[ORALCE]SQL 优化案例之 组合索引与排序
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组合索引与排序
步骤一 构造测试表
drop table TX1 purge; create table TX1 as select * from dba_objects; create index idx_object_id on TX1(owner,object_type); alter table TX1 modify owner not NULL; exec dbms_stats.gather_table_stats(‘SYS‘,‘TX1‘,cascade=>true);
测试一,索引无排序
SQL> set autotrace traceonly SQL> select /*+index(TX1,idx_object_id)*/ * from TX1 order by owner desc ,object_type asc; 73396 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 509348553 -------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time | -------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 73396 | 9461K| | 5932 (1)| 00:00:01 | | 1 | SORT ORDER BY | | 73396 | 9461K| 13M| 5932 (1)| 00:00:01 | | 2 | TABLE ACCESS BY INDEX ROWID BATCHED| TX1 | 73396 | 9461K| | 3627 (1)| 00:00:01 | | 3 | INDEX FULL SCAN | IDX_OBJECT_ID | 73396 | | | 259 (1)| 00:00:01 | -------------------------------------------------------------------------------------------------------------- Statistics ---------------------------------------------------------- 60 recursive calls 0 db block gets 3726 consistent gets 0 physical reads 0 redo size 4637967 bytes sent via SQL*Net to client 54269 bytes received via SQL*Net from client 4895 SQL*Net roundtrips to/from client 5 sorts (memory) 0 sorts (disk) 73396 rows processed
发现有许多内存排序
测试二,索引无有排序
drop index idx_object_id; create index idx_object_id on TX1(owner desc ,object_type asc); select /*+index(TX1,idx_object_id)*/ * from TX1 order by owner desc ,object_type asc; Execution Plan ---------------------------------------------------------- Plan hash value: 3824983714 --------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 73396 | 9461K| 3638 (1)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| TX1 | 73396 | 9461K| 3638 (1)| 00:00:01 | | 2 | INDEX FULL SCAN | IDX_OBJECT_ID | 73396 | | 270 (1)| 00:00:01 | --------------------------------------------------------------------------------------------- Statistics ---------------------------------------------------------- 1 recursive calls 0 db block gets 13190 consistent gets 268 physical reads 0 redo size 4610085 bytes sent via SQL*Net to client 54269 bytes received via SQL*Net from client 4895 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 73396 rows processed
测试结果,发现内存排序没有了,cost降低了不少
测试三,索引排序反转
select /*+index(TX1,idx_object_id)*/ * from TX1 order by owner asc ,object_type desc; SQL> select /*+index(TX1,idx_object_id)*/ * from TX1 order by owner asc ,object_type desc; 73396 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 509348553 -------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time | -------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 73396 | 9461K| | 5943 (1)| 00:00:01 | | 1 | SORT ORDER BY | | 73396 | 9461K| 13M| 5943 (1)| 00:00:01 | | 2 | TABLE ACCESS BY INDEX ROWID BATCHED| TX1 | 73396 | 9461K| | 3638 (1)| 00:00:01 | | 3 | INDEX FULL SCAN | IDX_OBJECT_ID | 73396 | | | 270 (1)| 00:00:01 | -------------------------------------------------------------------------------------------------------------- Statistics ---------------------------------------------------------- 1 recursive calls 0 db block gets 3634 consistent gets 0 physical reads 0 redo size 4637361 bytes sent via SQL*Net to client 54269 bytes received via SQL*Net from client 4895 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 73396 rows processed
发现有内存排序,cost 还是很高
测试四,加hit 索引反转
SQL> select /*+index_desc(TX1,idx_object_id)*/ * from TX1 order by owner asc ,object_type desc; 73396 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 1887026249 --------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 73396 | 9461K| 3638 (1)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| TX1 | 73396 | 9461K| 3638 (1)| 00:00:01 | | 2 | INDEX FULL SCAN DESCENDING| IDX_OBJECT_ID | 73396 | | 270 (1)| 00:00:01 | --------------------------------------------------------------------------------------------- Statistics ---------------------------------------------------------- 1 recursive calls 0 db block gets 13182 consistent gets 0 physical reads 0 redo size 4610122 bytes sent via SQL*Net to client 54274 bytes received via SQL*Net from client 4895 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 73396 rows processed
测试结果,发现没有内存排序,COST提高不少
总结，索引排序提高效率，可以在应用中使用
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