Question

当我通过分页查询二级索引时，随着数据的增长，查询速度变慢。
我以为pagination，无论您的数据量有多大，查询一页都需要花费相同的时间。真的吗？为什么我的查询变慢？

我的简化表是

CREATE TABLE closed_executions (
  domain_id            uuid,
  workflow_id          text,
  start_time           timestamp,
  workflow_type_name   text,
  PRIMARY KEY  ((domain_id), start_time)
) WITH CLUSTERING ORDER BY (start_time DESC)
  AND COMPACTION = {
    'class': 'org.apache.cassandra.db.compaction.LeveledCompactionStrategy'
  }
  AND GC_GRACE_SECONDS = 172800;

然后我将二级索引创建为

CREATE INDEX closed_by_type ON closed_executions (workflow_type_name);

我使用以下CQL查询

SELECT workflow_id, start_time, workflow_type_name 
FROM closed_executions 
WHERE domain_id = ? 
AND start_time >= ? 
AND start_time <= ? 
AND workflow_type_name = ?

和代码

query := v.session.Query(templateGetClosedWorkflowExecutionsByType,
        request.DomainUUID,
        common.UnixNanoToCQLTimestamp(request.EarliestStartTime),
        common.UnixNanoToCQLTimestamp(request.LatestStartTime),
        request.WorkflowTypeName).Consistency(gocql.One)
iter := query.PageSize(request.PageSize).PageState(request.NextPageToken).Iter()
// PageSize is 10, but could be thousand

环境：

MacBook Pro
卡桑德拉：3.11.0
GoCql：github.com/gocql/gocql master

观察：
1万行，秒之内
10万行，约3秒
1M行，约17秒

调试日志：

INFO  [ScheduledTasks:1] 2018-09-11 16:29:48,349 NoSpamLogger.java:91 - Some operations were slow, details available at debug level (debug.log)
DEBUG [ScheduledTasks:1] 2018-09-11 16:29:48,357 MonitoringTask.java:173 - 1 operations were slow in the last 5005 msecs:
<SELECT * FROM cadence_visibility.closed_executions WHERE workflow_type_name = code.uber.internal/devexp/cadence-bench/load/basic.stressWorkflowExecute AND token(domain_id, domain_partition) >= token(d3138e78-abe7-48a0-adb9-8c466a9bb3fa, 0) AND token(domain_id, domain_partition) <= token(d3138e78-abe7-48a0-adb9-8c466a9bb3fa, 0) AND start_time >= 2018-09-11 16:29-0700 AND start_time <= 1969-12-31 16:00-0800 LIMIT 10>, time 2747 msec - slow timeout 500 msec
DEBUG [COMMIT-LOG-ALLOCATOR] 2018-09-11 16:31:47,774 AbstractCommitLogSegmentManager.java:107 - No segments in reserve; creating a fresh one
DEBUG [ScheduledTasks:1] 2018-09-11 16:40:22,922 ColumnFamilyStore.java:899 - Enqueuing flush of size_estimates: 23.997MiB (2%) on-heap, 0.000KiB (0%) off-heap

相关裁判（我的问题没有答案）：

-编辑 tablestats返回

Total number of tables: 105

----------------
Keyspace : cadence_visibility
    Read Count: 19
    Read Latency: 0.5125263157894736 ms.
    Write Count: 3220964
    Write Latency: 0.04900822269357869 ms.
    Pending Flushes: 0
        Table: closed_executions
        SSTable count: 1
        SSTables in each level: [1, 0, 0, 0, 0, 0, 0, 0, 0]
        Space used (live): 20.3 MiB
        Space used (total): 20.3 MiB
        Space used by snapshots (total): 0 bytes
        Off heap memory used (total): 6.35 KiB
        SSTable Compression Ratio: 0.40192660515179696
        Number of keys (estimate): 3
        Memtable cell count: 28667
        Memtable data size: 7.35 MiB
        Memtable off heap memory used: 0 bytes
        Memtable switch count: 9
        Local read count: 9
        Local read latency: NaN ms
        Local write count: 327024
        Local write latency: NaN ms
        Pending flushes: 0
        Percent repaired: 0.0
        Bloom filter false positives: 0
        Bloom filter false ratio: 0.00000
        Bloom filter space used: 16 bytes
        Bloom filter off heap memory used: 8 bytes
        Index summary off heap memory used: 38 bytes
        Compression metadata off heap memory used: 6.3 KiB
        Compacted partition minimum bytes: 150
        Compacted partition maximum bytes: 62479625
        Compacted partition mean bytes: 31239902
        Average live cells per slice (last five minutes): NaN
        Maximum live cells per slice (last five minutes): 0
        Average tombstones per slice (last five minutes): NaN
        Maximum tombstones per slice (last five minutes): 0
        Dropped Mutations: 0 bytes

----------------

Answer 1

为什么分页不能扩展为主表？
您二级索引中的数据分散分页只会应用逻辑直到达到页码因为您的数据没有按时间聚类您仍然必须筛选很多行例如，在找到第一个10之前。

查询跟踪的确显示了分页的作用很晚。

为什么二级索引比较慢？
首先，Cassandra读取索引表以检索所有匹配行的主键，对于每个匹配行，它将读取原始表以获取数据。已知具有低基数指数的反模式。（参考https://www.datastax.com/dev/blog/cassandra-native-secondary-index-deep-dive）

随着数据增长，带有分页的Cassandra查询第二个索引变慢

1 个答案: