sql优化 consistent gets与cost解决思路

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| Id  | Operation                        | Name                     | Rows  | Bytes | Cost (%CPU)| Time     |
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|   0 | SELECT STATEMENT                 |                          |     1 |   142 |    22   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE                  |                          |     1 |   142 |            |       |
|   2 |   NESTED LOOPS                   |                          |     1 |   142 |    22   (0)| 00:00:01 |
|   3 |    NESTED LOOPS                  |                          |     1 |   111 |    20   (0)| 00:00:01 |
|   4 |     NESTED LOOPS                 |                          |     1 |    88 |    19   (0)| 00:00:01 |