驱动表的概念

http://www.itpub.net/thread-144375-1-1.html

驱动表是在使用多表嵌套连接时，会先全表扫描该驱动表，然后驱动表返回的结果集中一行一行去匹配被驱动表（可以利用索引），所以我们会选择小表做为驱动表，而被驱动使用索引进行连接。对于基于规则的优化系统，驱动表在FROM后是有次序设置的（最右边的是驱动表），而在基于成本的优化，ORACLE会选择最合适的驱动表。

 

 

The 'driving' table is the table we will join FROM -- that is JOIN TO other
tables.  For example, lets say you have the query:

select * from emp, dept where emp.deptno = dept.deptno;

In this case the driving table might be DEPT, we would fetch rows from DEPT in a
full scan and then find the rows in EMP that match.  DEPT is the driving table.

The choice of a driving table made using many factors.  For example, in the
above query if there was an index on DEPT(DEPTNO) but not EMP(DEPTNO), we would
probably use EMP as the driving table -- we would fetch rows from EMP and then
using the DEPT(DEPTNO) index -- find their matches in the DEPT table.  Table
sizes, cardinality of column values, and other things can affect the choice of a
driving table (eg: HINTS).

When using the Rule Based Optimizer -- the placement of tables in the FROM
clause is relevant.  We process the from clause from the RIGHT to the LEFT -- we
would tend to pick a driving table from the end of the FROM list.  There is a
hint in the Cost Based Optimizer to have this happen as well.

When using CBO -- the order of tables is not relevant (unless you hint it to
be).  We use the statistics and data dictionary to determine which table is best
to be used as the driving table.