Programmer's Guide to the Oracle Precompilers, 1.8 Go to Product Documentation Library
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Using Host Arrays

This chapter looks at using arrays to simplify coding and improve program performance. You learn how to manipulate Oracle data using arrays, how to operate on all the elements of an array with a single SQL statement, and how to limit the number of array elements processed. The following questions are answered:

What Is a Host Array?

An array is a collection of related data items, called elements, associated with a single variable name. When declared as a host variable, the array is called a host array. Likewise, an indicator variable declared as an array is called an indicator array. An indicator array can be associated with any host array.

Why Use Arrays?

Arrays can ease programming and offer improved performance. When writing an application, you are usually faced with the problem of storing and manipulating large collections of data. Arrays simplify the task of naming and referencing the individual items in each collection.

Using arrays can boost the performance of your application. Arrays let you manipulate an entire collection of data items with a single SQL statement. Thus, Oracle communication overhead is reduced markedly, especially in a networked environment. For example, suppose you want to insert information about 300 employees into the EMP table. Without arrays your program must do 300 individual INSERTs--one for each employee. With arrays, only one INSERT need be done.

Declaring Host Arrays

You declare host arrays in the Declare Section like simple host variables. You also dimension (set the size of) host arrays in the Declare Section. In the following example, you declare three host arrays and dimension them with 50 elements:

EXEC SQL BEGIN DECLARE SECTION;
    emp_name (50)    CHARACTER(20);
    emp_number (50)  INTEGER;
    salary (50)      REAL;
EXEC SQL END DECLARE SECTION;

Dimensioning Arrays

The maximum dimension of a host array is 32,767 elements. If you use a host array that exceeds the maximum, you get a "parameter out of range" runtime error. If you use multiple host arrays in a single SQL statement, their dimensions should be the same. Otherwise, an "array size mismatch" warning message is issued at precompile time. If you ignore this warning, the precompiler uses the smallest dimension for the SQL operation.

Restrictions

You cannot declare host arrays of pointers. Also, host arrays that might be referenced in a SQL statement are limited to one dimension. So, the two-dimensional array declared in the following example is invalid:

EXEC SQL BEGIN DECLARE SECTION;
    hi_lo_scores (25, 25)  INTEGER;   -- not allowed
EXEC SQL END DECLARE SECTION;

Using Arrays in SQL Statements

The Oracle Precompilers allow the use of host arrays in data manipulation statements. You can use host arrays as input variables in the INSERT, UPDATE, and DELETE statements and as output variables in the INTO clause of SELECT and FETCH statements.

Note: When MODE=ANSI14, array operations are not allowed. In other words, you can reference host arrays in a SQL statement only when MODE={ANSI|ANSI13|ORACLE}.

The syntax used for host arrays and simple host variables is nearly the same. One difference is the optional FOR clause, which lets you control array processing. Also, there are restrictions on mixing host arrays and simple host variables in a SQL statement.

The following sections illustrate the use of host arrays in data manipulation statements.

Selecting into Arrays

You can use host arrays as output variables in the SELECT statement. If you know the maximum number of rows the select will return, simply dimension the host arrays with that number of elements. In the following example, you select directly into three host arrays. Knowing the select will return no more than 50 rows, you dimension the arrays with 50 elements:

EXEC SQL BEGIN DECLARE SECTION;
    emp_name (50)    CHARACTER(20);
    emp_number (50)  INTEGER;
    salary (50)      REAL;
EXEC SQL END DECLARE SECTION;
EXEC SQL SELECT ENAME, EMPNO, SAL
    INTO :emp_name, :emp_number, :salary
    FROM EMP
    WHERE SAL > 1000;

In this example, the SELECT statement returns up to 50 rows. If there are fewer than 50 eligible rows or you want to retrieve only 50 rows, this method will suffice. However, if there are more than 50 eligible rows, you cannot retrieve all of them this way. If you reexecute the SELECT statement, it just returns the first 50 rows again, even if more are eligible. You must either dimension a larger array or declare a cursor for use with the FETCH statement.

