2015年5月20日19:14:43 – 排序
1. 查看排序区内存的大小以及设置
实际排序所用到的内存、磁盘的统计信息：
pga_aggregate_target：此参数用来指定所有session总计可以使用最大PGA内存 olap:50% oltp:20%
sqlSQL> show parameter pga_aggre NAME TYPE VALUE ------------------------------------ ---------------------- ------------------------------ pga_aggregate_target big integer 0 SQL> show parameter workarea_size_policy
workarea_size_policy：此参数用于开关PGA内存自动管理功能 auto:自动分配sort_area_size 属于workarea 如果需要经常排序就需要把这个值设置大点
sqlSQL> show parameter workarea_size_policy NAME TYPE VALUE ------------------------------------ ---------------------- ------------------------------ workarea_size_policy string AUTO
select name, value from v$sysstat where name like ‘sort%’; select * from v$pgastat;
sqlSQL> select name , value from v$sysstat where name like 'sort%'; NAME VALUE -------------------------------------------------------------------------------------------------------------------------------- ---------- sorts (memory) 4283 sorts (disk) 0 sorts (rows) 40823
2. 比较以下操作
select * from customers;
sqlSQL> set autotrace traceonly SQL> select * from customers; 已选择55500行。 已用时间: 00: 00: 01.93 执行计划 ---------------------------------------------------------- Plan hash value: 2008213504 ------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 55500 | 9810K| 406 (1)| 00:00:05 | | 1 | TABLE ACCESS FULL| CUSTOMERS | 55500 | 9810K| 406 (1)| 00:00:05 | ------------------------------------------------------------------------------- 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 5057 consistent gets 1455 physical reads 0 redo size 10855625 bytes sent via SQL*Net to client 41109 bytes received via SQL*Net from client 3701 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 55500 rows processed
select * from customers order by cust_last_name;
sqlSQL> select * from customers order by cust_last_name; 已选择55500行。 已用时间: 00: 00: 01.93 执行计划 ---------------------------------------------------------- Plan hash value: 2792773903 ---------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time | ---------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 55500 | 9810K| | 2612 (1)| 00:00:32 | | 1 | SORT ORDER BY | | 55500 | 9810K| 12M| 2612 (1)| 00:00:32 | | 2 | TABLE ACCESS FULL| CUSTOMERS | 55500 | 9810K| | 406 (1)| 00:00:05 | ---------------------------------------------------------------------------------------- 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 1459 consistent gets 1454 physical reads 0 redo size 6278979 bytes sent via SQL*Net to client 41109 bytes received via SQL*Net from client 3701 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 55500 rows processed
可以看到使用order by用到了额外的12M内存
3. 在cust_last_name创建b*索引
没建索引：
sqlexplain plan for select cust_last_name from customers order by cust_last_name;
sqlSQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------ Plan hash value: 2792773903 ---------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time | ---------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 55500 | 433K| | 610 (1)| 00:00:08 | | 1 | SORT ORDER BY | | 55500 | 433K| 880K| 610 (1)| 00:00:08 | | 2 | TABLE ACCESS FULL| CUSTOMERS | 55500 | 433K| | 405 (1)| 00:00:05 | ----------------------------------------------------------------------------------------
建立索引：
sqlcreate index lastname_idx on customers(cust_last_name);
sqlexplain plan for select /*+ index(c lastname_idx) */ cust_last_name from customers order by cust_last_name;
执行计划：
sqlSQL> explain plan for select /*+ index(c lastname_idx) */ cust_last_name from customers order by cust_last_name; 已解释。 已用时间: 00: 00: 00.00 SQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------ Plan hash value: 3470560620 --------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 55500 | 433K| 143 (1)| 00:00:02 | | 1 | INDEX FULL SCAN | LASTNAME_IDX | 55500 | 433K| 143 (1)| 00:00:02 | ---------------------------------------------------------------------------------
可以发现在一列建立索引后，对该列进行排序操作不需要再执行排序操作，
他会根据索引来进行查询(索引中本来就已经排好序)
结论
建立索引可以节省排序操作的时间。
4. 查询前十条记录
通常我们都会：
sqlselect cust_last_name from customers where rownum<=20 order by 1;
sqlSQL> select cust_last_name from customers where rownum<=20 order by 1; CUST_LAST_NAME -------------------------------------------------------------------------------- Everett Everett Everett Everett Everett Everett Everett Everett Everett Ruddy Ruddy CUST_LAST_NAME -------------------------------------------------------------------------------- Ruddy Ruddy Ruddy Ruddy Ruddy Ruddy Ruddy Ruddy Ruddy 已选择20行。
这样是错误的,因为在oracle中where永远都是先执行的也就先取20条再排序。
正确的做法是使用子查询：
sqlselect cust_last_name from (select * from customers order by cust_last_name) where rownum<10;
sqlSQL>select cust_last_name from (select * from customers order by cust_last_name) where rownum<10; CUST_LAST_NAME -------------------------------------------------------------------------------- Aaron Aaron Aaron Aaron Aaron Aaron Aaron Aaron Aaron Aaron Aaron CUST_LAST_NAME -------------------------------------------------------------------------------- Aaron Aaron Aaron Aaron Aaron Aaron Aaron Aaron Aaron 已选择20行。
没建索引：
