之前同事发过一个语句，实现的功能比較简单，相似group by的分组计数功能，由于where条件有like，又无法用group by来实现。SELECT a.N0,b.N1,c.N2,d.N3,e.N4,f.N5,g.N6,h.N7,i.N8,j.N9 from  (select count(*) N0 from tbl_loginfo_20141110 where keyrecord like '0%' or keyrecord like 'GJ_0%') a, (select count(*) N1 from tbl_loginfo_20141110 where keyrecord like '1%' or keyrecord like 'GJ_1%') b, (select count(*) N2 from tbl_loginfo_20141110 where keyrecord like '2%' or keyrecord like 'GJ_2%') c, (select count(*) N3 from tbl_loginfo_20141110 where keyrecord like '3%' or keyrecord like 'GJ_3%') d, (select count(*) N4 from tbl_loginfo_20141110 where keyrecord like '4%' or keyrecord like 'GJ_4%') e, (select count(*) N5 from tbl_loginfo_20141110 where keyrecord like '5%' or keyrecord like 'GJ_5%') f, (select count(*) N6 from tbl_loginfo_20141110 where keyrecord like '6%' or keyrecord like 'GJ_6%') g, (select count(*) N7 from tbl_loginfo_20141110 where keyrecord like '7%' or keyrecord like 'GJ_7%') h, (select count(*) N8 from tbl_loginfo_20141110 where keyrecord like '8%' or keyrecord like 'GJ_8%') i, (select count(*) N9 from tbl_loginfo_20141110 where keyrecord like '9%' or keyrecord like 'GJ_9%') j; 为了了解语句的性能，我做了例如以下相似的測试： select * from v$version;   --Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Productiondrop table a;create table a as select * from dba_objects where rownum<=50000;begin  for x in 1..6 loop    insert into a select * from a;  end loop;  commit;end;select count(*) from a;   --3200000select bytes/1024/1024 from user_segments where segment_name='A';  --357Malter system flush shared_pool;alter system flush buffer_cache;SELECT a.N0,b.N1,c.N2,d.N3,e.N4,f.N5,g.N6,h.N7,i.N8,j.N9 from  (select count(*) N0 from a where object_name like 'A%' or object_name like 'V%') a, (select count(*) N1 from a where object_name like 'B%' or object_name like 'V%') b, (select count(*) N2 from a where object_name like 'C%' or object_name like 'V%') c, (select count(*) N3 from a where object_name like 'D%' or object_name like 'V%') d, (select count(*) N4 from a where object_name like 'E%' or object_name like 'V%') e, (select count(*) N5 from a where object_name like 'F%' or object_name like 'V%') f, (select count(*) N6 from a where object_name like 'G%' or object_name like 'V%') g, (select count(*) N7 from a where object_name like 'H%' or object_name like 'V%') h, (select count(*) N8 from a where object_name like 'I%' or object_name like 'V%') i, (select count(*) N9 from a where object_name like 'J%' or object_name like 'V%') j;--58salter system flush shared_pool;alter system flush buffer_cache;--改写后select sum(case when object_name like 'A%' or object_name like 'V%' then 1 else 0 end) N0,sum(case when object_name like 'B%' or object_name like 'V%' then 1 else 0 end) N1, sum(case when object_name like 'C%' or object_name like 'V%' then 1 else 0 end) N2, sum(case when object_name like 'D%' or object_name like 'V%' then 1 else 0 end) N3, sum(case when object_name like 'E%' or object_name like 'V%' then 1 else 0 end) N4, sum(case when object_name like 'F%' or object_name like 'V%' then 1 else 0 end) N5, sum(case when object_name like 'G%' or object_name like 'V%' then 1 else 0 end) N6, sum(case when object_name like 'H%' or object_name like 'V%' then 1 else 0 end) N7, sum(case when object_name like 'I%' or object_name like 'V%' then 1 else 0 end) N8, sum(case when object_name like 'J%' or object_name like 'V%' then 1 else 0 end) N9from a; --19s--对照运行计划：--前者运行计划：SQL> explain plan for  2  SELECT a.N0,b.N1,c.N2,d.N3,e.N4,f.N5,g.N6,h.N7,i.N8,j.N9 from  3   (select count(*) N0 from a where object_name like 'A%' or object_name