If a SELECT INTO statement returns more rows than the number of elements you dimensioned, Oracle issues the error message

SQL-02112: SELECT...INTO returns too many rows

unless you specify SELECT_ERROR=NO. For more information about the option SELECT_ERROR, see page 6 - 34.

Batch Fetches

If you do not know the maximum number of rows a select will return, you can declare and open a cursor, then fetch from it in "batches." Batch fetches within a loop let you retrieve a large number of rows with ease. Each fetch returns the next batch of rows from the current active set. In the following example, you fetch in 20-row batches:

EXEC SQL BEGIN DECLARE SECTION;
    emp_number (20)  INTEGER;
    salary (20)      REAL;
EXEC SQL END DECLARE SECTION;
EXEC SQL DECLARE emp_cursor CURSOR FOR
    SELECT EMPNO, SAL FROM EMP;
EXEC SQL OPEN emp_cursor;
EXEC SQL WHENEVER NOT FOUND DO ...
LOOP
    EXEC SQL FETCH emp_cursor INTO :emp_number, :salary;
    --  process batch of rows
ENDLOOP;

Number of Rows Fetched

Each fetch returns, at most, the number of rows in the array dimension. Fewer rows are returned in the following cases:

The cumulative number of rows returned can be found in the third element of SQLERRD in the SQLCA, called SQLERRD(3) in this guide. This applies to each open cursor. In the following example, notice how the status of each cursor is maintained separately:

EXEC SQL OPEN cursor1;
EXEC SQL OPEN cursor2;
EXEC SQL FETCH cursor1 INTO :array_of_20;
    --  now running total in SQLERRD(3) is 20
EXEC SQL FETCH cursor2 INTO :array_of_30;
    --  now running total in SQLERRD(3) is 30, not 50
EXEC SQL FETCH cursor1 INTO :array_of_20;
    --  now running total in SQLERRD(3) is 40 (20 + 20)
EXEC SQL FETCH cursor2 INTO :array_of_30;
    --  now running total in SQLERRD(3) is 60 (30 + 30)

Restrictions

Using host arrays in the WHERE clause of a SELECT statement is allowed only in a subquery. (For an example, see "Using the WHERE Clause" [*].) Also, you cannot mix simple host variables with host arrays in the INTO clause of a SELECT or FETCH statement; if any of the host variables is an array, all must be arrays.

Table 9 - 1 shows which uses of host arrays are valid in a SELECT INTO statement:

INTO Clause WHERE Clause Valid?
array array no
scalar scalar yes
array scalar yes
scalar array no
Table 9 - 1. Host Arrays Valid in SELECT INTO

Fetching Nulls

When DBMS=V6, if you select or fetch a null into a host array that lacks an indicator array, no error is generated. So, when doing array selects and fetches, always use indicator arrays. That way, you can find nulls in the associated output host array. (To learn how to find nulls and truncated values, see "Using Indicator Variables" [*].)

When DBMS=V7, if you select or fetch a null into a host array that lacks an indicator array, Oracle stops processing, sets SQLERRD(3) to the number of rows processed, and issues the following error message: 

ORA-01405: fetched column value is NULL

Fetching Truncated Values

When DBMS=V7, if you select or fetch a truncated column value into a host array that lacks an indicator array, Oracle stops processing, sets SQLERRD(3) to the number of rows processed, and issues the following error message: 

ORA-01406: fetched column value was truncated

You can check SQLERRD(3) for the number of rows processed before the truncation occurred. The rows-processed count includes the row that caused the truncation error.

When MODE=ANSI, truncation is not considered an error, so Oracle continues processing.

Again, when doing array selects and fetches, always use indicator arrays. That way, if Oracle assigns one or more truncated column values to an output host array, you can find the original lengths of the column values in the associated indicator array.

Inserting with Arrays

You can use host arrays as input variables in an INSERT statement. Just make sure your program populates the arrays with data before executing the INSERT statement. If some elements in the arrays are irrelevant, you can use the FOR clause to control the number of rows inserted. See "Using the FOR Clause" [*].

