Question

这是对MySQL数据库的一些查询。（按历史记录）这是可行的，但我认为这是一个瓶颈。

SELECT
tb_real.One,
tb_real.Two,
tb_real.TimestampMs,
tb_real.Symbol,
tb_max.MaxTimestapmMs
FROM (SELECT
  Symbol,
  MAX(TimestampMs) AS MaxTimestapmMs
FROM times
WHERE Symbol = inSymbol
AND TimestampMs < inDateTo) AS tb_max
JOIN (SELECT
    TimestampMs,
    One,
    Two,
    Symbol
  FROM Times
  WHERE Symbol = inSymbol
  AND TimestampMs < inDateTo) AS tb_real
  ON tb_max.MaxTimestapmMs = tb_real.TimestampMs
;

有效，但速度足够慢。是否可以使其简单\快速？

Answer 1

似乎第一个子查询总是只在Symbol = inSymbol时才选择一行，因此您可以按如下所示重写它。并且确保您必须索引Symbol和TimestapmMs字段

SELECT
    One,
    Two,
    TimestampMs,
    Symbol,
    TimestapmMs
  FROM Times
  WHERE Symbol = inSymbol
  AND TimestampMs  =   (SELECT
                       MAX(TimestampMs)
                       FROM times
                       WHERE Symbol = inSymbol
                       AND TimestampMs < inDateTo)

如果TimestampMs字段是唯一的（只有一个记录具有MAX值），并且此查询始终只返回一个记录，则可以使用LIMIT 1来获取此记录：

    SELECT
        One,
        Two,
        TimestampMs,
        Symbol,
        TimestapmMs
      FROM Times
      WHERE Symbol = inSymbol
      AND TimestampMs < inDateTo
   ORDER BY TimestampMs DESC 
   LIMIT 1

如何更改查询以使其更快？

1 个答案: