In case you only have SQL*Plus as your editor, you can use the following codes to show the execution plan of your query.
First Step : create plan
EXPLAIN PLAN FOR
SELECT field1, field2
Second Step: show results
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);
You will need the necessary privileges to execute this. You’ll also need to have the PLAN table. If not, you’ll have to create it as that’s the placeholder where execution plan is written.
This technique is important especially when you don’t have a GUI-based SQL editor like PL/SQL Developer or TOAD.