我有一张Cassandra表，键看起来像这样：
PRIMARY KEY（（“k1”，“k2”），“c1”，“c2”），）具有聚类顺序 （“c1”DESC，“c2”DESC）;
当我完全约束查询时，它比我省略最后一个聚类键需要更长的时间。它还预先形成“添加到饲料记忆”，无约束查询不会。为什么是这样？我知道以前这个查询不会将条目添加到memtable中，因为我已经将事物添加到memtable中时运行了自定义代码。此代码应仅在插入或修改内容时运行，但在我仅查询项目时开始运行。
编辑：我应该提到两个查询返回1行并且它是相同的记录。
activity | timestamp | source | source_elapsed | client
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------+---------------+----------------+------------
Execute CQL3 query | 2017-09-05 18:09:37.456000 | **.***.**.237 | 0 | ***.**.*.4
Parsing select c2 from feed where k1 = 'AAA' and k2 = 'BBB' and c1 = '2017-09-05T16:09:00.222Z' and c2 = 'CCC'; [SharedPool-Worker-1] | 2017-09-05 18:09:37.456000 | **.***.**.237 | 267 | ***.**.*.4
Preparing statement [SharedPool-Worker-1] | 2017-09-05 18:09:37.456000 | **.***.**.237 | 452 | ***.**.*.4
Executing single-partition query on feed [SharedPool-Worker-3] | 2017-09-05 18:09:37.457000 | **.***.**.237 | 1253 | ***.**.*.4
Acquiring sstable references [SharedPool-Worker-3] | 2017-09-05 18:09:37.457000 | **.***.**.237 | 1312 | ***.**.*.4
Merging memtable contents [SharedPool-Worker-3] | 2017-09-05 18:09:37.457000 | **.***.**.237 | 1370 | ***.**.*.4
Key cache hit for sstable 22 [SharedPool-Worker-3] | 2017-09-05 18:09:37.463000 | **.***.**.237 | 6939 | ***.**.*.4
Key cache hit for sstable 21 [SharedPool-Worker-3] | 2017-09-05 18:09:37.463000 | **.***.**.237 | 7077 | ***.**.*.4
Key cache hit for sstable 12 [SharedPool-Worker-3] | 2017-09-05 18:09:37.463000 | **.***.**.237 | 7137 | ***.**.*.4
Key cache hit for sstable 6 [SharedPool-Worker-3] | 2017-09-05 18:09:37.463000 | **.***.**.237 | 7194 | ***.**.*.4
Key cache hit for sstable 3 [SharedPool-Worker-3] | 2017-09-05 18:09:37.463000 | **.***.**.237 | 7249 | ***.**.*.4
Merging data from sstable 10 [SharedPool-Worker-3] | 2017-09-05 18:09:37.463000 | **.***.**.237 | 7362 | ***.**.*.4
Key cache hit for sstable 10 [SharedPool-Worker-3] | 2017-09-05 18:09:37.463001 | **.***.**.237 | 7429 | ***.**.*.4
Key cache hit for sstable 9 [SharedPool-Worker-3] | 2017-09-05 18:09:37.463001 | **.***.**.237 | 7489 | ***.**.*.4
Key cache hit for sstable 4 [SharedPool-Worker-3] | 2017-09-05 18:09:37.463001 | **.***.**.237 | 7628 | ***.**.*.4
Key cache hit for sstable 7 [SharedPool-Worker-3] | 2017-09-05 18:09:37.463001 | **.***.**.237 | 7720 | ***.**.*.4
Defragmenting requested data [SharedPool-Worker-3] | 2017-09-05 18:09:37.463001 | **.***.**.237 | 7779 | ***.**.*.4
Adding to feed memtable [SharedPool-Worker-4] | 2017-09-05 18:09:37.464000 | **.***.**.237 | 7896 | ***.**.*.4
Read 1 live and 4 tombstone cells [SharedPool-Worker-3] | 2017-09-05 18:09:37.464000 | **.***.**.237 | 7932 | ***.**.*.4
Request complete | 2017-09-05 18:09:37.464092 | **.***.**.237 | 8092 | ***.**.*.4
activity | timestamp | source | source_elapsed | client
-------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------+---------------+----------------+------------
Execute CQL3 query | 2017-09-05 18:09:44.703000 | **.***.**.237 | 0 | ***.**.*.4
Parsing select c2 from feed where k1 = 'AAA' and k2 = 'BBB' and c1 = '2017-09-05T16:09:00.222Z'; [SharedPool-Worker-1] | 2017-09-05 18:09:44.704000 | **.***.**.237 | 508 | ***.**.*.4
Preparing statement [SharedPool-Worker-1] | 2017-09-05 18:09:44.704000 | **.***.**.237 | 717 | ***.**.*.4
Executing single-partition query on feed [SharedPool-Worker-2] | 2017-09-05 18:09:44.704000 | **.***.**.237 | 1377 | ***.**.*.4
Acquiring sstable references [SharedPool-Worker-2] | 2017-09-05 18:09:44.705000 | **.***.**.237 | 1499 | ***.**.*.4
Key cache hit for sstable 10 [SharedPool-Worker-2] | 2017-09-05 18:09:44.705000 | **.***.**.237 | 1730 | ***.**.*.4
Skipped 8/9 non-slice-intersecting sstables, included 5 due to tombstones [SharedPool-Worker-2] | 2017-09-05 18:09:44.705000 | **.***.**.237 | 1804 | ***.**.*.4
