column of named data types, all three calls must be invoked for the column; but to fetch multiple rows of scalar columns, OCIDefineArrayOfStruct() and OCIDefineByPos() are sufficient.
| See Also:
These more sophisticated define operations are covered in the section "Advanced Define Operations". |
Oracle also provides pre-defined C datatypes that map object type attributes.
| See Also:
Information about defining these datatypes (for example, OCIDate, OCINumber) can be found in Chapter 11, "Object-Relational Datatypes" |
The section "What is Defining?" discussed how a basic define operation is performed to create an association between a SQL select-list item and an output buffer in an application.
This section covers more advanced defined operations, including multi-step defines, and defines of named data types and REFs.
In some cases the define step requires more than just a call to OCIDefineByPos(). There are additional calls that define the attributes of an array fetch (OCIDefineArrayOfStruct()) or a named data type fetch (OCIDefineObject()). For example, to fetch multiple rows with a column of named data types, all the three calls must be invoked for the column; but to fetch multiple rows of scalar columns only OCIDefineArrayOfStruct() and OCIDefineByPos() are sufficient.
The following sections discuss specific information pertaining to different types of defines.
For information on defining named data type (object) output variables, refer to "Defining Named Datatype Output Variables".
For information on defining REF output variables, refer to "Defining REF Output Variables".
There are two ways of defining LOBs:
Both of these ways are discussed next.
Either a single locator or an array of locators can be defined in a single define call. In each case, the application must pass the address of a LOB locator and not the locator itself. For example, if an application has prepared a SQL statement like:
SELECT lob1 FROM some_table;
where lob1 is the LOB column and one_lob is a define variable corresponding to a LOB column with the following declaration:
OCILobLocator * one_lob;
Then the following sequence of steps would be used to bind the placeholder, and execute the statement
/* initialize single locator */ one_lob = OCIDescriptorAlloc(...OCI_DTYPE_LOB...); ... /* pass the address of the locator */ OCIDefineByPos(... 1, ...,(dvoid *) &one_lob,... SQLT_CLOB, ...); OCIStmtExecute(...,1,...) /* 1 is the iters parameter */
You could also do an array select using the same SQL SELECT statement. In this case, the application would include the following code:
OCILobLocator * lob_array[10]; ... for (i=0; i<10, i++) lob_array[i] = OCIDescriptorAlloc(...OCI_DTYPE_LOB...); /* initialize array of locators */ ... OCIDefineByPos(...,1, (dvoid *) lob_array,... SQLT_CLOB, ...); OCIDefineArrayOfStruct(...); OCIStmtExecute(...,10,...); /* 10 is the iters parameter */
Note that you must allocate descriptors with the OCIDescriptorAlloc() routine before they can be used. In the case of an array of locators, you must initialize each array element using OCIDescriptorAlloc(). Use OCI_DTYPE_LOB as the type parameter when allocating BLOBs, CLOBs, and NCLOBs. Use OCI_DTYPE_FILE when allocating BFILEs
Oracle allows nonzero defines for SELECTs of any size LOB. So you can select up to 4 gigabytes of data from a LOB column using OCIDefineByPos(), and PL/SQL defines. Because you can have multiple LOBs in a row, you can select up to 4 gigabytes of data from each one of those LOBs in the same SELECT statement.
