arraysize什么意思?

在发送给客户端。与之相关的另一个参数就是SDU(session data unit)，预抓取在PGA所占的内存大小和SDU大小相关，默认是8K.

    SQL> show arraysize         
    arraysize 15
    SQL> create table jason as select * from dba_objects;
    Table created.
    SQL>
    SQL>
    SQL> select count(1) from dba_objects;
      COUNT(1)
    ———-
         50042
    SQL>
    SQL>
    SQL> set autot trace
    SQL>
    SQL>
    SQL>
    SQL> select * from jason;
    50042 rows selected.
    Execution Plan
    ———————————————————-
    Plan hash value: 1258222954
    —————————————————————————
    | Id  | Operation         | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
    —————————————————————————
    |   0 | SELECT STATEMENT  |       | 44426 |  7679K|   155   (2)| 00:00:02 |
    |   1 |  TABLE ACCESS FULL| JASON | 44426 |  7679K|   155   (2)| 00:00:02 |
    —————————————————————————
    Note
    —–
       – dynamic sampling used for this statement
    Statistics
    ———————————————————-
              0  recursive calls
              0  db block gets
           3992  consistent gets
              0  physical reads
              0  redo size
        2544199  bytes sent via SQLwww.pizei.com*Net to client
          37188  bytes received via SQL*Net from client
           3338  SQL*Net roundtrips to/from client
              0  sorts (memory)
              0  sorts (disk)
          50042  rows processed
    SQL> set arraysize 400
    SQL>
    SQL>
    SQL>
    SQL> /
    SQL> /
    50042 rows selected.
    Execution Plan
    ———————————————————-
    Plan hash value: 1258222954
    —————————————————————————
    | Id  | Operation         | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
    —————————————————————————
    |   0 | SELECT STATEMENT  |       | 44426 |  7679K|   155   (2)| 00:00:02 |
    |   1 |  TABLE ACCESS FULL| JASON | 44426 |  7679K|   155   (2)| 00:00:02 |
    —————————————————————————
    Note
    —–
       – dynamic sampling used for this statement
    Statistics
    ———————————————————-
              0  recursive calls
              0  db block gets
            817  consistent gets
              0  physical reads
              0  redo size
        1956586  bytes sent via SQL*Net to client
           1867  bytes received via SQL*Net from client
            127  SQL*Net roundtrips to/from client
              0  sorts (memory)
              0  sorts (disk)
          50042  rows processed

可以看到当我们把arraysize从默认的15调整到400后consistent gets逻辑读从原来的3992下降到817，bytes received via SQL*Net from client相关的页游的值也有明显的下降。

    arraysize参数的设置对大数据的范围比如全表扫性能提高还是比较大的，所以在网络带宽允许的情况下适当调大arraysize和估算SDU的值，对减少
    逻辑读，减少网络交互性，从而介绍CPU的消耗帮助还是比较大的。