like 'V%') a,  4   (select count(*) N1 from a where object_name like 'B%' or object_name like 'V%') b,  5   (select count(*) N2 from a where object_name like 'C%' or object_name like 'V%') c,  6   (select count(*) N3 from a where object_name like 'D%' or object_name like 'V%') d,  7   (select count(*) N4 from a where object_name like 'E%' or object_name like 'V%') e,  8   (select count(*) N5 from a where object_name like 'F%' or object_name like 'V%') f,  9   (select count(*) N6 from a where object_name like 'G%' or object_name like 'V%') g, 10   (select count(*) N7 from a where object_name like 'H%' or object_name like 'V%') h, 11   (select count(*) N8 from a where object_name like 'I%' or object_name like 'V%') i, 12   (select count(*) N9 from a where object_name like 'J%' or object_name like 'V%') j;Explained.Elapsed: 00:00:00.15SQL> @getplan'general,outline,starts'Enter value for plan type:PLAN_TABLE_OUTPUT-----------------------------------------------------------------------------------------Plan hash value: 2527411742-------------------------------------------------------------------------------------| Id  | Operation                    | Name | Rows  | Bytes | Cost (%CPU)| Time     |-------------------------------------------------------------------------------------|   0 | SELECT STATEMENT             |      |     1 |   130 |   123K  (1)| 00:24:46 ||   1 |  NESTED LOOPS                |      |     1 |   130 |   123K  (1)| 00:24:46 ||   2 |   NESTED LOOPS               |      |     1 |   117 |   111K  (1)| 00:22:17 ||   3 |    NESTED LOOPS              |      |     1 |   104 | 99032   (1)| 00:19:49 ||   4 |     NESTED LOOPS             |      |     1 |    91 | 86653   (1)| 00:17:20 ||   5 |      NESTED LOOPS            |      |     1 |    78 | 74274   (1)| 00:14:52 ||   6 |       NESTED LOOPS           |      |     1 |    65 | 61895   (1)| 00:12:23 ||   7 |        NESTED LOOPS          |      |     1 |    52 | 49516   (1)| 00:09:55 ||   8 |         NESTED LOOPS         |      |     1 |    39 | 37137   (1)| 00:07:26 ||   9 |          NESTED LOOPS        |      |     1 |    26 | 24758   (1)| 00:04:58 ||  10 |           VIEW               |      |     1 |    13 | 12379   (1)| 00:02:29 ||  11 |            SORT AGGREGATE    |      |     1 |    66 |            |          ||* 12 |             TABLE ACCESS FULL| A    | 91587 |  5903K| 12379   (1)| 00:02:29 ||  13 |           VIEW               |      |     1 |    13 | 12379   (1)| 00:02:29 ||  14 |            SORT AGGREGATE    |      |     1 |    66 |            |          ||* 15 |             TABLE ACCESS FULL| A    |   137K|  8831K| 12379   (1)| 00:02:29 ||  16 |          VIEW                |      |     1 |    13 | 12379   (1)| 00:02:29 ||  17 |           SORT AGGREGATE     |      |     1 |    66 |            |          ||* 18 |            TABLE ACCESS FULL | A    | 85818 |  5531K| 12379   (1)| 00:02:29 ||  19 |         VIEW                 |      |     1 |    13 | 12379   (1)| 00:02:29 ||  20 |          SORT AGGREGATE      |      |     1 |    66 |            |          ||* 21 |           TABLE ACCESS FULL  | A    |   111K|  7158K| 12379   (1)| 00:02:29 ||  22 |        VIEW                  |      |     1 |    13 | 12379   (1)| 00:02:29 ||  23 |         SORT AGGREGATE       |      |     1 |    66 |            |          ||* 24 |          TABLE ACCESS FULL   | A    | 86539 |  5577K| 12379   (1)| 00:02:29 ||  25 |       VIEW                   |      |     1 |    13 | 12379   (1)| 00:02:29 ||  26 |        SORT AGGREGATE        |      |     1 |    66 |            |          ||* 27 |         