An example of inserting with host arrays follows:

EXEC SQL BEGIN DECLARE SECTION;
    emp_name (50)    CHARACTER(20);
    emp_number (50)  INTEGER;
    salary (50)      REAL;
EXEC SQL END DECLARE SECTION;
--  populate the host arrays
EXEC SQL INSERT INTO EMP (ENAME, EMPNO, SAL)
    VALUES (:emp_name, :emp_number, :salary);

The cumulative number of rows inserted can be found in SQLERRD(3).

Although functionally equivalent to the following statement, the INSERT statement in the last example is much more efficient because it issues only one call to Oracle:

FOR i = 1 TO array_dimension
    EXEC SQL INSERT INTO EMP (ENAME, EMPNO, SAL)
       VALUES (:emp_name[i], :emp_number[i], :salary[i]);
ENDFOR;

In this imaginary example (imaginary because host variables cannot be subscripted in a SQL statement), you use a FOR loop to access all array elements in sequential order.

Restrictions

You cannot use an array of pointers in the VALUES clause of an INSERT statement; all array elements must be data items. Also, mixing simple host variables with host arrays in the VALUES clause of an INSERT statement is not allowed; if any of the host variables is an array, all must be arrays.

Updating with Arrays

You can also use host arrays as input variables in an UPDATE statement, as the following example shows:

EXEC SQL BEGIN DECLARE SECTION;
    emp_number (50)  INTEGER;
    salary (50)      REAL;
EXEC SQL END DECLARE SECTION;
--  populate the host arrays
EXEC SQL UPDATE EMP SET SAL = :salary WHERE EMPNO = :emp_number;

The cumulative number of rows updated can be found in SQLERRD(3). The number does not include rows processed by an update cascade.

If some elements in the arrays are irrelevant, you can use the FOR clause to limit the number of rows updated.

The last example showed a typical update using a unique key (emp_number). Each array element qualified just one row for updating. In the following example, each array element qualifies multiple rows:

EXEC SQL BEGIN DECLARE SECTION;
    job_title (10)   CHARACTER(10);
    commission (50)  REAL;
EXEC SQL END DECLARE SECTION;
--  populate the host arrays
EXEC SQL UPDATE EMP SET COMM = :commission WHERE JOB = :job_title;

Restrictions

Mixing simple host variables with host arrays in the SET or WHERE clause of an UPDATE statement is not allowed. If any of the host variables is an array, all must be arrays. Furthermore, if you use a host array in the SET clause, you must use one in the WHERE clause. However, their dimensions and datatypes need not match.

You cannot use host arrays with the CURRENT OF clause in an UPDATE statement. For an alternative, see "Mimicking CURRENT OF" [*].

Table 9 - 2 shows which uses of host arrays are valid in an UPDATE statement:

SET Clause WHERE Clause Valid?
array array yes
scalar scalar yes
array scalar no
scalar array no
Table 9 - 2. Host Arrays Valid in UPDATE

Deleting with Arrays

You can also use host arrays as input variables in a DELETE statement. It is like executing the DELETE statement repeatedly using successive elements of the host array in the WHERE clause. Thus, each execution might delete zero, one, or more rows from the table. An example of deleting with host arrays follows:

EXEC SQL BEGIN DECLARE SECTION;
    ... 
   emp_number (50)  INTEGER;
 EXEC SQL END DECLARE SECTION;
--  populate the host array
EXEC SQL DELETE FROM EMP WHERE EMPNO = :emp_number;

The cumulative number of rows deleted can be found in SQLERRD(3). That number does not include rows processed by a delete cascade.

The last example showed a typical delete using a unique key (emp_number). Each array element qualified just one row for deletion. In the following example, each array element qualifies multiple rows:

EXEC SQL BEGIN DECLARE SECTION;
    ...
    job_title (10)  CHARACTER(10);
EXEC SQL END DECLARE SECTION;
--  populate the host array
EXEC SQL DELETE FROM EMP WHERE JOB = :job_title;

Restrictions

Mixing simple host variables with host arrays in the WHERE clause of a DELETE statement is not allowed; if any of the host variables is an array, all must be arrays. Also, you cannot use host arrays with the CURRENT OF clause in a DELETE statement. For an alternative, see "Mimicking CURRENT OF" [*].

