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); 
      } 
     } 
    } 
} 

我试过这个，但结果在独特的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：IsEqual是我的扩展方法不区分大小写比较字符串。

Answer 1

显然的瓶颈是其最里面的循环中执行的行

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

。与往常一样，它可以通过预先准备快速查找数据结构并在关键循环内部使用它进行优化。

对于你的情况，我建议是这样的：

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); 
}