使用LINQ優化嵌套循環

Question

您能否建議如何爲以下操作編寫優化LINQ查詢？

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); 
      } 
     } 
    } 
} 

我試過這個，但結果在獨特的迭代計數方面不匹配。

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是我的擴展方法不區分大小寫比較字符串。

Answer 1

顯然的瓶頸是其最裏面的循環中執行的行

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

。與往常一樣，它可以通過預先準備快速查找數據結構並在關鍵循環內部使用它進行優化。

對於你的情況，我建議是這樣的：

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); 
}