Using Indicator Arrays

You use indicator arrays to assign nulls to input host arrays and to detect null or truncated values in output host arrays. The following example shows how to insert with indicator arrays:

EXEC SQL BEGIN DECLARE SECTION;
    emp_number (50)   INTEGER;
    dept_number (50)  INTEGER;
    commission (50)   REAL;
    ind_comm (50)     SMALLINT;   -- indicator array
EXEC SQL END DECLARE SECTION;
--  populate the host arrays
--  populate the indicator array; to insert a null into 
--  the COMM column, assign -1 to the appropriate element in 
--  the indicator array
EXEC SQL INSERT INTO EMP (EMPNO, DEPTNO, COMM)
    VALUES (:emp_number, :dept_number, :commission:ind_comm);

The dimension of the indicator array cannot be smaller than the dimension of the host array.

Using the FOR Clause

You can use the optional FOR clause to set the number of array elements processed by any of the following SQL statements:

The FOR clause is especially useful in UPDATE, INSERT, and DELETE statements. With these statements you might not want to use the entire array. The FOR clause lets you limit the elements used to just the number you need, as the following example shows:

EXEC SQL BEGIN DECLARE SECTION;
    emp_name (100)  CHARACTER(20);
    salary (100)    REAL;
    rows_to_insert  INTEGER;
EXEC SQL END DECLARE SECTION;
--  populate the host arrays
set rows_to_insert = 25;       -- set FOR-clause variable
EXEC SQL FOR :rows_to_insert   -- will process only 25 rows
    INSERT INTO EMP (ENAME, SAL)
    VALUES (:emp_name, :salary);

The FOR clause must use an integer host variable to count array elements. For example, the following statement is illegal:

EXEC SQL FOR 25  -- illegal
    INSERT INTO EMP (ENAME, EMPNO, SAL)
    VALUES (:emp_name, :emp_number, :salary);

The FOR-clause variable specifies the number of array elements to be processed. Make sure the number does not exceed the smallest array dimension. Also, the number must be positive. If it is negative or zero, no rows are processed.

Restrictions

Two restrictions keep FOR clause semantics clear.: you cannot use the FOR clause in a SELECT statement or with the CURRENT OF clause.

In a SELECT Statement

If you use the FOR clause in a SELECT statement, you get the following error message:

PCC-E-0056:  FOR clause not allowed on SELECT statement at ...

The FOR clause is not allowed in SELECT statements because its meaning is unclear. Does it mean "execute this SELECT statement n times"? Or, does it mean "execute this SELECT statement once, but return n rows"? The problem in the former case is that each execution might return multiple rows. In the latter case, it is better to declare a cursor and use the FOR clause in a FETCH statement, as follows:

EXEC SQL FOR :limit FETCH emp_cursor INTO ...

With the CURRENT OF Clause

You can use the CURRENT OF clause in an UPDATE or DELETE statement to refer to the latest row returned by a FETCH statement, as the following example shows:

EXEC SQL DECLARE emp_cursor CURSOR FOR 
    SELECT ENAME, SAL FROM EMP WHERE EMPNO = :emp_number;
... 
EXEC SQL OPEN emp_cursor;
...
EXEC SQL FETCH emp_cursor INTO :emp_name, :salary;
...
EXEC SQL UPDATE EMP SET SAL = :new_salary
    WHERE CURRENT OF emp_cursor;

However, you cannot use the FOR clause with the CURRENT OF clause. The following statements are invalid because the only logical value of limit is 1 (you can only update or delete the current row once):

EXEC SQL FOR :limit UPDATE EMP SET SAL = :new_salary 
    WHERE CURRENT OF emp_cursor;
...
EXEC SQL FOR :limit DELETE FROM EMP 
    WHERE CURRENT OF emp_cursor;

Using the WHERE Clause

Oracle treats a SQL statement containing host arrays of dimension n like the same SQL statement executed n times with n different scalar variables (the individual array elements). The precompiler issues the following error message only when such treatment is ambiguous:

PCC-S-0055: Array <name> not allowed as bind variable at ...

