近期有朋友对于单个表上的index各种情况比较模糊,这里对于单个表上,单个index出现的大多数情况进行了总结性测试,给出了测试结果,至于为什么出现这样的试验结果未做过多解释,给读者留下思考的空间.本篇文章仅仅是为了测试hint对index的影响,而不是说明走各种index方式的好坏.参考: INDEX FULL SCAN vs INDEX FAST FULL SCAN
创建表模拟测试
SQL> create table t_xifenfei as select object_id,object_name from dba_objects;Table created.SQL> create index i_t_object_id on t_xifenfei(object_id); Index created.SQL> exec dbms_stats.gather_table_stats(USER,'T_XIFENFEI',cascade=>true); PL/SQL procedure successfully completed.SQL> desc t_xifenfei Name Null? Type ----------------------------------------- -------- ---------------------------- OBJECT_ID NUMBER OBJECT_NAME VARCHAR2(128) |
TABLE ACCESS FULL
SQL> SET AUTOT TRACE EXP STATSQL> SELECT OBJECT_ID FROM T_XIFENFEI;49838 rows selected.Execution Plan----------------------------------------------------------Plan hash value: 548923532--------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |--------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 49838 | 243K| 57 (2)| 00:00:01 || 1 | TABLE ACCESS FULL| T_XIFENFEI | 49838 | 243K| 57 (2)| 00:00:01 |--------------------------------------------------------------------------------Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 3544 consistent gets 0 physical reads 0 redo size 721203 bytes sent via SQL*Net to client 36927 bytes received via SQL*Net from client 3324 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 49838 rows processedSQL> SELECT /*+ INDEX(T i_t_object_id) */ OBJECT_ID FROM T_XIFENFEI;49838 rows selected.Execution Plan----------------------------------------------------------Plan hash value: 548923532--------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |--------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 49838 | 243K| 57 (2)| 00:00:01 || 1 | TABLE ACCESS FULL| T_XIFENFEI | 49838 | 243K| 57 (2)| 00:00:01 |--------------------------------------------------------------------------------Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 3544 consistent gets 0 physical reads 0 redo size 721203 bytes sent via SQL*Net to client 36927 bytes received via SQL*Net from client 3324 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 49838 rows processed |
从上面的执行计划中可知,此时走了全表扫描. 由于我们需要查询的列为object_id,因此理论上只需要读取索引就应该可以返回所有数据,而此时为什么是全表扫描呢？ 这是因为NULL值与索引的特性所决定的.即null值不会被存储到B树索引.因此应该为表 t_xifenfei 的列 object_id 添加 not null 约束.
INDEX FAST FULL SCAN
SQL> alter table t_xifenfei modify(object_id not null); Table altered.SQL> SELECT object_id from t_xifenfei;49838 rows selected.Execution Plan----------------------------------------------------------Plan hash value: 2036340805--------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |--------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 49838 | 243K| 27 (4)| 00:00:01 || 1 | INDEX FAST FULL SCAN| I_T_OBJECT_ID | 49838 | 243K| 27 (4)| 00:00:01 |--------------------------------------------------------------------------------------Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 3432 consistent gets 0 physical reads 0 redo size 721203 bytes sent via SQL*Net to client 36927 bytes received via SQL*Net from client 3324 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 49838 rows processed |
INDEX FAST FULL SCAN:当在高速缓存中没有找到所需的索引块时,则根据db_file_multiblock_read_count的值进行多块读操作.对于索引的分支结构只是简单的获取,然后扫描所有的叶结点.其结果是导致索引结构没有访问,获取的数据没有根据索引键的顺序排序.INDEX FAST FULL SCAN使用multiblock_read,故产生db file scattered reads 事件.