Consider the following SQL statements which will be used in the examples that follow:
CREATE TABLE lob_tab (C1 CLOB, C2 CLOB);
void select_define_before_execute() /* A function in an OCI program */ { /* The following is allowed */ ub1 buffer1[8000]; ub1 buffer2[8000]; text *select_sql = "SELECT c1, c2 FROM lob_tab"; OCIStmtPrepare(stmthp, errhp, select_sql, strlen((char*)select_sql), (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT); OCIDefineByPos(stmthp, &defhp[0], errhp, 1, (dvoid *)buffer1, 8000, SQLT_LNG, 0, 0, 0, (ub4) OCI_DEFAULT); OCIDefineByPos(stmthp, &defhp[1], errhp, 2, (dvoid *)buffer2, 8000, SQLT_LNG, 0, 0, 0, (ub4) OCI_DEFAULT); OCIStmtExecute(svchp, stmthp, errhp, 1, 0, OCI_DEFAULT); }
void select_execute_before_define() { /* The following is allowed */ ub1 buffer1[8000]; ub1 buffer2[8000]; text *select_sql = "SELECT c1, c2 FROM lob_tab"; OCIStmtPrepare(stmthp, errhp, select_sql, strlen((char*)select_sql), (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT); OCIStmtExecute(svchp, stmthp, errhp, 0, 0, OCI_DEFAULT); OCIDefineByPos(stmthp, &bindhp[0], errhp, 1, (dvoid *)buffer1, 8000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIDefineByPos(stmthp, &bindhp[1], errhp, 2, (dvoid *)buffer2, 8000, SQLT_LNG, 0, 0, 0, 0, 0, (ub4) OCI_DEFAULT); OCIStmtFetch(stmthp, errhp, 1, OCI_FETCH_NEXT, OCI_DEFAULT); }
void select() { /* Piecewise, callback and array select operations similar to * the allowed regular select operations are also allowed */ }
You do not use the define calls to define output variables for select-list items in a SQL SELECT statement in a PL/SQL block. You must use OCI bind calls instead.
| See Also:
See the section "Information for Named Datatype and REF Defines, and PL/SQL OUT Binds" for more information about defining PL/SQL output variables. |
When performing a piecewise fetch, an initial call to OCIDefineByPos() is required. An additional call to OCIDefineDynamic() is necessary if the application will use callbacks rather than the standard polling mechanism for fetching data.
| See Also:
See the section "Runtime Data Allocation and Piecewise Operations" for more information. |
When using arrays of structures, an initial call to OCIDefineByPos() is required. An additional call to OCIDefineArrayOfStruct() is necessary to set up additional parameters, including the skip parameter necessary for arrays of structures operations.
The arrays of structures functionality of OCI can simplify the processing of multi-row, multi-column operations. The OCI programmer can create a structure of related scalar data items and then fetch values from the database into an array of these structures or insert values into the database from an array of these structures.
For example, an application may need to fetch multiple rows of data from three columns named NAME, AGE, and SALARY. The OCI application could include the definition of a structure containing separate fields to hold the NAME, AGE and SALARY data from one row in the database table. The application would then fetch data into an array of these structures.
In order to perform a multi-row, multi-column operation using an array of structures, the developer associates each column involved in the operation with a field in a structure. This association, which is part of the OCIDefineArrayOfStruct() and OCIBindArrayOfStruct() calls, specifies where fetched data will be stored, or where inserted or updated data will be found.
Figure 5-2, "Fetching Data Into an Array of Structures" is a graphical representation of this process. In the figure, an application fetches various fields from a database row into a single structure in an array of structures. Each column being fetched corresponds to one of the fields in the structure.
When you split column data across an array of structures, it is no longer contiguous. The single array of structures stores data as though it were composed of several interleaved arrays of scalars. Because of this fact, developers must specify a "skip parameter" for each field they are binding or defining. This skip parameter specifies the number of bytes that need to be skipped in the array of structures before the same field is encountered again. In general this will be equivalent to the byte size of one structure.
The figure below demonstrates how a skip parameter is determined. In this case the skip parameter is the sum of the sizes of the fields field1, field2, andfield3, which is 8 bytes. This equals the size of one structure.
On some systems it may be necessary to set the skip parameter to be sizeof(one_array_element) rather than sizeof(struct). This is because some compilers may insert padding into a structure. For example, consider an array of C structures consisting of two fields, a ub4 and a ub1.
struct demo { ub4 field1; ub1 field2; }; struct demo demo_array[MAXSIZE];
Some compilers insert three bytes of padding after the ub1 so that the ub4 which begins the next structure in the array is properly aligned. In this case, the following statement may return an incorrect value:
skip_parameter = sizeof(struct demo);
On some systems this will produce a proper skip parameter of eight. On other systems, skip_parameter will be set to five bytes by this statement. In this case, use the following statement to get the correct value for the skip parameter:
skip_parameter = sizeof(demo_array[0]);
The ability to work with arrays of structures is an extension of the functionality for binding and defining arrays of program variables. Programmers can also work with standard arrays (as opposed to arrays of structures). When specifying a standard array operation, the related skip will be equal to the size of the datatype of the array under consideration. For example, for an array declared as
text emp_names[4][20];
the skip parameter for the bind or define operation will be 20. Each data element in the array is then recognized as a separate unit, rather than being part of a structure.
