在主鍵字段中選擇具有最大值的行的最快方法

Question

我有一張桌子：

CREATE TABLE spots(
    datetime TIMESTAMP,
    market VARCHAR(15),
    spot NUMERIC(10, 5),
    PRIMARY KEY (market, datetime)
);

我需要選擇一行p_market與最大值datetime ，該申請是小於或等於p_datetime ，所以我有兩個選擇采用這樣的：

SELECT * FROM spots WHERE market = p_market AND datetime = (
    SELECT MAX(datetime) FROM spots WHERE market = p_market AND datetime <= p_datetime
);

和

SELECT * FROM spots WHERE market = p_market AND datetime <= p_datetime
ORDER BY datetime DESCENDING LIMIT 1;

因此，問題是-從性能角度來看哪個變體更好。

Answer 1

Postgres支持window函數 ，從理論上講，應該在處理行時以遞增方式計算它，因此它應該在大型數據集上提供一些性能提升：

SELECT * 
FROM   (SELECT *,
               RANK() OVER (ORDER BY datetime DESC) AS rk
        FROM   spots
        WHERE  market = p_market AND datetime <= p_datetime) t
FROM   spots 
WHERE  rk = 1