sqlPLAN_TABLE_OUTPUT -------------------------------------------------------------------------------------------------------- Plan hash value: 1285511559 --------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time | --------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 9 | 198 | | 2612 (1)| 00:00:32 | |* 1 | COUNT STOPKEY | | | | | | | | 2 | VIEW | | 55500 | 1192K| | 2612 (1)| 00:00:32 | |* 3 | SORT ORDER BY STOPKEY| | 55500 | 9810K| 12M| 2612 (1)| 00:00:32 | | 4 | TABLE ACCESS FULL | CUSTOMERS | 55500 | 9810K| | 406 (1)| 00:00:05 | --------------------------------------------------------------------------------------------- PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter(ROWNUM<10) 3 - filter(ROWNUM<10) 已选择17行。
建立索引后：
sqlPLAN_TABLE_OUTPUT ---------------------------------------------------------------------------------------------------- Plan hash value: 3026242074 ---------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 9 | 198 | 4 (0)| 00:00:01 | |* 1 | COUNT STOPKEY | | | | | | | 2 | VIEW | | 9 | 198 | 4 (0)| 00:00:01 | | 3 | INDEX FULL SCAN| LASTNAME_IDX | 9 | | 2 (0)| 00:00:01 | ---------------------------------------------------------------------------------- PLAN_TABLE_OUTPUT ---------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter(ROWNUM<10) 已选择15行。
建立索引后,子查询效率明显提高
同时select cust_last_name from customers where rownum<=10 order by cust_last_name;和上面的结果已经大不相同。
sqlSQL> select cust_last_name from customers where rownum<=10 order by cust_last_name; CUST_LAST_NAME -------------------------------------------------------------------------------- Aaron Aaron Aaron Aaron Aaron Aaron Aaron Aaron Aaron Aaron 已选择10行。
执行计划：
sqlSQL> explain plan for 2 select cust_last_name from customers where rownum<=10 order by cust_last_name; 已解释。 已用时间: 00: 00: 00.00 SQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT --------------------------------------------------------------------------------------------- Plan hash value: 2820001957 --------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 10 | 80 | 2 (0)| 00:00:01 | |* 1 | COUNT STOPKEY | | | | | | | 2 | INDEX FULL SCAN| LASTNAME_IDX | 55500 | 433K| 2 (0)| 00:00:01 | --------------------------------------------------------------------------------- Predicate Information (identified by operation id): PLAN_TABLE_OUTPUT --------------------------------------------------------------------------------------------- --------------------------------------------------- 1 - filter(ROWNUM<=10) 已选择14行。
竟然不是刚刚那样乱的。
结论
建立索引能够对提高子查询的性能，并且查询前十条记录不再需要使用子查询， 因为建立好索引后已经是排好序的，只要根据索引从表中拿出前十条记录即可。
5. 分组与索引
首先先建立一个表
sqlSQL> create table s as select * from sales; 表已创建。
s表与sales表的数据完全是一致的但s表没有sales表上的索引。
然后我们看一下执行计划：
sqlSQL> explain plan for 2 select cust_id,avg(amount_sold) from s group by cust_id; 已解释。 已用时间: 00: 00: 00.23 SQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------ Plan hash value: 1912481676 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 785K| 19M| 1271 (4)| 00:00:16 | | 1 | HASH GROUP BY | | 785K| 19M| 1271 (4)| 00:00:16 | | 2 | TABLE ACCESS FULL| S | 785K| 19M| 1236 (1)| 00:00:15 | --------------------------------------------------------------------------- Note PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------ ----- - dynamic sampling used for this statement (level=2) 已选择13行。
我们再看一下sales表（即建立了索引的表）:
sqlSQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------------- Plan hash value: 2820001957 --------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 10 | 80 | 2 (0)| 00:00:01 | |* 1 | COUNT STOPKEY | | | | | | | 2 | INDEX FULL SCAN| LASTNAME_IDX | 55500 | 433K| 2 (0)| 00:00:01 | --------------------------------------------------------------------------------- Predicate Information (identified by operation id): PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------------- --------------------------------------------------- 1 - filter(ROWNUM<=10) 已选择14行。
结论
group by的一列上建立索引group by 性能更佳。
下面是关于order by 升序降序的执行计划可以看出建立索引升降序对排序不会影响性能
升序:
sqlSQL> select cust_last_name from customers where rownum <= 10 order by cust_last_name; SQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------------- Plan hash value: 2820001957 --------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 10 | 80 | 2 (0)| 00:00:01 | |* 1 | COUNT STOPKEY | | | | | | | 2 | INDEX FULL SCAN| LASTNAME_IDX | 55500 | 433K| 2 (0)| 00:00:01 | --------------------------------------------------------------------------------- Predicate Information (identified by operation id): PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------------- --------------------------------------------------- 1 - filter(ROWNUM<=10) 已选择14行。
降序：
sqlSQL> select cust_last_name from customers where rownum <= 10 order by cust_last_name; SQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------------------------- Plan hash value: 1596600344 -------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 9 | 72 | 2 (0)| 00:00:01 | |* 1 | COUNT STOPKEY | | | | | | | 2 | INDEX FULL SCAN DESCENDING| LASTNAME_IDX | 55500 | 433K| 2 (0)| 00:00:01 | -------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------------------------- --------------------------------------------------- 1 - filter(ROWNUM<10) 已选择14行。
