In Oracle PL/SQL, what is a PL/SQL table (index-by or associative array) and what is its main use inside a block?

Introduction / Context:
This question focuses on PL/SQL tables, which in older terminology refer to index-by tables and in modern Oracle PL/SQL correspond to associative arrays. Interviewers ask about them to see whether you understand how to work with in-memory collections of data inside PL/SQL blocks, procedures, and functions, instead of relying only on permanent database tables. Knowing this helps you write efficient procedural code that can manipulate sets of values quickly.

Given Data / Assumptions:
• The environment is Oracle PL/SQL, where index-by tables and associative arrays are available. • The term PL/SQL table refers to a collection type defined in PL/SQL, not a physical SQL table. • The question wants the primary purpose of such a structure inside a PL/SQL block. • No numeric calculation is required; only conceptual understanding of collections is needed.

Concept / Approach:
A PL/SQL table is a collection type that stores multiple elements in memory. You can think of it as a one-dimensional array indexed by either integers or, in the case of associative arrays, by string keys. Unlike permanent tables, PL/SQL tables exist only within the scope of the PL/SQL block that declares them. They are useful when you want to fetch multiple rows from the database once, operate on them repeatedly, or pass them between subprograms without constant round trips to the database. A correct definition must emphasize that PL/SQL tables are in-memory collections used for processing data inside PL/SQL code, not disk based objects like normal tables or indexes.

Step-by-Step Solution:
Step 1: Recall that PL/SQL adds procedural features and collection types on top of SQL. Step 2: Remember that PL/SQL tables (index-by tables) are declared in PL/SQL using TYPE and TABLE OF syntax. Step 3: Note that they are stored in memory and exist only for the lifetime of the PL/SQL block, procedure, or function. Step 4: Recognize that they act like arrays or associative arrays that can hold many values or records for internal processing. Step 5: Choose the option that correctly describes them as memory-resident collections used to store and process multiple rows inside PL/SQL code.

Verification / Alternative check:
Think of a common usage pattern: you declare a PL/SQL table of records, fetch a set of rows from a query into it, loop over the collection to perform calculations, and possibly write back aggregated results. At no point is this structure stored permanently on disk like a normal table, nor does it behave like an index defined over a table. It is purely an in-memory data structure belonging to PL/SQL. Therefore, the option describing a PL/SQL table as a memory-resident collection similar to a one-dimensional array used inside PL/SQL blocks is the most accurate summary.

Why Other Options Are Wrong:
Option b incorrectly claims that a PL/SQL table is a physical table stored permanently on disk, which is actually what normal SQL tables are. Option c confuses PL/SQL tables with indexes, which are separate disk based structures used to speed up data access. Option d suggests that PL/SQL tables are files used for backup, which is not related to their purpose. Option e describes a system view that shows sessions, not an in-memory collection type programmable in PL/SQL.

Common Pitfalls:
A frequent misunderstanding is assuming that anything called a table must be a permanent database object. In PL/SQL, the word table in this older term refers to a collection, not to a physical SQL table. Another pitfall is mixing up PL/SQL tables with nested tables or VARRAY types, which are also collection types but have different storage and usage patterns. Finally, some learners forget that PL/SQL tables disappear once the block finishes, which means they are not suitable for long term data storage but are ideal for quick in-memory processing and passing data between subprograms.

Final Answer:
The correct choice is A PL/SQL table is a memory-resident collection (similar to a one-dimensional array) used to store and process multiple rows of data inside PL/SQL blocks., because it captures the in-memory, collection based, block scoped nature and principal use of PL/SQL tables.