向cassandra表添加列是否立即完成? [英] Does adding a column to a cassandra table complete instantly?

问题描述

我们计划在现有的cassandra表中添加一列类型列表，该表的数据文件大小约为350 GB.在应用架构更改时，我们可以暂时暂停所有读取/写入操作几分钟.

We plan to add a column of type list to an existing cassandra table which data file size is about 350 GB. We can temporarily halt all the read/write for a few minutes while applying the schema change.

我们的理解是，在应用模式更改时，cassandra不会锁定表，但是请确保我们的DBA希望对数据文件大小为25 GB的表进行实验.但是，在运行非生产cassandra服务器的小型服务器上，要达到这种大小，将需要3-4周的时间来增长(具有更多并发插入会开始导致超时问题).

Our understanding is that cassandra does not lock a table when applying schema changes, but to be sure our DBA wants to do an experiment on a table with datafile at 25 GB in size. However it will take 3-4 weeks to grow in such size on a small server where a non-production cassandra server is running (having more concurrent inserts starts to cause time out issues).

有人知道无论基础数据文件大小如何，向现有cassandra表中添加列都会立即返回?

Does anyone know that adding a column to an existing cassandra table returns promptly regardless the underlying data file size?

谢谢

推荐答案

在Cassandra中添加列只是将列的元信息添加到内部表中，以保留架构信息.进行此更改后，不会对现有数据进行任何修改-当磁盘上没有用于数据的数据(对于任何列，不仅是添加的内容)时，Cassandra只会将null而不是列值放进去-当数据为返回调用者，而不是在文件中添加null.

Adding a column in Cassandra is just an addition of the column's meta-information to internal table that keeps schema information. No modification of existing data happens when this change is done - Cassandra will simply put null instead of the column value when there is no data for it on the disk (for any column, not only what was added) - this happens when the data is returned to caller, not by adding null to the files.

类似地，删除列不会修改现有数据-而是将新条目添加到 system_schema.dropped_columns 表中，并且从磁盘读取相应的数据后，它们会被过滤掉.

Similarly, deletion of the column doesn't modify the existing data - instead a new entry is added to system_schema.dropped_columns table, and corresponding data is filtered out after they are read from the disk.

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