开启Oracle跟踪的语句set autotrace on要在sqlplus环境中运行，在PL/SQL Developer中执行始终弹出cannot set autotrace。加了索引的SQL，查询速度会很快！
Oracle统计信息，还不明白是什么意思
9 recursive calls
0 db block gets
61 consistent gets
4 physical reads
0 redo size
420 bytes sent via SQL*NET to client
400 bytes received via SQL*NET from client
2 SQL*NET roundtrips to/from client
0 sorts(memory)
0 sorts(disk)
1 rows processed
