Question

I’ve come across both terms, but they sound synonymous. Is there a distinction between the two?

Answer 1

An “index-by table” is Oracle’s term for “associative array”. These are arrays that contain elements that you can address (or index by) either an integer or string. They’re probably called that because of the use of the INDEX BY keywords when defining the array.

An abbreviation of the example given in the Oracle documentation:

DECLARE  
  TYPE population_type IS TABLE OF NUMBER INDEX BY VARCHAR2(64);
  country_population population_type;
  howmany NUMBER;
BEGIN
  country_population('Greenland') := 100000; -- Creates new entry
  howmany := country_population('Greenland');
  ...

You can create index-by tables containing records, where records are essentially a structure containing multiple types. A record, for example, often contains the same types as a row in a table.

Again, from the Oracle documentation:

DECLARE
   TYPE EmpTabTyp IS TABLE OF employees%ROWTYPE
      INDEX BY PLS_INTEGER;
   emp_tab EmpTabTyp;
BEGIN
   /* Retrieve employee record. */
   SELECT * INTO emp_tab(100) FROM employees
     WHERE employee_id = 100;
END;

Here, emp_tab is an index-by table, indexed by integers, containing records of employees%ROWTYPE.