SQL & PL/SQL :: Describe Results / Define Output And Fetch Rows Of A Query

There are several stages for sql processing in 10g2 database concept document.The following stages are necessary for each type of statement processing:

■ Stage 1: Create a Cursor
■ Stage 2: Parse the Statement
■ Stage 5: Bind Any Variables
■ Stage 7: Run the Statement
■ Stage 9: Close the Cursor
Optionally, you can include another stage:
■ Stage 6: Parallelize the Statement

Queries (SELECTs) require several additional stages, as shown in Figure 241:

■ Stage 3: Describe Results of a Query
■ Stage 4: Define Output of a Query
■ Stage 8: Fetch Rows of a Query

Stage 3: Describe Results of a Query The describe stage is necessary only if the characteristics of a query's result are not known; for example, when a query is entered interactively by a user. In this case, the describe stage determines the characteristics (datatypes, lengths, and names) of a query's result.

Stage 4: Define Output of a Query In the define stage for queries, you specify the location, size, and datatype of variables defined to receive each fetched value. These variables are called define variables. Oracle performs datatype conversion if necessary.

I still don't understand what's Stage 3: Describe Results of a Query and Stage 4: Define Output of a Query.