Two OCI calls must be used when performing operations involving arrays of structures: OCIBindArrayOfStruct() (for binding fields in arrays of structures for input variables) and OCIDefineArrayOfStruct() (for defining arrays of structures for output variables).
| See Also:
See the descriptions of |
The implementation of arrays of structures also supports the use of indicator variables and return codes. OCI application developers can declare parallel arrays of column-level indicator variables and return codes, corresponding to the arrays of information being fetched, inserted, or updated. These arrays can have their own skip parameters, which are specified during a call to OCIBindArrayOfStruct() or OCIDefineArrayOfStruct().
You can set up arrays of structures of program values and indicator variables in many ways. For example, consider an application that fetches data from three database columns into an array of structures containing three fields. You can set up a corresponding array of indicator variable structures of three fields, each of which is a column-level indicator variable for one of the columns being fetched from the database.
|
Note: A one-to-one relationship between the fields in an indicator struct and the number of select-list items is not necessary. |
| See Also:
See "Indicator Variables" for more information about indicator variables. |
The OCI supports the use of the RETURNING clause with SQL INSERT, UPDATE, and DELETE statements. This section outlines the rules an OCI application must follow to correctly implement DML statements with the RETURNING clause.
|
Note: For more information about the use of the |
| See Also:
For a complete code example, see the demonstration programs included with your Oracle installation. For additional information, refer to Appendix B, "OCI Demonstration Programs" |
Using the RETURNING clause with a DML statement nonzero you to essentially combine two SQL statements into one, possibly saving you a server round trip. This is accomplished by adding an extra clause to the traditional UPDATE, INSERT, and DELETE statements. The extra clause effectively adds a query to the DML statement.
In the OCI, the values are returned to the application through the use of OUT bind variables. The rules for binding these variables are described in the next section. In the following examples, the bind variables are indicated by the preceding colon, such as :out1. These examples assume the existence of a table called table1, which contains three columns: col1, col2, and col3.
For example, the following statement inserts new values into the database and then retrieves the column values of the affected row from the database, allowing your application to work with inserted rows.
INSERT INTO table1 VALUES (:1, :2, :3,) RETURNING col1, col2, col3 INTO :out1, :out2, :out3
The next example uses the UPDATE statement. This statement updates the values of all columns whose col1 value falls within a certain range, and then returns the affected rows to the application, allowing the application to see which rows were modified.
UPDATE table1 SET col1 = col1 + :1, col2 = :2, col3 = :3 WHERE col1 >= :low AND col1 <= :high RETURNING col1, col2, col3 INTO :out1, :out2, :out3
The following DELETE statement deletes the rows whose col1 value falls within a certain range, and then returns the data from those rows so that the application can check them.
DELETE FROM table1 WHERE col1 >= :low AND col2 <= :high RETURNING col1, col2, col3 INTO :out1, :out2, :out3
Note that in both the UPDATE and DELETE examples there is the possibility that the statement will affect multiple rows in the table. Additionally, a DML statement could be executed multiple times in a single OCIExecute() statement. Because of this possibility for multiple returning values, an OCI application may not know how much data will be returned at runtime. As a result, the variables corresponding to the RETURNING...INTO placeholders must be bound in OCI_DATA_AT_EXEC mode. It is an additional requirement that the application must define its own dynamic data handling callbacks rather than using the OCI_DATA_AT_EXEC polling mechanism.
|
Note: Even if the application can be sure that it will only get a single value back in the |
The returning clause can be particularly useful when working with LOBs. Normally, an application must insert an empty LOB locator into the database, and then SELECT it back out again to operate on it. Using the RETURNING clause, the application can combine these two steps into a single statement:
INSERT INTO some_table VALUES (:in_locator) RETURNING lob_column INTO :out_locator
An OCI application implements the placeholders in the RETURNING clause as pure OUT bind variables. However, all binds in the RETURNING clause are initially IN and must be properly initialized. To provide a valid value, you can provide a NULL indicator and set that indicator to -1 (NULL).
