表很小的时候，究竟走全表扫描还是走索引？

jiangjh62

走索引比走全表成本要小，那么执行计划就会优先走索引。（CBO）


小表就一定是走全表？

Yong Huang

> 小表就一定是走全表？

No. As I said in message #13, it depends on cost calculation. If you want to know why the cost is "counter-intuitive", think about the index. Remember there's a clustering factor. It's quite possible to create a test case where a small table has an index whose clustering factor is almost the same as the number of rows in the table. Test the SQL in that case  and let us know.

Yong Huang

muchun_fly

执行一下这个语句：

select empno,dbms_rowid.rowid_block_number(rowid) from emp

xuqingwei

这两天忙，没有来itpub。非常感谢各位的帮助！

我对Cost计算的理解还是太肤浅了，这两天闲，要好好恶补一下。