TABLE ACCESS FULL    | A    | 91587 |  5903K| 12379   (1)| 00:02:29 ||  28 |      VIEW                    |      |     1 |    13 | 12379   (1)| 00:02:29 ||  29 |       SORT AGGREGATE         |      |     1 |    66 |            |          ||* 30 |        TABLE ACCESS FULL     | A    |   228K|    14M| 12379   (1)| 00:02:29 ||  31 |     VIEW                     |      |     1 |    13 | 12379   (1)| 00:02:29 ||  32 |      SORT AGGREGATE          |      |     1 |    66 |            |          ||* 33 |       TABLE ACCESS FULL      | A    | 87981 |  5670K| 12379   (1)| 00:02:29 ||  34 |    VIEW                      |      |     1 |    13 | 12379   (1)| 00:02:29 ||  35 |     SORT AGGREGATE           |      |     1 |    66 |            |          ||* 36 |      TABLE ACCESS FULL       | A    | 84376 |  5438K| 12379   (1)| 00:02:29 ||  37 |   VIEW                       |      |     1 |    13 | 12379   (1)| 00:02:29 ||  38 |    SORT AGGREGATE            |      |     1 |    66 |            |          ||* 39 |     TABLE ACCESS FULL        | A    |   112K|  7251K| 12379   (1)| 00:02:29 |-------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------  12 - filter("OBJECT_NAME" LIKE 'J%' OR "OBJECT_NAME" LIKE 'V%')  15 - filter("OBJECT_NAME" LIKE 'I%' OR "OBJECT_NAME" LIKE 'V%')  18 - filter("OBJECT_NAME" LIKE 'H%' OR "OBJECT_NAME" LIKE 'V%')  21 - filter("OBJECT_NAME" LIKE 'G%' OR "OBJECT_NAME" LIKE 'V%')  24 - filter("OBJECT_NAME" LIKE 'F%' OR "OBJECT_NAME" LIKE 'V%')  27 - filter("OBJECT_NAME" LIKE 'E%' OR "OBJECT_NAME" LIKE 'V%')  30 - filter("OBJECT_NAME" LIKE 'D%' OR "OBJECT_NAME" LIKE 'V%')  33 - filter("OBJECT_NAME" LIKE 'C%' OR "OBJECT_NAME" LIKE 'V%')  36 - filter("OBJECT_NAME" LIKE 'B%' OR "OBJECT_NAME" LIKE 'V%')  39 - filter("OBJECT_NAME" LIKE 'A%' OR "OBJECT_NAME" LIKE 'V%')          --后者运行计划：  SQL> explain plan for  2  select  3  sum(case when object_name like 'A%' or object_name like 'V%' then 1 else 0 end) N0,  4  sum(case when object_name like 'B%' or object_name like 'V%' then 1 else 0 end) N1,  5  sum(case when object_name like 'C%' or object_name like 'V%' then 1 else 0 end) N2,  6  sum(case when object_name like 'D%' or object_name like 'V%' then 1 else 0 end) N3,  7  sum(case when object_name like 'E%' or object_name like 'V%' then 1 else 0 end) N4,  8  sum(case when object_name like 'F%' or object_name like 'V%' then 1 else 0 end) N5,  9  sum(case when object_name like 'G%' or object_name like 'V%' then 1 else 0 end) N6, 10  sum(case when object_name like 'H%' or object_name like 'V%' then 1 else 0 end) N7, 11  sum(case when object_name like 'I%' or object_name like 'V%' then 1 else 0 end) N8, 12  sum(case when object_name like 'J%' or object_name like 'V%' then 1 else 0 end) N9 13  from a;Explained.Elapsed: 00:00:00.01SQL> @getplan'general,outline,starts'Enter value for plan type:PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------Plan hash value: 3918351354---------------------------------------------------------------------------| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |---------------------------------------------------------------------------|   0 | SELECT STATEMENT   |      |     1 |    66 | 12349   (1)| 00:02:29 ||   1 |  SORT AGGREGATE    |      |     1 |    66 |            |          ||   2 |   TABLE ACCESS FULL| A    |  3097K|   194M| 12349   (1)| 00:02:29 |---------------------------------------------------------------------------Note-----   - dynamic sampling used for this statement (level=2)  能够看出，前者10次全表扫描，后者1次全表扫描。

从而时间上也大大减少了。由58s减少到19s。 优化这个sql主要还是思路的转换。难点在于如何把10次全表扫描转化成1次全表扫描。

在OLAP中，能够加并行使sql速度更快。