Connected to Oracle Database 11g Enterprise Edition Release 11.2.0.1.0
Connected as [email protected]:1521/orcl
SQL>
SQL> DROP TABLE t PURGE;
Table dropped
SQL> create table t as select * from dba_objects;
Table created
SQL> create index idx_t on t(object_id);
Index created
SQL> UPDATE t SET object_id = 500 WHERE object_id <= 5000 AND object_id >= 1000;
3989 rows updated
SQL> UPDATE t SET object_id = NULL WHERE object_id < 800 AND object_id >= 400;
4367 rows updated
SQL> BEGIN
2 DBMS_STATS.GATHER_TABLE_STATS(ownname => 'TEST',
3 tabname => 'T',
4 estimate_percent => 100,
5 method_opt => 'for all columns size auto',
6 degree => DBMS_STATS.AUTO_DEGREE,
7 cascade => TRUE);
8 END;
9 /
PL/SQL procedure successfully completed
SQL> EXPLAIN PLAN FOR SELECT owner FROM t WHERE object_id < 1000;
Explained
SQL> SELECT * FROM TABLE(dbms_xplan.display());
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 1594971208
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 882 | 9702 | 17 (0)| 00:00
| 1 | TABLE ACCESS BY INDEX ROWID| T | 882 | 9702 | 17 (0)| 00:00
|* 2 | INDEX RANGE SCAN | IDX_T | 882 | | 3 (0)| 00:00
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"<1000)
14 rows selected
SQL> SET serveroutput ON
SQL> DECLARE
2 v_blevel user_indexes.blevel%TYPE;
3 v_leaf_blocks user_indexes.leaf_blocks%TYPE;
4 v_effective_index_selectivity NUMBER(18,4);
5 v_clustering_factor user_indexes.clustering_factor%TYPE;
6 v_effective_table_selectivity NUMBER(18,4);
7 v_num_rows user_tables.num_rows%TYPE;
8 v_num_distinct user_tab_col_statistics.num_distinct%TYPE;
9 v_num_nulls user_tab_col_statistics.num_nulls%TYPE;
10 v_high_value PLS_INTEGER;
11 v_low_value PLS_INTEGER;
12 v_cost PLS_INTEGER;
13 v_selected_rows PLS_INTEGER;
14 v_count NUMBER := 1000;
15 BEGIN
16 SELECT leaf_blocks, blevel, clustering_factor
17 INTO v_leaf_blocks, v_blevel, v_clustering_factor
18 FROM user_indexes
19 WHERE index_name = 'IDX_T';
20
21 SELECT b.num_rows,
22 a.num_distinct,
23 a.num_nulls,
24 utl_raw.cast_to_number(high_value) high_value,
25 utl_raw.cast_to_number(low_value) low_value
26 INTO v_num_rows, v_num_distinct, v_num_nulls, v_high_value, v_low_value
27 FROM user_tab_col_statistics a, user_tables b
28 WHERE a.table_name = b.table_name
29 AND a.table_name = upper('T')
30 AND a.column_name = 'OBJECT_ID';
31
32 v_effective_index_selectivity := ((v_count - v_low_value) / (v_high_value - v_low_value));
33 v_selected_rows := ceil(((v_count - v_low_value) / (v_high_value - v_low_value)) * (v_num_rows - v_num_nulls));
34 v_effective_table_selectivity := ((v_count - v_low_value) / (v_high_value - v_low_value));
35
36 v_cost := v_blevel +
37 ceil(v_leaf_blocks * (v_count - v_low_value) / (v_high_value - v_low_value) +
38 ceil(v_clustering_factor * (v_count - v_low_value) / (v_high_value - v_low_value)));
39
40 dbms_output.put_line('leaf_blocks==>' || v_leaf_blocks);
41 dbms_output.put_line('blevel==>' || v_blevel);
42 dbms_output.put_line('clustering_factor==>' || v_clustering_factor);
43 dbms_output.put_line('v_high_value==>' || v_num_rows);
44 dbms_output.put_line('high_value==>' || v_high_value);
45 dbms_output.put_line('low_value==>' || v_low_value);
46 dbms_output.put_line('num_rows==>' || v_num_rows);
47 dbms_output.put_line('num_nulls==>' || v_num_nulls);
48 dbms_output.put_line('selected_rows==>' || v_selected_rows);
49 dbms_output.put_line('effective_index_selectivity==>' || v_effective_index_selectivity);
50 dbms_output.put_line('effective_table_selectivity==>' || v_effective_table_selectivity);
51 dbms_output.put_line('cost==>' || v_cost);
52 END;
53 /
leaf_blocks==>155
blevel==>1
clustering_factor==>1072
v_high_value==>72925
high_value==>77556
low_value==>2
num_rows==>72925
num_nulls==>4368
selected_rows==>883
effective_index_selectivity==>.0129
effective_table_selectivity==>.0129
cost==>17
PL/SQL procedure successfully completed
