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。

巴特西