Postgres的：獲得最大值和最小值，和時間戳他們發生

Question

我跑的Postgres 9.2，並有溫度和時間標記，每分鐘一個時間戳毫秒時代時間的表：

weather=# \d weather_data 
     Table "public.weather_data" 
    Column |  Type  | Modifiers 
-------------+--------------+----------- 
timestamp | bigint  | not null 
sensor_id | integer  | not null 
temperature | numeric(4,1) | 
humidity | integer  | 
date  | date   | not null 
Indexes: 
    "weather_data_pkey" PRIMARY KEY, btree ("timestamp", sensor_id) 
    "weather_data_date_idx" btree (date) 
    "weather_data_humidity_idx" btree (humidity) 
    "weather_data_sensor_id_idx" btree (sensor_id) 
    "weather_data_temperature_idx" btree (temperature) 
    "weather_data_time_idx" btree ("timestamp") 
Foreign-key constraints: 
    "weather_data_sensor_id_fkey" FOREIGN KEY (sensor_id) REFERENCES weather_sensors(sensor_id) 

weather=# select * from weather_data order by timestamp desc; 
    timestamp | sensor_id | temperature | humidity | date  
---------------+-----------+-------------+----------+------------ 
1483272420000 |   2 |  22.3 |  57 | 2017-01-01 
1483272420000 |   1 |  24.9 |  53 | 2017-01-01 
1483272360000 |   2 |  22.3 |  57 | 2017-01-01 
1483272360000 |   1 |  24.9 |  58 | 2017-01-01 
1483272300000 |   2 |  22.4 |  57 | 2017-01-01 
1483272300000 |   1 |  24.9 |  57 | 2017-01-01 
[...] 

我有這個現有的查詢得到的高點和每一天的低點，但不是具體時間是高還是低發生：

WITH t AS (
    SELECT date, highest, lowest 
    FROM (
     SELECT date, max(temperature) AS highest 
     FROM weather_data 
     WHERE sensor_id = (SELECT sensor_id FROM weather_sensors WHERE sensor_name = 'outdoor') 
     GROUP BY date 
     ORDER BY date ASC 
    ) h 
    INNER JOIN (
     SELECT date, min(temperature) AS lowest 
     FROM weather_data 
     WHERE sensor_id = (SELECT sensor_id FROM weather_sensors WHERE sensor_name = 'outdoor') 
     GROUP BY date 
     ORDER BY date ASC 
    ) l 
    USING (date) 
    ORDER BY date DESC 
) 
SELECT * from t ORDER BY date ASC; 

有一點超過兩個百萬行的數據庫，它需要〜1.2秒運行，這不是 太糟糕了。我想現在得到的具體時間，高或低的是，我想出了這個利用窗口函數，這確實工作，但需要〜5.6秒時：

SELECT h.date, high_time, high_temp, low_time, low_temp FROM (
    SELECT date, high_temp, high_time FROM (
     SELECT date, temperature AS high_temp, timestamp AS high_time, row_number() 
     OVER (PARTITION BY date ORDER BY temperature DESC, timestamp DESC) 
     FROM weather_data 
     WHERE sensor_id = (SELECT sensor_id FROM weather_sensors WHERE sensor_name = 'outdoor') 
    ) highs 
    WHERE row_number = 1 
) h 
INNER JOIN (
    SELECT * FROM (
     SELECT date, temperature AS low_temp, timestamp AS low_time, row_number() 
     OVER (PARTITION BY date ORDER BY temperature ASC, timestamp DESC) 
     FROM weather_data 
     WHERE sensor_id = (SELECT sensor_id FROM weather_sensors WHERE sensor_name = 'outdoor') 
    ) lows 
    WHERE row_number = 1 
) l 
ON h.date = l.date 
ORDER BY h.date ASC; 

有一些相對簡單的除我可以做的第一個查詢不會增加大量的執行時間？我假設有，但我認爲我處於這個問題太久的地步了！

Answer 1

SELECT 
     DISTINCT ON (zdate) zdate 
     , first_value(ztimestamp) OVER www AS stamp_at_min 
     , first_value(temperature) OVER www AS tmin 
     , last_value(ztimestamp) OVER www AS stamp_at_max 
     , last_value(temperature) OVER www AS tmax 
FROM weather_data 
WHERE sensor_id = 2 
WINDOW www AS (PARTITION BY zdate ORDER BY temperature, ztimestamp 
       ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING 
       ) 
     ; 

---

• 前綴ž日期和ž時間戳
• 我加ztimestamp到排序作爲決勝

Answer 2

這確實與您的第二個查詢，但只需要在weather_data表中的單個掃描：

select date, 
     max(case when high_rn = 1 then timestamp end) as high_time, 
     max(case when high_rn = 1 then temperature end) as high_temp, 
     max(case when low_rn = 1 then timestamp end) as low_time, 
     max(case when low_rn = 1 then temperature end) as low_temp 
from (
    select timestamp, temperature, date, 
     row_number() OVER (PARTITION BY date ORDER BY temperature DESC, timestamp DESC) as high_rn, 
     row_number() OVER (PARTITION BY date ORDER BY temperature ASC, timestamp DESC) as low_rn 
    from weather_data 
    where sensor_id = ... 
) t 
where (high_rn = 1 or low_rn = 1) 
group by date; 

它使用條件聚集做一個交叉表（又名「轉動」）的結果查詢只包含最低和最高溫度。

---

^{無關，而是：date和timestamp是列名可怕。首先是因爲它們是關鍵字，但更重要的是因爲它們沒有記錄列的實際含義。它是「到期日期」嗎？ 「閱讀日期」？ 「處理日期」？}