For example, assuming the declarations

EXEC SQL BEGIN DECLARE SECTION;
    mgr_number (50)  INTEGER;
    job_title (50)   CHARACTER(20);
EXEC SQL END DECLARE SECTION;

it would be ambiguous if the statement

EXEC SQL SELECT MGR INTO :mgr_number FROM EMP
    WHERE JOB = :job_title;

were treated like the imaginary statement

FOR i = 1 TO 50
    SELECT MGR INTO :mgr_number[i] FROM EMP
        WHERE JOB = :job_title[i];
ENDFOR;

because multiple rows might meet the WHERE-clause search condition, but only one output variable is available to receive data. Therefore, an error message is issued.

On the other hand, it would not be ambiguous if the statement

EXEC SQL UPDATE EMP SET MGR = :mgr_number
    WHERE EMPNO IN (SELECT EMPNO FROM EMP WHERE JOB = :job_title);

were treated like the imaginary statement

FOR i = 1 TO 50
    UPDATE EMP SET MGR = :mgr_number[i]
        WHERE EMPNO IN
            (SELECT EMPNO FROM EMP WHERE JOB = :job_title[i]);
ENDFOR;

because there is a mgr_number in the SET clause for each row matching job_title in the WHERE clause, even if each job_title matches multiple rows. All rows matching each job_title can be SET to the same mgr_number. So, no error message is issued.

Mimicking the CURRENT OF Clause

You use the CURRENT OF cursor clause in a DELETE or UPDATE statement to refer to the latest row fetched from the cursor. However, you cannot use CURRENT OF with host arrays. 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 BEGIN DECLARE SECTION;
    emp_name (25)   CHARACTER(20);
    job_title (25)  CHARACTER(15);
    old_title (25)  CHARACTER(15);
    row_id (25)     CHARACTER(18);
EXEC SQL END DECLARE SECTION;
...
EXEC SQL DECLARE emp_cursor CURSOR FOR
    SELECT ENAME, JOB, ROWID FROM EMP;
...
EXEC SQL OPEN emp_cursor;
EXEC SQL WHENEVER NOT FOUND GOTO ...
...
LOOP
    EXEC SQL FETCH emp_cursor
        INTO :emp_name, :job_title, :row_id;
    ...
    EXEC SQL DELETE FROM EMP
        WHERE JOB = :old_title AND ROWID = :row_id;
    EXEC SQL COMMIT WORK;
ENDLOOP;

However, the fetched rows are not locked because no FOR UPDATE OF clause is used. So, you might get inconsistent results if another user changes a row after you read it but before you delete it.

Using SQLERRD(3)

For INSERT, UPDATE, DELETE, and SELECT INTO statements, SQLERRD(3) records the number of rows processed. For FETCH statements, it records the cumulative sum of rows processed.

When using host arrays with FETCH, to find the number of rows returned by the most recent iteration, subtract the current value of SQLERRD(3) from its previous value (stored in another variable). In the following example, you determine the number of rows returned by the most recent fetch:

EXEC SQL BEGIN DECLARE SECTION;
    emp_number (100)  INTEGER;
    emp_name (100)    CHARACTER(20);
EXEC SQL END DECLARE SECTION;
...
    rows_to fetch     INTEGER;
    rows_before       INTEGER;
    rows_this_time    INTEGER;
...
EXEC SQL DECLARE emp_cursor CURSOR FOR
    SELECT EMPNO, ENAME
    FROM EMP
    WHERE DEPTNO = 30;
EXEC SQL OPEN emp_cursor;
EXEC SQL WHENEVER NOT FOUND CONTINUE;
...
--  initialize loop variables
set rows_to_fetch = 20;   -- number of rows in each "batch"
set rows_before = 0;      -- previous value of sqlerrd(3)
set rows_this_time = 20;
WHILE rows_this_time = rows_to_fetch
    LOOP
        EXEC SQL FOR :rows_to_fetch
            FETCH emp_cursor
            INTO :emp_number, :emp_name;
        set rows_this_time = sqlca.sqlerrd(3) - rows_before;
        set rows_before = sqlca.sqlerrd(3);
    ENDLOOP;
ENDWHILE;

SQLERRD(3) is also useful when an error occurs during an array operation. Processing stops at the row that caused the error, so SQLERRD(3) gives the number of rows processed successfully.


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