pipelineDB里Combine用法

combine only works on aggregate columns that belong to continuous views.

创建CONTINUOUS

CREATE CONTINUOUS VIEW v AS
      SELECT g::integer, AVG(x::integer) FROM stream GROUP BY g;
CREATE CONTINUOUS VIEW

　　

插入数据

INSERT INTO stream (g, x) VALUES (0, 10), (0, 10), (0, 10), (0, 10), (0, 10);
INSERT INTO stream (g, x) VALUES (1, 20);

　　

查询结果

pipeline=# SELECT * FROM v;
 g |         avg
---+---------------------
 0 | 10.0000000000000000
 1 | 20.0000000000000000
(2 rows)

　　

pipeline=# SELECT avg(avg) FROM v;
         avg
---------------------
 15.0000000000000000
(1 row)

　　

使用Combine查询

pipeline=# SELECT combine(avg) FROM v;
       combine
---------------------
 11.6666666666666667
(1 row)

直接对表表v求平均值，只会对对CQL结果进行再次求平均值。

如果拿Combine，那么会对总的6条数据来进行求平均值。