SELECT MIN(object_id),MAX(object_id) FROM TABLE 如何优化

<pre name="code" class="sql">索引和NULL值

create table test10(id int,name char(10))


begin
for i in 1 .. 100000
loop
insert into test10 values(i,'a'||i);
commit;
end loop;
end;

create index test10_idx1 on test10(id);

BEGIN
  DBMS_STATS.GATHER_TABLE_STATS(ownname          => 'TEST',
                                tabname          => 'TEST10',
                                estimate_percent => 100,
                                method_opt       => 'for all columns size repeat',
                                no_invalidate    => FALSE,
                                degree           => 8,
                                cascade          => TRUE);
END;

insert into test10 values(null,'a');

1.
SQL> select count(*) from test10;


执行计划
----------------------------------------------------------
Plan hash value: 260470369

---------------------------------------------------------------------
| Id  | Operation	   | Name   | Rows  | Cost (%CPU)| Time     |
---------------------------------------------------------------------
|   0 | SELECT STATEMENT   |	    |	  1 |	 66   (2)| 00:00:01 |
|   1 |  SORT AGGREGATE    |	    |	  1 |		 |	    |
|   2 |   TABLE ACCESS FULL| TEST10 |	100K|	 66   (2)| 00:00:01 |
---------------------------------------------------------------------


统计信息
----------------------------------------------------------
	  0  recursive calls
	  0  db block gets
	293  consistent gets
	  0  physical reads
	  0  redo size
	425  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

SQL> select count(id) from test10;   


执行计划
----------------------------------------------------------
Plan hash value: 3206018756

-------------------------------------------------------------------------------------
| Id  | Operation	      | Name	    | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      | 	    |	  1 |	  5 |	 50   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE       | 	    |	  1 |	  5 |		 |	    |
|   2 |   INDEX FAST FULL SCAN| TEST10_IDX1 |	100K|	488K|	 50   (0)| 00:00:01 |
-------------------------------------------------------------------------------------


统计信息
----------------------------------------------------------
	  0  recursive calls
	  0  db block gets
	228  consistent gets
	  0  physical reads
	  0  redo size
	424  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


2.
SQL> select id from test10;

已选择100001行。


执行计划
----------------------------------------------------------
Plan hash value: 4117858598

----------------------------------------------------------------------------
| Id  | Operation	  | Name   | Rows  | Bytes | Cost (%CPU)| Time	   |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |	   |   100K|   488K|	66   (2)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| TEST10 |   100K|   488K|	66   (2)| 00:00:01 |
----------------------------------------------------------------------------


统计信息
----------------------------------------------------------
	  0  recursive calls
	  0  db block gets
       6943  consistent gets
	  0  physical reads
	  0  redo size
    1455977  bytes sent via SQL*Net to client
      73741  bytes received via SQL*Net from client
       6668  SQL*Net roundtrips to/from client
	  0  sorts (memory)
	  0  sorts (disk)
     100001  rows processed

SQL>  select id from test10 where id is not null;

已选择100000行。


执行计划
----------------------------------------------------------
Plan hash value: 1361564665

------------------------------------------------------------------------------------
| Id  | Operation	     | Name	   | Rows  | Bytes | Cost (%CPU)| Time	   |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |		   |   100K|   488K|	50   (0)| 00:00:01 |
|*  1 |  INDEX FAST FULL SCAN| TEST10_IDX1 |   100K|   488K|	50   (0)| 00:00:01 |
------------------------------------------------------------------------------------

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

   1 - filter("ID" IS NOT NULL)


统计信息
----------------------------------------------------------
	  1  recursive calls
	  0  db block gets
       6880  consistent gets
	  0  physical reads
	  0  redo size
    1455975  bytes sent via SQL*Net to client
      73741  bytes received via SQL*Net from client
       6668  SQL*Net roundtrips to/from client
	  0  sorts (memory)
	  0  sorts (disk)
     100000  rows processed

3.
SQL> select max(id) from test10;


执行计划
----------------------------------------------------------
Plan hash value: 2324431888

------------------------------------------------------------------------------------------
| Id  | Operation		   | Name	 | Rows  | Bytes | Cost (%CPU)| Time	 |
------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	   |		 |     1 |     5 |    66   (2)| 00:00:01 |
|   1 |  SORT AGGREGATE 	   |		 |     1 |     5 |	      | 	 |
|   2 |   INDEX FULL SCAN (MIN/MAX)| TEST10_IDX1 |   100K|   488K|	      | 	 |
------------------------------------------------------------------------------------------


统计信息
----------------------------------------------------------
	  0  recursive calls
	  0  db block gets
	  2  consistent gets
	  0  physical reads
	  0  redo size
	422  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
如果希望直接从索引返回数据,由于索引不存储NULL值,则需要显示说明 is not null

4.
SQL>     select max(id),min(id) from test10; 


执行计划
----------------------------------------------------------
Plan hash value: 260470369

-----------------------------------------------------------------------------
| Id  | Operation	   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |	    |	  1 |	  5 |	 66   (2)| 00:00:01 |
|   1 |  SORT AGGREGATE    |	    |	  1 |	  5 |		 |	    |
|   2 |   TABLE ACCESS FULL| TEST10 |	100K|	488K|	 66   (2)| 00:00:01 |
-----------------------------------------------------------------------------


统计信息
----------------------------------------------------------
	  1  recursive calls
	  0  db block gets
	293  consistent gets
	  0  physical reads
	  0  redo size
	486  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


为什么不走索引呢？

这条语句要同时获得两个值, oracle的INDEX FULL SCAN (MIN/MAX)是无法一次取到两个值的


SQL>  select max(id),min(id) from test10 where id is not null;


执行计划
----------------------------------------------------------
Plan hash value: 3206018756

-------------------------------------------------------------------------------------
| Id  | Operation	      | Name	    | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      | 	    |	  1 |	  5 |	 50   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE       | 	    |	  1 |	  5 |		 |	    |
|*  2 |   INDEX FAST FULL SCAN| TEST10_IDX1 |	100K|	488K|	 50   (0)| 00:00:01 |
-------------------------------------------------------------------------------------

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

   2 - filter("ID" IS NOT NULL)


统计信息
----------------------------------------------------------
	  1  recursive calls
	  0  db block gets
	228  consistent gets
	  0  physical reads
	  0  redo size
	486  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




SQL> select (select max(id)from test10),(select min(id)from test10) from dual;


执行计划
----------------------------------------------------------
Plan hash value: 3694443619

------------------------------------------------------------------------------------------
| Id  | Operation		   | Name	 | Rows  | Bytes | Cost (%CPU)| Time	 |
------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	   |		 |     1 |	 |     2   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE 	   |		 |     1 |     5 |	      | 	 |
|   2 |   INDEX FULL SCAN (MIN/MAX)| TEST10_IDX1 |   100K|   488K|	      | 	 |
|   3 |  SORT AGGREGATE 	   |		 |     1 |     5 |	      | 	 |
|   4 |   INDEX FULL SCAN (MIN/MAX)| TEST10_IDX1 |   100K|   488K|	      | 	 |
|   5 |  FAST DUAL		   |		 |     1 |	 |     2   (0)| 00:00:01 |
------------------------------------------------------------------------------------------


统计信息
----------------------------------------------------------
	  1  recursive calls
	  0  db block gets
	  4  consistent gets
	  0  physical reads
	  0  redo size
	522  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)