An application must adhere to the following rules when working with bind variables in a RETURNING clause:
RETURNING clause placeholders in OCI_DATA_AT_EXEC mode using OCIBindByName() or OCIBindByPos(), followed by a call to OCIBindDynamic() for each placeholder.
Note: The OCI only supports the callback mechanism for RETURNING clause binds. The polling mechanism is not supported.
RETURNING clause placeholders, you must supply a valid out bind function as the ocbfp parameter of the OCIBindDynamic() call. This function must provide storage to hold the returned data.icbfp parameter of OCIBindDynamic() call should provide a "dummy" function which returns NULL values when called.piecep parameter of OCIBindDynamic() must be set to OCI_ONE_PIECE.RETURNING clause, such as no duplication between bind variables in the DML section and the RETURNING section of the statement.The out bind function provided to OCIBindDynamic() must be prepared to receive partial results of a statement in the event of an error. For example, if the application has issued a DML statement which should be executed 10 times, and an error occurs during the fifth iteration, the server will still return the data from iterations 1 through 4. The callback function would still be called to receive data for the first four iterations.
The RETURNING clause can also be used to return a REF to an object which is being inserted into or updated in the database. The following SQL statement shows how this could be used.
UPDATE EXTADDR E SET E.ZIP = '12345', E.STATE='AZ' WHERE E.STATE = 'CA' AND E.ZIP='95117' RETURNING REF(E), ZIP INTO :addref, :zip
This statement updates several attributes of an object in an object table and then returns a REF to the object (along with the scalar ZIP code) in the RETURNING clause.
Binding the REF output variable in an OCI application requires three steps:
OCIBindByName()REF (including the TDO) is set with OCIBindObject()OCIBindDynamic()The following pseudocode shows a function which performs the binds necessary for the above example.
sword bind_output(stmthp, bndhp, errhp) OCIStmt *stmthp; OCIBind *bndhp[]; OCIError *errhp; { ub4 i; /* get TDO for BindObject call */ if (OCITypeByName(envhp, errhp, svchp, (CONST text *) 0, (ub4) 0, (CONST text *) "ADDRESS_OBJECT", (ub4) strlen((CONST char *) "ADDRESS_OBJECT"), (CONST text *) 0, (ub4) 0, OCI_DURATION_SESSION, OCI_TYPEGET_HEADER, &addrtdo)) { return OCI_ERROR; } /* initial bind call for both variables */ if (OCIBindByName(stmthp, &bndhp[2], errhp, (text *) ":addref", (sb4) strlen((char *) ":addref"), (dvoid *) 0, (sb4) sizeof(OCIRef *), SQLT_REF, (dvoid *) 0, (ub2 *)0, (ub2 *)0, (ub4) 0, (ub4 *) 0, (ub4) OCI_DATA_AT_EXEC) || OCIBindByName(stmthp, &bndhp[3], errhp, (text *) ":zip", (sb4) strlen((char *) ":zip"), (dvoid *) 0, (sb4) MAXZIPLEN, SQLT_CHR, (dvoid *) 0, (ub2 *)0, (ub2 *)0, (ub4) 0, (ub4 *) 0, (ub4) OCI_DATA_AT_EXEC)) { return OCI_ERROR; } /* object bind for REF variable */ if (OCIBindObject(bndhp[2], errhp, (OCIType *) addrtdo, (dvoid **) &addrref[0], (ub4 *) 0, (dvoid **) 0, (ub4 *) 0)) { return OCI_ERROR; } for (i = 0; i < MAXCOLS; i++) pos[i] = i; /* dynamic binds for both RETURNING variables */ if (OCIBindDynamic(bndhp[2], errhp, (dvoid *) &pos[0], cbf_no_data, (dvoid *) &pos[0], cbf_get_data) || OCIBindDynamic(bndhp[3], errhp, (dvoid *) &pos[1], cbf_no_data, (dvoid *) &pos[1], cbf_get_data)) { return OCI_ERROR; } return OCI_SUCCESS; }
When a callback function is called, the OCI_ATTR_ROWS_RETURNED attribute of the bind handle tells the application the number of rows being returned in that particular iteration. Thus, when the callback is called the first time in a particular iteration (that is, index=0), you can allocate space for all the rows which will be returned for that bind variable. When the callback is called subsequently (with index>0) within the same iteration, you can merely increment the buffer pointer to the correct memory within the allocated space to retrieve the data.