Key cache hit for sstable 22 [SharedPool-Worker-2] | 2017-09-05 18:09:44.705000 | **.***.**.237 | 1858 | ***.**.*.4
Key cache hit for sstable 21 [SharedPool-Worker-2] | 2017-09-05 18:09:44.705000 | **.***.**.237 | 1908 | ***.**.*.4
Key cache hit for sstable 12 [SharedPool-Worker-2] | 2017-09-05 18:09:44.705000 | **.***.**.237 | 1951 | ***.**.*.4
Key cache hit for sstable 6 [SharedPool-Worker-2] | 2017-09-05 18:09:44.705001 | **.***.**.237 | 2002 | ***.**.*.4
Key cache hit for sstable 3 [SharedPool-Worker-2] | 2017-09-05 18:09:44.705001 | **.***.**.237 | 2037 | ***.**.*.4
Merged data from memtables and 6 sstables [SharedPool-Worker-2] | 2017-09-05 18:09:44.705001 | **.***.**.237 | 2252 | ***.**.*.4
Read 1 live and 4 tombstone cells [SharedPool-Worker-2] | 2017-09-05 18:09:44.705001 | **.***.**.237 | 2307 | ***.**.*.4
Request complete | 2017-09-05 18:09:44.705458 | **.***.**.237 | 2458 | ***.**.*.4
cqlsh> show version [cqlsh 5.0.1 | Cassandra 3.7 | CQL spec 3.4.2 | Native protocol v4]
答案 0 :(得分：6)
这是一个很好的问题，你（帮助）提供了我们回答它所需的所有信息！
您的第一个查询是点查找（因为您指定了两个群集键）。第二个是切片。
如果我们查看跟踪，跟踪的明显区别是：
Skipped 8/9 non-slice-intersecting sstables, included 5 due to tombstones
这是一个非常好的提示，我们正在采用两种不同的读取路径。您可以使用它来编码潜水，但长话短说，the filter you use for your point read means you'll query the memtable/sstables in different order - 对于点读取，我们按时间戳排序，对于切片，我们将尝试首先消除不相交的sstables。
代码中的注释提示 - 第一个：
/**
* Do a read by querying the memtable(s) first, and then each relevant sstables sequentially by order of the sstable
* max timestamp.
*
* This is used for names query in the hope of only having to query the 1 or 2 most recent query and then knowing nothing
* more recent could be in the older sstables (which we can only guarantee if we know exactly which row we queries, and if
* no collection or counters are included).
* This method assumes the filter is a {@code ClusteringIndexNamesFilter}.
*/
第二个：
/*
* We have 2 main strategies:
* 1) We query memtables and sstables simulateneously. This is our most generic strategy and the one we use
* unless we have a names filter that we know we can optimize futher.
* 2) If we have a name filter (so we query specific rows), we can make a bet: that all column for all queried row
* will have data in the most recent sstable(s), thus saving us from reading older ones. This does imply we
* have a way to guarantee we have all the data for what is queried, which is only possible for name queries
* and if we have neither collections nor counters (indeed, for a collection, we can't guarantee an older sstable
* won't have some elements that weren't in the most recent sstables, and counters are intrinsically a collection
* of shards so have the same problem).
*/
在您的情况下，如果返回的行恰好在memtable中，则第一次（点）读取会更快。此外，由于您有8个sstables，您可能正在使用STCS或TWCS - 如果您使用LCS，则可能是您将该分区压缩为~5 sstables，并且您（再次）具有更可预测的读取性能。
我之前知道这个查询不会将条目添加到memtable中，因为我已经将事情添加到memtable中时运行了自定义代码。此代码应仅在插入或修改内容时运行，但在我仅查询项目时开始运行。
默认情况下，读取路径都不应该向memtable添加任何内容，除非您正在阅读修复（即，除非副本之间的值不匹配，或触发后台读取修复机会）。请注意，切片查询比点查询更可能不匹配，因为它是基于扫描的 - 您将使用匹配值c1 = '2017-09-05T16:09:00.222Z'读取修复任何/所有删除标记（逻辑删除）
编辑：我错过了跟踪中的一行：
Defragmenting requested data
这表明您正在使用STCS并触及太多的sstables，因此将整个分区复制到memtable中以使将来的读取速度更快。当你开始触摸太多的sstables时，这在STCS中是一个鲜为人知的优化，你可以使用LCS来解决它。
答案 1 :(得分：0)
您正在将苹果与橙子进行比较
您要求所有匹配条件的行的第一个查询
public static void ProcessQueueMessage([QueueTrigger("queue3")] string thumbnail, TextWriter log)
{
string instanceid = Environment.GetEnvironmentVariable("WEBSITE_INSTANCE_ID");
log.Write( "Current instance ID : " + instanceid);
}
这里的额外条件是c2 =＆＃39; CCC＆＃39;所以cassandra需要在返回符合这些条件的行中做更多的工作。
在第二个查询中，您放松了在c2上匹配的条件，因此您可以看到不同的性能行为。
假设您有1000行符合条件k1 =＆＃39; AAA＆＃39;和k2 =＆＃39; BBB＆＃39;和c1 =＆＃39; 2017-09-05T16：09：00.222Z＆＃39;。 添加c2的条件可能只返回4行（可能需要检查c2条件的所有行），因为删除条件将在匹配k1，k2和c1后开始流式传输结果。
k1 = 'AAA' and k2 = 'BBB' and c1 = '2017-09-05T16:09:00.222Z' and c2 = 'CCC'
此外，在检查性能时，您需要多次运行相同的查询以避免任何缓存行为。