Originally posted by thg
Thanks!

It seems that these data are static. Unlike the 'explain plan' command, the 'analyze table...' command doesn't really help me to optimize my query.
Huh? Explain plan and Analyze table are two different commands. "EXPLAIN PLAN" explains how the optimizer will execute your query. "ANALYZE TABLE" will compute statistics on your table which the optimizer will use to make it's choices. EXPLAIN PLAN without fresh statistics from ANALYZE will be useless (more or less).