运行计划中cost计算方法

概念：

blevel：二元高度=索引高度-1

clustering_factor：集群因子，通过索引扫面得出的要查询table的blocks数量，clustering_factor接近table的blocks说明table是依照索引顺序存储的，clustering_factor接近table行数说明table相比于依照索引顺序。用一种更为乱序的存储的。

索引扫描的计算公式：

cost =

blevel +

ceil(leaf_blocks *effective index selectivity) +

ceil(clustering_factor * effective table selectivity)

一下測试是在rule based optimizer used条件下

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

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

SELECT b.num_rows, a.num_distinct, a.num_nulls, utl_raw.cast_to_number(high_value) AS high_value, utl_raw.cast_to_number(low_value) AS low_value

, b.num_rows - a.num_nulls AS "NUM_ROWS-NUM_NULLS", utl_raw.cast_to_number(high_value) - utl_raw.cast_to_number(low_value) AS "HIGH_VALUE-LOW_VALUE"

FROM dba_tab_col_statistics a, dba_tables b

WHERE a.owner = b.owner

AND a.table_name = b.table_name

AND a.owner = 'SCOTT'

AND a.table_name = upper('TEST')

AND a.column_name = 'OBJECT_ID'

NUM_ROWS  
NUM_DISTINCT NUM_NULLSHIGH_VALUELOW_VALUENUM_ROWS-NUM_NULLSHIGH_VALUE-LOW_VALUE

50736 5073515382025073553818

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。

运行计划
----------------------------------------------------------
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)