在主键字段中选择具有最大值的行的最快方法

Question

我有一张桌子：

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

我需要选择一行p_market与最大值datetime ，该申请是小于或等于p_datetime ，所以我有两个选择采用这样的：

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，问题是-从性能角度来看哪个变体更好。

Answer 1

Postgres支持window函数 ，从理论上讲，应该在处理行时以递增方式计算它，因此它应该在大型数据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