执行计划中cost计算方法

概念：

blevel：二元高度=索引高度-1

clustering_factor：集群因子，扫描index scan得出的要扫描的表中block数，clustering_factor<=table blocks

索引扫描的计算公式：

blevel +

ceil(clustering_factor * effective table selectivity)

SQL> select leaf_blocks,blevel,clustering_factor from dba_indexes where index_name='IDX_T';

LEAF_BLOCKS     BLEVEL CLUSTERING_FACTOR
----------- ---------- -----------------
        112          1               776

NUM_ROWS   NUM_DISTINCT NUM_NULLSHIGH_VALUE LOW_VALUENUM_ROWS-NUM_NULLS HIGH_VALUE-LOW_VALUE

50736 507351 538202 5073553818

effective index selectivity=(limit-low_value)/(high_value-low_value)

SQL> select (1000-2)/(53820-2) selectivity from dual;

SELECTIVITY
-----------
0.018543982

SQL> SELECT OWNER FROM TEST WHERE OBJECT_ID<1000;

已选择953行。


执行计划
----------------------------------------------------------
Plan hash value: 1810195980

---------------------------------------------------------------------
| Id  | Operation                   | Name  | Rows  | Bytes | Cost  |
---------------------------------------------------------------------
|   0 | SELECT STATEMENT            |       |   941 | 10351 |    19 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST  |   941 | 10351 |    19 |
|*  2 |   INDEX RANGE SCAN          | IDX_T |   941 |       |     4 |
---------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("OBJECT_ID"<1000)

1.回表io=ceil(clustering_factor * effective table selectivity)=19-4=15

2.blevel +ceil(leaf_blocks *effective index selectivity)

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执行计划中cost计算方法

原文：http://blog.csdn.net/hughwang1216/article/details/38627629