Oracle全表扫描及其执行计划(full table scan)(四)
for owner: scott
AVG LEAF BLKS AVG DATA BLKS
BLEV IDX_NAME LEAF_BLKS DST_KEYS PER KEY PER KEY CLUST_FACT LAST_ANALYZED TB_BLKS TB_ROWS
---- -------------- ---------- ---------- ------------- ------------- ---------- ------------------ ---------- ----------
1 BIG_TABLE_PK 208 100000 1 1 1483 20130524 10:45:51 1515 100000
--数据库参数设置
scott@ORA11G> show parameter optimizer_index_
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
optimizer_index_caching integer 0
optimizer_index_cost_adj integer 100
scott@ORA11G> show parameter optimizer_mode
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
optimizer_mode string ALL_ROWS
b、查询返回20%数据行的情形
scott@ORA11G> alter system flush buffer_cache;
scott@ORA11G> select sum(object_id),avg(object_id) from big_table where id between 20000 and 40000;
Execution Plan
----------------------------------------------------------
Plan hash value: 3098837282 -- 执行计划中，使用了索引范围扫描
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 1 | SORT AGGREGATE | | 1 | 18 | | |
| 2 | TABLE ACCESS BY INDEX ROWID| BIG_TABLE | 20046 | 352K| 341 (0)| 00:00:05 |
|* 3 | INDEX RANGE SCAN | BIG_TABLE_PK | 20046 | | 43 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("ID">=20000 AND "ID"<=40000)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
351 consistent gets
351 physical reads
0 redo size
427 bytes sent via SQL*Net to client
349 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
scott@ORA11G> alter system flush buffer_cache;
scott@ORA11G> select /*+ full(big_table) */ sum(object_id),avg(object_id) from big_table where id between 20000 and 40000;
Execution Plan
----------------------------------------------------------
Plan hash value: 599409829 ---- 使用了提示执行为全表扫描
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 18 | 413 (1)| 00:00:05 |
| 1 | SORT AGGREGATE | | 1 | 18 | | |