INDEX RANGE SCAN
SQL> select object_id from t_xifenfei where object_id<10;8 rows selected.Execution Plan----------------------------------------------------------Plan hash value: 2197008162----------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |----------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 2 | 10 | 2 (0)| 00:00:01 ||* 1 | INDEX RANGE SCAN| I_T_OBJECT_ID | 2 | 10 | 2 (0)| 00:00:01 |----------------------------------------------------------------------------------Predicate Information (identified by operation id):--------------------------------------------------- 1 - access("OBJECT_ID"<10)Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 3 consistent gets 0 physical reads 0 redo size 499 bytes sent via SQL*Net to client 385 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 8 rows processedSQL> select /*+ index_ffs(t i_t_object_id) */ object_id from t_xifenfei t where object_id<10;8 rows selected.Execution Plan----------------------------------------------------------Plan hash value: 2036340805--------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |--------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 2 | 10 | 27 (4)| 00:00:01 ||* 1 | INDEX FAST FULL SCAN| I_T_OBJECT_ID | 2 | 10 | 27 (4)| 00:00:01 |--------------------------------------------------------------------------------------Predicate Information (identified by operation id):--------------------------------------------------- 1 - filter("OBJECT_ID"<10)Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 118 consistent gets 0 physical reads 0 redo size 499 bytes sent via SQL*Net to client 385 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 8 rows processed1这里可以看出index_ffs已经生效,但是对于这样的情况hint index_ffs效率一般来说不会太高.<br><strong>INDEX FULL SCAN</strong>1SQL> SELECT /*+ INDEX(T i_t_object_id) */ object_id from t_xifenfei t;49838 rows selected.Execution Plan----------------------------------------------------------Plan hash value: 431110666----------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |----------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 49838 | 243K| 113 (2)| 00:00:02 || 1 | INDEX FULL SCAN | I_T_OBJECT_ID | 49838 | 243K| 113 (2)| 00:00:02 |----------------------------------------------------------------------------------Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 3426 consistent gets 0 physical reads 0 redo size 721203 bytes sent via SQL*Net to client 36927 bytes received via SQL*Net from client 3324 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 49838 rows processed |
INDEX FULL SCAN:完全按照索引存储的顺序依次访问整个索引树.当访问到叶结点之后,按照双向链表方式读取相连节点的值.换言之,对于索引上所有的数据是按照有序的方式来读取的.如果索引块没有在高速缓存中被找到时,则需要从数据文件中单块进行读取.对于需要读取大量数据的全索引扫描而言,这将使其变得低效.INDEX FULL SCAN使用single read,故产生db file sequential reads事件.新版的Oracle支持db file parallel reads方式.
HINT INDEX不会使用INDEX FAST FULL SCAN功能.
INDEX列ORDER BY
SQL> SELECT OBJECT_ID FROM T_XIFENFEI order by object_id ;49838 rows selected.Execution Plan----------------------------------------------------------Plan hash value: 431110666----------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |----------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 49838 | 243K| 113 (2)| 00:00:02 || 1 | INDEX FULL SCAN | I_T_OBJECT_ID | 49838 | 243K| 113 (2)| 00:00:02 |----------------------------------------------------------------------------------Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 3426 consistent gets 0 physical reads 0 redo size 721203 bytes sent via SQL*Net to client 36927 bytes received via SQL*Net from client 3324 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 49838 rows processedSQL> SELECT OBJECT_ID FROM T_XIFENFEI order by object_id desc;49838 rows selected.Execution Plan----------------------------------------------------------Plan hash value: 2808014233--------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |--------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 49838 | 243K| 113 (2)| 00:00:02 || 1 | INDEX FULL SCAN DESCENDING| I_T_OBJECT_ID | 49838 | 243K| 113 (2)| 00:00:02 |--------------------------------------------------------------------------------------------Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 3427 consistent gets 0 physical reads 0 redo size 721203 bytes sent via SQL*Net to client 36927 bytes received via SQL*Net from client 3324 