postgresql

Question

我有 table 看起来像这样的：

我正在尝试 INNER JOIN 这两个 table，这样我就会得到这样的结果：

        time         | block_height | differential_pressure |
---------------------+--------------+-----------------------+
 2018-09-08 11:14:10 |         83.7 |                286.84 |
 2018-09-08 11:14:10 |         83.6 |                282.14 |
 2018-09-08 11:14:11 |         83.4 |                298.35 |
 2018-09-08 11:14:12 |         83.1 |                298.23 |
 2018-09-08 11:14:12 |         82.9 |                294.76 |
 2018-09-08 11:14:13 |         82.7 |                288.37 |

但是当我运行下面的查询时：

SELECT * FROM rt_block_height 
INNER JOIN rt_differential_pressure 
ON rt_block_height.time = rt_differential_pressure.time;

这是我得到的：

我不明白这是怎么回事。似乎添加了一些随机的附加行，但我不知道为什么会这样。原来的table只有6行，但是查询出来的table returns 10.

我不知道此信息是否有帮助，但这是一个 TimescaleDB Hypertable。这是 table 创建的源代码：

CREATE TABLE IF NOT EXISTS public.rt_BLOCK_HEIGHT
(
"time" timestamp without time zone,
BLOCK_HEIGHT double precision
)
WITH (
OIDS = FALSE
)
TABLESPACE pg_default;

ALTER TABLE public.rt_BLOCK_HEIGHT
OWNER to postgres;
SELECT create_hypertable('rt_BLOCK_HEIGHT', 'time');

Answer 1

您的时间列不是唯一的。

对于 2018-09-08 11:14:10 时间戳，您有：

block_heightA = 83.7
block_heightB = 83.6
differential_pressureA = 286.84
differential_pressureB = 282.14

因此，当您进行连接时，您将得到两个 2 元素集的笛卡尔积：

2018-09-08 11:14:10 block_heightA differential_pressureA
2018-09-08 11:14:10 block_heightA differential_pressureB
2018-09-08 11:14:10 block_heightB differential_pressureA
2018-09-08 11:14:10 block_heightB differential_pressureB

要获得您想要的结果，您必须决定如何处理每个时间戳的重复值。例如，您可以计算平均值：

SELECT
  grouped_block_height.time,
  avg_block_height,
  avg_differential_pressure
FROM (
  SELECT time, avg(block_height) as avg_block_height
  FROM rt_block_height
  GROUP BY time
) as grouped_block_height 
INNER JOIN (
  SELECT time, avg(differential_pressure) as avg_differential_pressure
  FROM rt_differential_pressure
  GROUP BY time
) as grouped_differential_pressure
ON grouped_block_height.time = grouped_differential_pressure.time;

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