If you use SET AUTOTRACE TRACEONLY EXPLAIN you'll get the execution plan without executing the query. If you use SET AUTOTRACE TRACEONLY you are asking for the statistics as well as the execution plan and the query has to be executed to get the statistics.

Please note that this is only true for queries. Other statements are executed even if you have used SET AUTOTRACE TRACEONLY EXPLAIN.

I thought the TRACEONLY option would just return the explain plan? However, when I run this it appears that data is being retrieved due to buffer cache hit rate going down the pan!

Does this option just not return the data to your screen? I would like to be able to generate an explain plan in SQL without actually executing the query. Any ideas?