Because objects in a database can change constantly, you must update statistics regularly so that they accurately describe these objects. Oracle Database automatically maintains optimizer statistics.
Oracle recommends that you enable automatic optimizer statistics collection. In this case, the database automatically collects optimizer statistics for tables with absent or stale statistics. If fresh statistics are required for a table, then the database collects them both for the table and associated indexes.
Automatic collection eliminates many manual tasks associated with managing the optimizer. It also significantly reduces the risks of generating poor execution plans because of missing or stale statistics.
Automatic optimizer statistics collection calls the DBMS_STATS.GATHER_DATABASE_STATS_JOB_PROC procedure. This internal procedure operates similarly to the DBMS_STATS.GATHER_DATABASE_STATS procedure using the GATHER AUTO option. The main difference is that GATHER_DATABASE_STATS_JOB_PROC prioritizes database objects that require statistics, so that objects that most need updated statistics are processed first, before the maintenance window closes.
另外，从优化器记录的信息看：
Optimizer statistics include the following:
Table statistics
Number of rows
Number of blocks
Average row length
Column statistics
Number of distinct values (NDV) in column
Number of nulls in column
Data distribution (histogram)
Extended statistics
Index statistics
Number of leaf blocks
Levels
Clustering factor
System statistics
I/O performance and utilization
CPU performance and utilization
你可以核对这些相关的信息，只要正确收集了，应该就是准的。
关键还在于语句以及对应的执行计划。
