Using PL/SQL Collections to Optimize Data Retrieval

Introduction

Using PL/SQL collections, you can combine master and detail tables in a single SELECT statement, converting detail records into a collection type. This significantly improves performance for data-intensive applications by reducing the number of round trips to the database. This avoids the overhead of duplicating the master record for each detail record.

Consider this SELECT statement in a simplified HR application:

SELECT
    d.department_id,
    d.department_name,
    e.employee_id,
    e.first_name,
    e.last_name
FROM departments d
INNER JOIN employees e
    ON d.department_id = e.department_id;

Here, departments is the master table and employees is the detail table. For each department, there are zero or more employees.

Now, consider this example using PL/SQL collections:

SET SERVEROUTPUT ON

CREATE OR REPLACE TYPE employees_ids_t AS TABLE OF NUMBER(6);
/

DECLARE
    CURSOR dep_emp_cur IS
        SELECT
                d.department_id,
                d.department_name,
            CAST(MULTISET(SELECT e.employee_id
                        FROM employees e
                        WHERE d.department_id = e.department_id)
            AS employees_ids_t) AS emp_collection
         FROM departments d;
    dep_emp_row dep_emp_cur%ROWTYPE;
    row_index PLS_INTEGER;

BEGIN
    OPEN dep_emp_cur;

    LOOP
        FETCH dep_emp_cur INTO dep_emp_row;

        EXIT WHEN dep_emp_cur%NOTFOUND;

        DBMS_OUTPUT.put(dep_emp_row.department_id || ', '
                || dep_emp_row.department_name || ': employees: ');

        row_index := dep_emp_row.emp_collection.FIRST;
        LOOP
            EXIT WHEN row_index IS NULL;

            DBMS_OUTPUT.put(dep_emp_row.emp_collection(row_index) || ', ');

            row_index := dep_emp_row.emp_collection.NEXT(row_index);

        END LOOP;

        DBMS_OUTPUT.put_line('');

    END LOOP;

    CLOSE dep_emp_cur;

END;
/

The simple JOIN repeats the master table for each detail record. However, the PL/SQL code produces a different result: it combines the master and detail data into single rows, eliminating master record repetition. This approach is significantly more efficient. (Images illustrating the difference would be included here if available.)

This demonstrates the power of using PL/SQL collections for efficient data retrieval and processing. By consolidating data from multiple tables into a single result set, you greatly reduce database load and improve application performance.

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