关于并行查询调优涉及的相关知识[收集中]

1、

并行查询、直接加载（direct load）大表等时，会发生一致性直接读取（consistent gets direct），即服务器进程会绕过高速缓存（buffer cache）直接从磁盘上的数据文件读取数据块，发送给客户端的用户进程。

2、V$PQ_TQSTAT

V$PQ_TQSTAT has a row for each query server process that it reads from or writes to in each table queue. A table queue connecting 10 consumer processes to 10 producer processes has 20 rows in the view.Total the bytes column and group by TQ_ID, and the table queue identifier, to obtain the total number of bytes sent through each table queue. Compare this to the optimizer estimates; large variations might indicate a need toanalyze the data using a larger sample.

Compute the variance of bytes grouped by TQ_ID. Large variances indicateworkload imbalances. You should investigate large variances to determine whether the producers start out with unequal distributions of data, or whether the distribution itself is skewed. If the data itself is skewed, this might indicate a low cardinality, or low number of distinct values.

参考：http://docs.oracle.com/cd/E11882_01/server.112/e25523/parallel006.htm

3、协调者进程是要等所有的并行服务器进程执行完毕将结果集都已经传送到协调者进程这里后才会将整合的结果集发送给客户端进程的。

4、并行处理的时间=从属【并行】并行服务器进程中最慢的那个所用的处理以及传送数据给协调者进程的时间+协调者进程汇总整合结果的时间