count(*)在平常工作中，使用到的頻率很高，是否會走索引，對性能影響不小！但是不是所有的count(*)都能走索引!小記下
create table t3
(
sid number not null primary key,
sno number,
sname varchar2(10)
)
tablespace test;
declare
maxrecords constant int:=100000;
i int :=1;
begin
for i in 1..maxrecords loop
insert into t3 values(i,i,'ocpyang');
end loop;
dbms_output.put_line(' 成功錄入數據！ ');
commit;
end;
/
declare
maxrecords constant int:=200000;
i int :=100001;
begin
for i in 100001..maxrecords loop
insert into t3(sid,sname) values(i,'ocpyang');
end loop;
dbms_output.put_line(' 成功錄入數據！ ');
commit;
end;
/
create index index_sno on t3(sno);
exec dbms_stats.gather_table_stats('SYS','T3',cascade=>TRUE);
***********
1.count
***********
SQL> set autotrace traceonly explain stat;
SQL> select count(*) from t3;
執行計劃
----------------------------------------------------------
Plan hash value: 463314188
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 2 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T3 | 82 | 2 (0)| 00:00:01 |
-------------------------------------------------------------------
Note
-----
- SQL plan baseline "SQL_PLAN_27gnhfjz9qahj14fae16c" used for this statement
統計信息
----------------------------------------------------------
55 recursive calls
38 db block gets
521 consistent gets
19 physical reads
14676 redo size
527 bytes sent via SQL*Net to client
520 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--通過全表掃描實現的.
SQL> select count(*) from t1 where sid is not null;
執行計劃
----------------------------------------------------------
Plan hash value: 1551730033
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 68 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FAST FULL SCAN| SYS_C0023596 | 85899 | 1090K| 68 (0)| 00:00:01 |
--------------------------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement (level=2)
- SQL plan baseline "SQL_PLAN_4xztry6akgpqqf2d247c8" used for this statement
統計信息
----------------------------------------------------------
4 recursive calls
0 db block gets
310 consistent gets
0 physical reads
0 redo size
527 bytes sent via SQL*Net to client
520 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--通過索引實現的.