sql - 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 table :

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

我需要为 p_market 选择具有 datetime 最大值的行，它小于或等于 p_datetime，所以我有两个选择:

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，哪种变体更好。

Answer 1

Postgres 支持 window function ，理论上应该在处理行时增量计算，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