同时查询最大最小值的效率问题

来源：互联网发布：linux mysql5.7 编辑：程序博客网时间：2024/05/21 19:37

有时候我们需要同时查询一部分数据中的最大和最小值，如何才能最快取得我们要的结果呢？

我们知道索引已经对数据进行了排序，所以这时走索引效率是最高的，下面看一个例子

--在一个有将近两百万数据的表中创建日期索引，需要同时取出日期的最大值和最小值：

SQL> select COUNT(*) from USER_OBJECTS_TMP t;   COUNT(*)----------   1933312CREATE INDEX user_objects_create_dt ON user_objects_tmp(created) TABLESPACE tbs_lubinsu_idx;SQL> desc user_objects_tmpName           Type          Nullable Default Comments -------------- ------------- -------- ------- -------- OBJECT_NAME    VARCHAR2(128) Y                         SUBOBJECT_NAME VARCHAR2(30)  Y                         OBJECT_ID      NUMBER        Y                         DATA_OBJECT_ID NUMBER        Y                         OBJECT_TYPE    VARCHAR2(19)  Y                         CREATED        DATE          Y                         LAST_DDL_TIME  DATE          Y                         TIMESTAMP      VARCHAR2(19)  Y                         STATUS         VARCHAR2(7)   Y                         TEMPORARY      VARCHAR2(1)   Y                         GENERATED      VARCHAR2(1)   Y                         SECONDARY      VARCHAR2(1)   Y                         ;

--在CREATED字段创建了索引，如果直接使用MAX和MIN函数来获取最大最小值，如下所示：
--消耗时间：Elapsed: 00:00:00.54，从执行计划我们可以看出改语句并没有走索引，因为这里没有指定谓词。

SQL> set timing onSQL> set autotrace traceonlySQL> set linesize 200  SQL> SELECT MIN(created), MAX(created) AS MIN FROM user_objects_tmp;Elapsed: 00:00:00.54Execution Plan----------------------------------------------------------Plan hash value: 3066201300---------------------------------------------------------------------------------------| Id  | Operation          | Name             | Rows  | Bytes | Cost (%CPU)| Time     |---------------------------------------------------------------------------------------|   0 | SELECT STATEMENT   |                  |     1 |     9 |  1718   (4)| 00:00:21 ||   1 |  SORT AGGREGATE    |                  |     1 |     9 |            |          ||   2 |   TABLE ACCESS FULL| USER_OBJECTS_TMP |  1600K|    13M|  1718   (4)| 00:00:21 |---------------------------------------------------------------------------------------Note-----   - dynamic sampling used for this statementStatistics----------------------------------------------------------          5  recursive calls          0  db block gets       8639  consistent gets        659  physical reads          0  redo size        481  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

--如果指定谓词之后，则结果如下，并没有因此而提高效率，
--因为同时指定最大MAX最小MIN值并不能走索引的 INDEX FULL SCAN (MIN/MAX) 扫描，
--对于INDEX FAST FULL SCAN的读取方式是：从段头开始，读取包含位图块，ROOT BLOCK，所有的BRANCH BLOCK，LEAF BLOCK，
--读取的顺序完全有物理存储位置决定，并采取多块读，每次读取DB_FILE_MULTIBLOCK_READ_COUNT个块。
--查询某个表记录总数的时候，往往基于PRIMARY KEY的INDEX FAST FULL SCAN是最有效的，但是在这里并不是最合适的。
--INDEX FULL SCAN的读取方式是：先定位到索引树的根节点，然后分支，再读取叶子节点，根据叶子节点的双向链表顺序读取，
--这种读取方式是顺序的并且也是经过排序的，所以我们希望走这种方式：

SQL> SELECT MIN(created), MAX(created) AS MIN FROM user_objects_tmp a WHERE a.created IS NOT NULL;Elapsed: 00:00:00.62Execution Plan----------------------------------------------------------Plan hash value: 3784617757------------------------------------------------------------------------------------------------| Id  | Operation             | Name                   | Rows  | Bytes | Cost (%CPU)| Time     |------------------------------------------------------------------------------------------------|   0 | SELECT STATEMENT      |                        |     1 |     9 |  1661   (1)| 00:00:20 ||   1 |  SORT AGGREGATE       |                        |     1 |     9 |            |          ||*  2 |   INDEX FAST FULL SCAN| USER_OBJECTS_CREATE_DT |  1600K|    13M|  1661   (1)| 00:00:20 |------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   2 - filter("A"."CREATED" IS NOT NULL)Note-----   - dynamic sampling used for this statementStatistics----------------------------------------------------------          5  recursive calls          0  db block gets       8647  consistent gets        267  physical reads          0  redo size        481  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

--既然放在一起查询不行，我们将两个函数分开来做：
--分别让其走索引的全表扫描：
--执行时间:Elapsed: 00:00:00.10

SQL> SELECT MIN, MAX  2  FROM   (SELECT MIN(created) AS MIN FROM user_objects_tmp) a,  3         (SELECT MAX(created) AS MAX FROM user_objects_tmp) b;Elapsed: 00:00:00.10Execution Plan----------------------------------------------------------Plan hash value: 4210122108-------------------------------------------------------------------------------------------------------| Id  | Operation                    | Name                   | Rows  | Bytes | Cost (%CPU)| Time     |-------------------------------------------------------------------------------------------------------|   0 | SELECT STATEMENT             |                        |     1 |    18 |  3437   (4)| 00:00:42 ||   1 |  NESTED LOOPS                |                        |     1 |    18 |  3437   (4)| 00:00:42 ||   2 |   VIEW                       |                        |     1 |     9 |  1718   (4)| 00:00:21 ||   3 |    SORT AGGREGATE            |                        |     1 |     9 |            |          ||   4 |     INDEX FULL SCAN (MIN/MAX)| USER_OBJECTS_CREATE_DT |  1600K|    13M|            |          ||   5 |   VIEW                       |                        |     1 |     9 |  1718   (4)| 00:00:21 ||   6 |    SORT AGGREGATE            |                        |     1 |     9 |            |          ||   7 |     INDEX FULL SCAN (MIN/MAX)| USER_OBJECTS_CREATE_DT |  1600K|    13M|            |          |-------------------------------------------------------------------------------------------------------Note-----   - dynamic sampling used for this statementStatistics----------------------------------------------------------        195  recursive calls          0  db block gets        159  consistent gets         72  physical reads          0  redo size        472  bytes sent via SQL*Net to client        385  bytes received via SQL*Net from client          2  SQL*Net roundtrips to/from client          5  sorts (memory)          0  sorts (disk)          1  rows processed--可见，这种情况效率是最高的SELECT MIN, MAXFROM   (SELECT MIN(created) AS MIN FROM user_objects_tmp) a,       (SELECT MAX(created) AS MAX FROM user_objects_tmp) b;