【Oracle index】组合索引如何选择前导列的几点考虑

选择组合索引的前导列，必须根据具体的业务（SQL）写法和列的数据分布不同而不同，很多书或网上都说，前导列要选择高选择性的，但是，脱离具体的业务，这些是没有意义的，本文就举一些常见的例子来分析下如何正确选择前导列,以抛砖引玉，实际应用中，有更多复杂的情况需要具体分析。

1.都是等值条件的列，谁做前导列都一样

DROP TABLE t;
CREATE TABLE t 
AS
SELECT * FROM dba_objects;
CREATE INDEX idx1_t ON t(owner,object_id);
CREATE INDEX idx2_t ON t(object_id,owner);
BEGIN
  dbms_stats.gather_table_stats(ownname => USER,tabname => 'T',estimate_percent => 100,cascade => TRUE);
END;
 /
dingjun123@ORADB>  SELECT COUNT(DISTINCT owner),COUNT(DISTINCT object_id),COUNT(*) FROM t;
COUNT(DISTINCTOWNER) COUNT(DISTINCTOBJECT_ID)   COUNT(*)
-------------------- ------------------------ ----------
                  33                    75250      75251
1 row selected.

owner有33个不同的值，object_id有75250,显然object_id的选择性更好。但是下面的查询，应用idx1_t与idx2_t的性能一样(COST与CONSISTENT GETS一样）。

dingjun123@ORADB>  SELECT/*+index(t idx1_t)*/ * FROM t
  2   WHERE owner='DINGJUN123' AND object_id=75677;
1 row selected.
Elapsed: 00:00:00.00
Execution Plan
----------------------------------------------------------
Plan hash value: 2071967826
--------------------------------------------------------------------------------------
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |        |     1 |    97 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T      |     1 |    97 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX1_T |     1 |       |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OWNER"='DINGJUN123' AND "OBJECT_ID"=75677)
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          4  consistent gets
          0  physical reads
          0  redo size
       1403  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed


dingjun123@ORADB>   SELECT/*+index(t idx2_t)*/ * FROM t
  2   WHERE owner='DINGJUN123' AND object_id=75677;
1 row selected.
Elapsed: 00:00:00.01
Execution Plan
----------------------------------------------------------
Plan hash value: 3787301248
--------------------------------------------------------------------------------------
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |        |     1 |    97 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T      |     1 |    97 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX2_T |     1 |       |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OBJECT_ID"=75677 AND "OWNER"='DINGJUN123')
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          4  consistent gets
          0  physical reads
          0  redo size
       1403  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

      虽然如此，但是要记住，这个索引不是仅仅给这2条SQL使用的，事实上可能我们有的查询谓词只有owner或object_id,这时候得考虑使用owner作为前导列还是使用object_id作为前导列。
    还有其他引用owner,object_id的情况，比如GROUP BY ,ORDER BY,甚至SELECT...都需要进行整体的分析，这样才能建立最佳的索引。
 
2.有的列是大于(等于)或小于(等于）或者是like 模糊匹配等不等条件，有的列是等值的条件，等值的一般作为前导列更好
--做5次，增加几十万行SYS的进去
INSERT INTO t SELECT * FROM t WHERE owner='SYS';
COMMIT;
--重新收集统计信息（省略）

dingjun123@ORADB> SELECT * FROM t
  2  WHERE owner='DINGJUN123'
  3  AND object_id>=107889;
1 row selected.
Elapsed: 00:00:00.01
Execution Plan
----------------------------------------------------------
Plan hash value: 2071967826
--------------------------------------------------------------------------------------
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |        |     1 |    96 |     4   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T      |     1 |    96 |     4   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX1_T |     1 |       |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OWNER"='DINGJUN123' AND "OBJECT_ID">=107889 AND "OBJECT_ID" IS
              NOT NULL)
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          5  consistent gets
          0  physical reads
          0  redo size
       1399  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

      上面的SQL走idx1_t,注意观察谓词，只有access,说明索引完全被利用上，很显然因为owner是前导列，而且是等值查询，按照前导列查询，然后只要分析索引的第2列object_id,当发现不满足条件object_id>=107889之后就停止了，索引扫描没有浪费。

dingjun123@ORADB> SELECT/*+index(t idx2_t)*/ * FROM t
  2  WHERE owner='DINGJUN123'
  3  AND object_id>=107889;
1 row selected.
Elapsed: 00:00:00.01
Execution Plan
----------------------------------------------------------
Plan hash value: 3787301248
--------------------------------------------------------------------------------------
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |        |     1 |    96 |     4   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T      |     1 |    96 |     4   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX2_T |     1 |       |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OBJECT_ID">=107889 AND "OWNER"='DINGJUN123' AND "OBJECT_ID" IS
              NOT NULL)
       filter("OWNER"='DINGJUN123')
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          5  consistent gets
          0  physical reads
          0  redo size
       1399  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