OCI provides additional functionality for single-row DML operations and array DML operations in which each iteration returns more than one row. To take advantage of this feature, the client application must specify an OUT buffer in the bind call which is at least as big as the iteration count specified in the OCIStmtExecute() call. This is in addition to the method by which bind buffers are provided through callbacks.
When the statement executes, if any of the iterations returns more than one row, then the application receives an OCI_SUCCESS_WITH_INFO return code. In this case, the DML operation is successfully completed. At this point the application may choose to roll back the transaction or ignore the warning.
This section discusses issues involving character conversions between the client and the server.
Oracle provides support for character data in the database, and OCI provides support for binding and defining character data. If a database column containing character data is defined to be an NCHAR/NVARCHAR2 column, then a bind or define involving that column must take into account special considerations for dealing with character set specifications.
These considerations are necessary in case the width of the client character set is different from that on the server, and also for proper character conversion between the client and server. During conversion of data between different character sets, the size of the data may grow or shrink as much as fourfold. Care must be taken to insure that buffers provided to hold the data are of sufficient size.
In some cases, it may also be easier for an application to deal with NCHAR/NVARCHAR2 data in terms of numbers of characters, rather than numbers of bytes (which is the usual case).
Each OCI bind and define handle has OCI_ATTR_CHARSET_FORM and OCI_ATTR_CHARSET_ID attributes associated with it. An application can set these attributes with the OCIAttrSet() call in order to specify the character form and character set ID of the bind/define buffer.
The form attribute (OCI_ATTR_CHARSET_FORM) indicates the character set the client buffer is in, for binds, and the character set in which to store fetched data, for defines. It has two possible values:
The default value is SQLCS_IMPLICIT, which implies the database character set for the bind or define buffer and the character data in the buffer is converted to the server database character set. SQLCS_NCHAR implies national character set ID for the bind or define buffer and the client buffer data is converted to the server national character set.
If the character set ID attribute (OCI_ATTR_CHARSET_ID) is not specified, then the default value of the database or national character set ID of the client is used, depending on the value of form. They are the values specified in the NLS_LANG and NLS_NCHAR environment variables.
| See Also:
For more information about |
As the result of implicit conversion between database character sets and national character sets, OCI can support cross binding and cross defining between CHAR and NCHAR. For example, even though the OCI_ATTR_CHARSET_FORM attribute is set to be SQLCS_NCHAR, OCI enables converting data to the database character set if the data is inserted into a CHAR column.
You can set the character sets through the OCIEnvCreateNLS() function parameters charset and ncharset. Both of these parameters can be set as OCI_UTF16ID. charset controls coding of the metadata and CHAR data. ncharset controls coding of NCHAR data. The function OCINlsEnvironmentVariableGet() returns the character set from NLS_LANG and the national character set from NLS_NCHAR.
Here is a pseudocode example of the use of these functions:
OCIEnv *envhp; ub2 ncsid = 2; /* we8dec */ ub2 hdlcsid, hdlncsid; OraText thename[20]; utext *selstmt = UTF16("SELECT ename FROM emp"); /* make a UTF16 statement */ OCIStmt *stmthp; OCIDefine *defhp; OCIError *errhp; OCIEnvNlsCreate(OCIEnv **envhp, ..., OCI_UTF16ID, ncsid); ... OCIStmtPrepare(stmthp, ..., selstmt, ...); /* prepare UTF16 statement */ OCIDefineByPos(stmthp, defnp, ..., 1, thename, sizeof(thename), SQLT_CHR,...); OCINlsEnvironmentVariableGet(&hdlcsid, 0, OCI_NLS_CHARSET_ID, 0, 0); OCIAttrSet(defnp, ..., &hdlcsid, 0, OCI_ATTR_CHARSET_ID, errhp); /* change charset id to NLS_LANG setting*/ ...
