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Cassandra long query time and adding to memtable when keys are fully trimmed

I have a Cassandra table and the keys look like this:

PRIMARY KEY (("k1", "k2"), "c1", "c2"),) WITH CLUSTER ORDER ("c1" DESC, "c2" DESC);

When I completely restrain the request, it takes significantly longer than if I left the last clustering key. It also transforms "Add to memtable feed", which the unconditional request does not. Why is this? I know that earlier this request did not add an entry to memtable, as I have custom code when things are added to memtable. This code should only run when everything is inserted or modified, but run when I only requested items.

Edit: I should have mentioned that both queries return 1 row, and this is the same record.

  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]
+6
source share
2 answers

This is a great question, and you (using) provided all the information we need to answer!

Your first query is a point search (since you specify both clustering keys). The second is a slice.

If we look at the trail, the obvious difference is in your track:

Skipped 8/9 non-slice-intersecting sstables, included 5 due to tombstones

, . , , , , , memtable/sstables - , , sstables.

, - :

/**
 * 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}.
 */

:

    /*
     * 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).
     */

, , () . , 8 sstables, , , STCS TWCS - LCS, , ~ 5 sstables, ( ) .

, memtable, , memtable. , , , .

- , ( , ). , FAR , - - / ( ) c1 = '2017-09-05T16:09:00.222Z'

: :

Defragmenting requested data

, STCS sstables, , . STCS, sstables, LCS.

+6

  • , , k1 = 'AAA' and k2 = 'BBB' and c1 = '2017-09-05T16:09:00.222Z' and c2 = 'CCC' c2 = 'CCC', cassandra , , .

  • c2, , .

, 1000 , k1 = 'AAA' k2 = 'BBB' c1 = '2017-09-05T16: 09: 00.222Z'. c2 4 (, c2 ), , k1, k2 c1.

  • ,

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'

, , - .

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