      强制使用idx2_t,object_id是前导列，谓词有access,还有filter,说明索引没有被完全利用上，这是因为object_id的不是等值查询，满足object_id>=107889的，按照顺序搜索
所以，中间可能有一些不满足owner='DINGJUN123'的，还要filter掉。
这种查询和不等值条件作为前导列的查询，一旦object_id>=107889不满足owner='DINGJUN123'的很多，那么必然造成过多不必要的索引搜索，COST与逻辑读会上升很快，
从而性能急剧下降，因为本例子基本都满足owner条件，所以没有啥浪费。但是下面的例子：

dingjun123@ORADB> SELECT * FROM t
  2  WHERE owner='DINGJUN123'
  3  AND object_id>=100;
2540 rows selected.
Elapsed: 00:00:00.15
Execution Plan
----------------------------------------------------------
Plan hash value: 2071967826
--------------------------------------------------------------------------------------
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |        |  2539 |   238K|   499   (0)| 00:00:06 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T      |  2539 |   238K|   499   (0)| 00:00:06 |
|*  2 |   INDEX RANGE SCAN          | IDX1_T |  2539 |       |    12   (0)| 00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OWNER"='DINGJUN123' AND "OBJECT_ID">=100 AND "OBJECT_ID" IS
              NOT NULL)
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        527  consistent gets
         21  physical reads
          0  redo size
     268134  bytes sent via SQL*Net to client
       2275  bytes received via SQL*Net from client
        171  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
       2540  rows processed

还是使用idx1_t,没有问题。看下面的强制使用idx2_t的。

dingjun123@ORADB> SELECT/*+index(t idx2_t)*/ * FROM t
  2  WHERE owner='DINGJUN123'
  3  AND object_id>=100;

2540 rows selected.
Elapsed: 00:00:00.33
Execution Plan
----------------------------------------------------------
Plan hash value: 3787301248
--------------------------------------------------------------------------------------
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |        |  2539 |   238K|  3762   (1)| 00:00:46 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T      |  2539 |   238K|  3762   (1)| 00:00:46 |
|*  2 |   INDEX RANGE SCAN          | IDX2_T |  2539 |       |  3274   (1)| 00:00:40 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OBJECT_ID">=100 AND "OWNER"='DINGJUN123' AND "OBJECT_ID" IS
              NOT NULL)
       filter("OWNER"='DINGJUN123')
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3763  consistent gets
          0  physical reads
          0  redo size
     268134  bytes sent via SQL*Net to client
       2275  bytes received via SQL*Net from client
        171  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
       2540  rows processed

dingjun123@ORADB> SELECT COUNT(*) FROM t WHERE object_id >= 100;
  COUNT(*)
----------
   1032649

 SELECT COUNT(*) FROM t WHERE object_id >= 100;
返回1032649行，但是WHERE owner='DINGJUN123' AND object_id>=100 只返回2540行，要filter掉百万行，轮询索引，造成了极大的浪费。


3.如果都是比较，都是<,>之类的表达式
  这种情况，前导列，根据谓词，选择条件能够定位最接近处理结果的基数，并能够减少索引后filter的工作，因为必然有一列是要走access之后的filter,最好是filter能够过滤较少数据，不要做过多过滤。
  
例如：

dingjun123@ORADB> SELECT * FROM t
  2  WHERE owner>='DINGJUN123'
  3  AND object_id>=107872;

37 rows selected.
Elapsed: 00:00:00.00
Execution Plan
----------------------------------------------------------
Plan hash value: 3787301248
--------------------------------------------------------------------------------------
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |        |   205 | 19680 |    43   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T      |   205 | 19680 |    43   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX2_T |   205 |       |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OBJECT_ID">=107872 AND "OWNER">='DINGJUN123' AND "OBJECT_ID"
              IS NOT NULL)
       filter("OWNER">='DINGJUN123')

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
         14  consistent gets
          0  physical reads
          0  redo size
       6468  bytes sent via SQL*Net to client
        438  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         37  rows processed

dingjun123@ORADB> SELECT COUNT(*) FROM t WHERE object_id>=107872;
  COUNT(*)
----------
        37
1 row selected.

  关闭index SKIP SCAN,因为owner种类很少，oracle选择skip SCAN
 alter session set "_optimizer_skip_scan_enabled" = false;
 

dingjun123@ORADB> SELECT/*+index(t idx1_t)*/ * FROM t
  2  WHERE owner>='DINGJUN123'
  3  AND object_id>=107872;
37 rows selected.
Elapsed: 00:00:00.23
Execution Plan
----------------------------------------------------------
Plan hash value: 2071967826
--------------------------------------------------------------------------------------
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |        |   205 | 19680 |  3740   (1)| 00:00:45 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T      |   205 | 19680 |  3740   (1)| 00:00:45 |
|*  2 |   INDEX RANGE SCAN          | IDX1_T |   205 |       |  3700   (1)| 00:00:45 |
--------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OWNER">='DINGJUN123' AND "OBJECT_ID">=107872 AND "OWNER" IS
              NOT NULL)
       filter("OBJECT_ID">=107872)
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3688  consistent gets
          0  physical reads
          0  redo size
       6468  bytes sent via SQL*Net to client
        438  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         37  rows processed

因为owner>='DINGJUN123'返回大量行，但是事实结果很少，只有几十行，过滤object_id>=107872,需要做大量工作，逻辑读和COST增大千倍+，性能低下。


后记：
     当然如何选择前导列的顺序很复杂，得全盘考虑对应的谓词，SELECT的列等要素，还要考虑ORDER BY ,GROUP BY等列，比如3列组合索引，如何考虑顺序。
     后续会补充更多的组合索引如何创建的要点。