combine only works on aggregate columns that belong to continuous views.
创建CONTINUOUS
1 2 3 | CREATE CONTINUOUS VIEW v AS SELECT g::integer, AVG(x::integer) FROM stream GROUP BY g;CREATE CONTINUOUS VIEW |
插入数据
1 2 | 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
1 2 3 4 5 6 | pipeline=# SELECT * FROM v; g | avg---+--------------------- 0 | 10.0000000000000000 1 | 20.0000000000000000(2 rows) |
1 2 3 4 5 | pipeline=# SELECT avg(avg) FROM v; avg --------------------- 15.0000000000000000(1 row) |
使用Combine查询
1 2 3 4 5 | 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。
