我有一个卡桑德拉表，钥匙看起来像这样：
主键（（" K1"，" K2"），" C1"，" C2"），带有聚类顺序 （" C1" desc，" C2" desc）;
当我完全限制查询时，它比忽略最后一个聚类键的时间要长得多。它还预示了"添加到可记忆的添加"，而无约束的查询不会。为什么是这样？我以前知道此查询不会将条目添加到可记忆的内容中，因为当我将物品添加到记忆中时，我的自定义代码运行。此代码只有在插入或修改的内容时才能运行，但在我只查询项目时就开始运行。
编辑：我应该提到两个查询返回1行，这是相同的记录。
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]
这是一个很好的问题，您（有益地）提供了我们需要回答的所有信息！
您的第一个查询是一个查找（因为您指定了两个群集键）。第二个是切片。
如果我们看迹线，则痕迹的明显区别是：
Skipped 8/9 non-slice-intersecting sstables, included 5 due to tombstones
这是一个很好的暗示，我们正在采用两种不同的阅读路径。您可以使用它来编码潜水，但是长话短说，您用于观点读取的过滤器意味着您会以不同的顺序查询可记忆的/sstables-对于点读取，我们按时间戳进行排序，为了切片，我们会尝试首先消除非交流的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}.
*/
和第二个：
/*
* 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记忆中，则第一个（点）读取将更快。另外，由于您有8个Sstables，因此您可能使用STC或TWC-如果使用LCS，则可能会将该分区压缩到〜5个Sstables中，并且（再次）具有更可预测的读取性能。
我以前知道此查询不会将条目添加到可记忆的内容中，因为当我将物品添加到记忆中时，我的自定义代码运行。此代码只有在插入或修改的内容时才能运行，但仅在我只查询项目时就开始运行。
否则默认情况下，
都不应将任何内容添加到记忆中，除非您正在阅读修复（也就是说，除非副本之间的值不匹配或背景读取修复机会是触发的）。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 - 您将读取任何/所有/所有删除标记（墓碑），具有c1 = '2017-09-05T16:09:00.222Z'
编辑：我错过了跟踪中的一行：
Defragmenting requested data
这表明您使用的是STC并触摸了太多的Sstables，因此将整个分区复制到可记忆中，以使未来的读数更快。这是STC中鲜为人知的优化，当您开始触摸太多的Sstables时，您可以使用LCS围绕它进行工作。
您将苹果与橙色进行比较
-
您要求所有行匹配条件的第一个查询
k1 = 'AAA' and k2 = 'BBB' and c1 = '2017-09-05T16:09:00.222Z' and c2 = 'CCC'这里的额外条件是C2 ='CCC'，因此Cassandra需要做更多的工作来返回与这些条件相匹配的行。 -
在第二个查询中，您正在放松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行与条件K1 ='AAA'和K2 ='BBB'和C1 ='2017-09-05T16：09：00.2222Z'。添加C2的条件可能仅返回4行（可能需要检查所有行中的所有行中的所有行），其中删除条件将在匹配K1，K2和C1时开始流式传输结果。
。- 如果您真的想比较，则可以比较 之间的性能
k1 = 'AAA' and k2 = 'BBB' and c1 = '2017-09-05T16:09:00.222Z' and c2 = 'CCC'
OR
k1 = 'AAA' and k2 = 'BBB' and c1 = '2017-09-05T16:09:00.222Z' and c2 = 'XXX'
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。