Explaining problem SQL queries in Oracle

Connect to sqlplus

set pagesize 2000
set linesize 2000
@$ORACLE_HOME/rdbms/admin/utlxplan.sql
explain plan for <paste in full query here>;
@$ORACLE_HOME/rdbms/admin/utlxpls.sql

Or in SQL Developer (The Java GUI from Oracle)

Paste your entire query into a blank worksheet, then run “Execute Explain Plan” (F6)