使用 LINQ 优化嵌套循环

Question

您能否建议如何为以下操作编写优化的 LINQ 查询？

foreach (DataRow entry1 in table1.Rows)
{
    var columnA = entry1["ColumnA"] as string;
    if (!string.IsNullOrEmpty(columnA))
    {
        foreach (string entry2 in table2)
        {
            var dataExists = table3.Any(rows3 =>
                !string.IsNullOrEmpty(rows3[entry2] as string)
                && columnA.IsEqual(rows3["ColumnB"] as string));
            if (dataExists)
            {
                entry1[entry2] = Compute(columnA, entry2);
            }
        }
    }
}

我尝试了这个，但结果在唯一迭代计数方面不匹配。

var t2t3Pair = from entry2 in table2
    let entry3 = table3.FirstOrDefault(x =>
        !string.IsNullOrEmpty(x[entry2] as string))
    where entry3 != null
    select new { entry2, entry3 };

var t1t3Pair = from pair in t2t3Pair
    from entry1 in table1.AsEnumerable()
    let columnA = entry1["ColumnA"] as string
    where !string.IsNullOrEmpty(columnA)
       && columnA.IsEqual(pair.entry3["ColumnB"] as string)
    select new { Entry1Alias = entry1, Entry2Alias = pair.entry2 };

foreach (var pair in t1t3Pair)
{
    var columnA = (string)pair.Entry1Alias["ColumnA"];
    pair.Entry1Alias[pair.Entry2Alias] = Compute(columnA, pair.Entry2Alias);
}

注意：IsEqual 是我在不区分大小写的情况下比较字符串的扩展方法。

Answer 1

显然瓶颈是线路

var dataExists = table3.Any(rows3 =>
    !string.IsNullOrEmpty(rows3[entry2] as string)
    && columnA.IsEqual(rows3["ColumnB"] as string));

在最里面的循环中执行。

像往常一样，可以通过提前准备一个快速查找数据结构并在关键循环中使用它来优化它。

对于您的情况，我建议如下：

var dataExistsMap = table3.AsEnumerable()
    .GroupBy(r => r["ColumnB"] as string)
    .Where(g => !string.IsNullOrEmpty(g.Key))
    .ToDictionary(g => g.Key, g => new HashSet<string>(
        table2.Where(e => g.Any(r => !string.IsNullOrEmpty(r[e] as string)))
    // Include the proper comparer if your IsEqual method is using non default string comparison
    //, StringComparer.OrdinalIgnoreCase
    )
);

foreach (DataRow entry1 in table1.Rows)
{
    var columnA = entry1["ColumnA"] as string;
    if (string.IsNullOrEmpty(columnA)) continue;
    HashSet<string> dataExistsSet;
    if (!dataExistsMap.TryGetValue(columnA, out dataExistsSet)) continue;
    foreach (string entry2 in table2.Where(dataExistsSet.Contains))
        entry1[entry2] = Compute(columnA, entry2);
}