Every bind handle has a OCI_ATTR_MAXDATA_SIZE attribute. This attribute specifies the number of bytes to be allocated on the server to accommodate the client-side bind data after any necessary character set conversions.
|
Note: Character set conversions performed when data is sent to the server may result in the data expanding or contracting, so its size on the client may not be the same as its size on the server. |
An application will typically set OCI_ATTR_MAXDATA_SIZE to the maximum size of the column or the size of the PL/SQL variable, depending on how it is used. Oracle issues an error if OCI_ATTR_MAXDATA_SIZE is not a large enough value to accommodate the data after conversion, and the operation will fail.
The following scenarios demonstrate some examples of the use of the OCI_ATTR_MAXDATA_SIZE attribute:
In this case there are implicit bind conversions taking place on the data. The recommended value of OCI_ATTR_MAXDATA_SIZE in this case would be the size of the source buffer multiplied by the worst-case expansion between the client and server character sets.
In either of these cases, the recommended value of OCI_ATTR_MAXDATA_SIZE is the size of the column.
In this case, the recommended value of OCI_ATTR_MAXDATA_SIZE is the size of the PL/SQL variable.
Bind and define handles have an attribute, OCI_ATTR_MAXCHAR_SIZE, associated with them. An application can use this attribute to work with data in terms of number of characters, rather than number of bytes.
For binds, the OCI_ATTR_MAXCHAR_SIZE attribute sets the number of characters that an application reserves on the server to store the data being bound. This works together with the OCI_ATTR_MAXDATA_SIZE attribute, and the nonzero minimum of their derived byte length is used.
For example, if OCI_ATTR_MAXDATA_SIZE is set to 100, and OCI_ATTR_MAXCHAR_SIZE is set to 0, then the maximum possible size of the data on the server after conversion is 100 bytes. However, if OCI_ATTR_MAXDATA_SIZE is set to 300, and OCI_ATTR_MAXCHAR_SIZE is set to a nonzero value, such as 100, then if the character set has 2 bytes/character, the maximum possible allocated size is 200 bytes, which is the minimum of 300 bytes and 2*100 bytes.
For defines, the OCI_ATTR_MAXCHAR_SIZE attribute specifies the maximum number of characters that the client application allows in the return buffer. Its derived byte length overrides the maxlength parameter specified in the OCIDefineByPos() call.
Update or insert operations are done through variable binding. When binding variables, specify OCI_ATTR_MAXCHAR_SIZE and/or OCI_ATTR_MAXDATA_SIZE in the bind handle to specify character and byte constraints to be used while inserting data in the server.
These attributes are defined as:
If neither of these two attributes is set, OCI expands the buffer using its best estimates.
Do not set OCI_ATTR_MAXDATA_SIZE for OUT binds or for PL/SQL binds.
Only set OCI_ATTR_MAXDATA_SIZE for INSERT or UPDATE statements.
If the underlying column was created using character length semantics, then it is preferable to specify the constraint using OCI_ATTR_MAXCHAR_SIZE. In this case, as long as the actual buffer contains less characters that specified in OCI_ATTR_MAXCHAR_SIZE, no constraints are violated at OCI level.
If the underlying column was created using byte length semantics, then use OCI_ATTR_MAXDATA_SIZE in the bind handle to specify the byte constraint on the server. If you also specify an OCI_ATTR_MAXCHAR_SIZE value, then this constraint is additionally imposed when allocating the receiving buffer on the server side.
For dynamic SQL, you can use the explicit describe to get OCI_ATTR_DATA_SIZE and or OCI_ATTR_CHAR_SIZE in parameter handles as a guide for setting OCI_ATTR_MAXDATA_SIZE and OCI_ATTR_MAXCHAR_SIZE attributes in bind handles. Furthermore, it is always a safer practice to specify OCI_ATTR_MAXDATA_SIZE and OCI_ATTR_MAXCHAR_SIZE to be no more than the actual column width in bytes, and characters.