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 49838 rows processedSQL> SELECT /*+ index_ffs(t i_t_object_id) */ object_id from t_xifenfei t order by object_id;49838 rows selected.Execution Plan----------------------------------------------------------Plan hash value: 2527678987-----------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |-----------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 49838 | 243K| | 185 (4)| 00:00:03 || 1 | SORT ORDER BY | | 49838 | 243K| 1192K| 185 (4)| 00:00:03 || 2 | INDEX FAST FULL SCAN| I_T_OBJECT_ID | 49838 | 243K| | 27 (4)| 00:00:01 |-----------------------------------------------------------------------------------------------Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 117 consistent gets 0 physical reads 0 redo size 721203 bytes sent via SQL*Net to client 36927 bytes received via SQL*Net from client 3324 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 49838 rows processed |
对于index 列排序,默认情况下会使用INDEX FULL SCAN/INDEX FULL SCAN DESCENDING而不选择使用INDEX FAST FULL SCAN,因为INDEX FAST FULL SCAN获得数据后,还需要做一次SORT ORDER BY操作
INDEX FAST FULL SCAN+SORT AGGREGATE
SQL> SELECT count(object_id) FROM T_XIFENFEI;Execution Plan----------------------------------------------------------Plan hash value: 3095383276-------------------------------------------------------------------------------| Id | Operation | Name | Rows | Cost (%CPU)| Time |-------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 27 (4)| 00:00:01 || 1 | SORT AGGREGATE | | 1 | | || 2 | INDEX FAST FULL SCAN| I_T_OBJECT_ID | 49838 | 27 (4)| 00:00:01 |-------------------------------------------------------------------------------Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 117 consistent gets 0 physical reads 0 redo size 421 bytes sent via SQL*Net to client 385 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processedSQL> SELECT /*+ INDEX(T i_t_object_id) */ count(object_id) FROM T_XIFENFEI t;Execution Plan----------------------------------------------------------Plan hash value: 3079973526--------------------------------------------------------------------------| Id | Operation | Name | Rows | Cost (%CPU)| Time |--------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 113 (2)| 00:00:02 || 1 | SORT AGGREGATE | | 1 | | || 2 | INDEX FULL SCAN| I_T_OBJECT_ID | 49838 | 113 (2)| 00:00:02 |--------------------------------------------------------------------------Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 111 consistent gets 0 physical reads 0 redo size 421 bytes sent via SQL*Net to client 385 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed |
sort aggregate通常发生在使用一些聚合函数的时候,sum(),avg(),min(),max(),count()等等,实际上sort aggregate不做真正的sort,并不会用到排序空间,而是通过一个全局变量＋全表或全索引扫描来实现.这样的操作在默认情况下使用INDEX FAST FULL SCAN
INDEX FULL SCAN (MIN/MAX)
SQL> SELECT max(object_id) FROM T_XIFENFEI;Execution Plan----------------------------------------------------------Plan hash value: 2939893782--------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |--------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 5 | 2 (0)| 00:00:01 || 1 | SORT AGGREGATE | | 1 | 5 | | || 2 | INDEX FULL SCAN (MIN/MAX)| I_T_OBJECT_ID | 49838 | 243K| 2 (0)| 00:00:01 |--------------------------------------------------------------------------------------------Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 2 consistent gets 0 physical reads 0 redo size 419 bytes sent via SQL*Net to client 385 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processedSQL> SELECT /*+ index_ffs(t i_t_object_id) */ max(object_id) FROM T_XIFENFEI t;Execution Plan----------------------------------------------------------Plan hash value: 2939893782--------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |--------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 5 | 27 (4)| 00:00:01 || 1 | SORT AGGREGATE | | 1 | 5 | | || 2 | INDEX FULL SCAN (MIN/MAX)| I_T_OBJECT_ID | 49838 | 243K| 27 (4)| 00:00:01 |--------------------------------------------------------------------------------------------Statistics---------------------------------------------------------- 1 recursive calls 0 db block gets 2 consistent gets 0 physical reads 0 redo size 419 bytes sent via SQL*Net to client 385 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed |
对于这样的查询INDEX FULL SCAN (MIN/MAX)明显是最优,但是此处奇怪的是使用了index_ffs提示无效,如果有知道的朋友,麻烦告知原因.