在程序处理分页时，有时需要使用count(*)查询，而如果表很大的时候，用此方法无疑瓶颈所在，今天发现了一个解决办法就是如果这个表有唯一列的话（比如主键），那我们可以使用主键，强制走索引来缩短查询时间。
主键：id
索引：SYS_C0029630

SQL> select /*+ index(tt SYS_C0029630) */count(id) from tt;

 COUNT(ID)
----------
     75651

Execution Plan
----------------------------------------------------------
Plan hash value: 3525147495

-------------------------------------------------------------------------
| Id  | Operation        | Name         | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------------
|   0 | SELECT STATEMENT |              |     1 |   213   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE  |              |     1 |            |          |
|   2 |   INDEX FULL SCAN| SYS_C0029630 | 88573 |   213   (0)| 00:00:01 |
-------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)

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

SQL> desc tt;
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 ID                                        NOT NULL NUMBER(38)
 NAME                                               VARCHAR2(2)