SUM/AVG的優(yōu)化
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drop table t purge;
create table t as select * from dba_objects;
create index idx1_object_id on t(object_id);
set autotrace on
set linesize 1000
set timing on
select sum(object_id) from t;
執(zhí)行計劃
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 49 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FAST FULL SCAN| IDX1_OBJECT_ID | 92407 | 1173K| 49 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
統(tǒng)計信息
----------------------------------------------------------
0 recursive calls
0 db block gets
170 consistent gets
0 physical reads
0 redo size
432 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--比較一下假如不走索引的代價，體會一下這個索引的重要性
select /*+full(t)*/ sum(object_id) from t;
SUM(OBJECT_ID)
--------------
2732093100
執(zhí)行計劃
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 292 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | TABLE ACCESS FULL| T | 92407 | 1173K| 292 (1)| 00:00:04 |
---------------------------------------------------------------------------
統(tǒng)計信息
----------------------------------------------------------
0 recursive calls
0 db block gets
1047 consistent gets
0 physical reads
0 redo size
432 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--起來類似的比如AVG，和SUM是一樣的，如下：
select avg(object_id) from t;
AVG(OBJECT_ID)
--------------
37365.5338
執(zhí)行計劃
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 49 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FAST FULL SCAN| IDX1_OBJECT_ID | 92407 | 1173K| 49 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
統(tǒng)計信息
----------------------------------------------------------
0 recursive calls
0 db block gets
170 consistent gets
0 physical reads
0 redo size
448 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--不知大家注意到?jīng)]，這里的試驗已經(jīng)告訴我們了，OBJECT_ID列是否為空，也不影響SUM/AVG等聚合的結(jié)果。
重慶分公司
重慶分公司
