索引唯一扫描,索引范围扫描,索引全扫描,索引快速全扫描和索引跳跃式扫描。
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索引唯一扫描:
SQL> create table employee(gender varchar2(1),employee_id number); Table created. SQL> insert into employee values('F',99); 1 row created. SQL> insert into employee values('F',100); 1 row created. SQL> insert into employee values('M',101); 1 row created. SQL> insert into employee values('M',102); 1 row created. SQL> insert into employee values('M',103); 1 row created. SQL> insert into employee values('M',104); 1 row created. SQL> insert into employee values('M',105); 1 row created. SQL> insert into employee values('F',106); 1 row created. SQL> commit; Commit complete. SQL> create unique index idx_unqi_emp on employee(employee_id); Index created. SQL> select * from employee where employee_id=100; G EMPLOYEE_ID - ----------- F 100 SQL> set lines 200 pagesize 1000 SQL> select plan_table_output from table(dbms_xplan.display_cursor(null,null,'ALL')); PLAN_TABLE_OUTPUT --------------------------------------------------------------------------------- SQL_IDbum8qv24s6tqp, child number 0 ------------------------------------- select * from employee where employee_id=100 Plan hash value: 1037614268 -------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 1 (100)| | | 1 | TABLE ACCESS BY INDEX ROWID| EMPLOYEE | 1 |15 | 1 (0)| 00:00:01 | |* 2 | INDEX UNIQUE SCAN | IDX_UNQI_EMP | 1 | | 0 (0)| | --------------------------------------------------------------------------------- 31 rows selected.
索引范围扫描:
SQL> drop index idx_unqi_emp; Index dropped. SQL> create index idx_unqi_emp on employee(employee_id); Index created. SQL> select * from employee where employee_id=100; G EMPLOYEE_ID - ----------- F 100 SQL> select plan_table_output from table(dbms_xplan.display_cursor(null,null,'ALL')); PLAN_TABLE_OUTPUT ----------------------------------------------------------------------------------------------------------------------------------------- SQL_IDbum8qv24s6tqp, child number 0 select * from employee where employee_id=100 Plan hash value: 407794244 -------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 2 (100)| | | 1 | TABLE ACCESS BY INDEX ROWID| EMPLOYEE | 1 |15 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX_UNQI_EMP | 1 | | 1 (0)| 00:00:01 | --------------------------------------------------------------------------------------------
索引快速全扫描:
SQL> begin 2 for i in 1..5000 loop 3 insert into employee values('F',i); 4 end loop; 5 commit; 6 end; 7 / PL/SQL procedure successfully completed. SQL> begin 2 for i in 5001..10000 loop 3 insert into employee values('M',i); 4 end loop; 5 commit; 6 end; 7 /
PL/SQL procedure successfully completed.
SQL> select gender,count(*) from employee group by gender; G COUNT(*) - ---------- M5000 F5000
BEGIN DBMS_STATS.GATHER_TABLE_STATS(ownname => 'SCOTT', tabname => 'EMPLOYEE', estimate_percent => 100, method_opt => 'for all columns size repeat', no_invalidate => FALSE, degree => 8, granularity => 'ALL', cascade => TRUE); END;
PL/SQL procedure successfully completed. SQL> set autot trace SQL> select employee_id from employee; 10000 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 2119105728 ------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 10000 | 40000 | 7 (0)| 00:00:01 | | 1 | TABLE ACCESS FULL| EMPLOYEE | 10000 | 40000 | 7 (0)| 00:00:01 | ------------------------------------------------------------------------------
提示走索引,无效,因为employee_id有null值:
SQL> create index idx_emp_1 on employee(employee_id); Index created. SQL> select /*+ index(employee idx_emp_1) */ employee_id from employee; 10000 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 2119105728 ------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 10000 | 40000 | 7 (0)| 00:00:01 | | 1 | TABLE ACCESS FULL| EMPLOYEE | 10000 | 40000 | 7 (0)| 00:00:01 | ------------------------------------------------------------------------------
建立组合索引,或许把employee_id限制为非空:
SQL> select /*+ index(employee idx_emp_1) */ employee_id from employee; 10000 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 438557521 ------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 10000 | 40000 | 25 (0)| 00:00:01 | | 1 | INDEX FULL SCAN | IDX_EMP_1 | 10000 | 40000 | 25 (0)| 00:00:01 | ------------------------------------------------------------------------------
索引跳跃扫描: SQL> create index idx_emp_1 on employee(gender,employee_id); Index created. SQL> select * from employee where employee_id=109; Execution Plan----------------------------------------------------------Plan hash value: 2039022311 ------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 6 | 3 (0)| 00:00:01 ||* 1 | INDEX SKIP SCAN | IDX_EMP_1 | 1 | 6 | 3 (0)| 00:00:01 |------------------------------------------------------------------------------ Predicate Information (identified by operation id):--------------------------------------------------- 1 - access("EMPLOYEE_ID"=109) filter("EMPLOYEE_ID"=109) Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 6 consistent gets 3 physical reads 0 redo size 600 bytes sent via SQL*Net to client 524 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed