<pre name="code" class="sql"><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)