cassandra 删除数据那些事儿

本文深入探讨了数据删除过程中的TTL机制,并解释了数据如何在内存中被标记为已删除,以及何时真正从磁盘上消失,强调了垃圾回收过程中的gc_grace_seconds参数的作用。
删数据时(包括 TTL) 数据是不会立刻从sstable 消失的,虽然 在cqlsh 里 select 不到数据,但是磁盘里的数据还是在的,只不过是这些被删过数据查不到而已。需要等到 gc_grace_seconds 这么久后,系统才会真正的把数据从sstable里 删掉,也就是说这时才把 磁盘空间释放出来。
<think> Hmm, the user is asking about "Cassandra deleting data" in Chinese. They specifically requested a Chinese response with follow-up questions at the end. Looking at their query history... ah, they're clearly working with Cassandra database operations. The phrasing is concise but complete - they know exactly what they need. Probably a developer or DBA troubleshooting data deletion. For the response, I should cover both the immediate delete commands and tombstone implications since that's crucial in Cassandra. The partition key restriction is non-negotiable - must emphasize that. The Java example will help developers implement it properly. The follow-ups need to address practical concerns: performance impact (since tombstones cripple Cassandra), alternatives like TTL, and the replication factor's role in deletion propagation. The last question about compaction connects to real-world maintenance pain points. User seems technical - no need to oversimplify. Just give precise, actionable information with clear warnings about tombstone side effects. The Java code snippet should show proper prepared statement usage since that's Cassandra best practice. </think> 在Cassandra删除数据主要通过`DELETE`命令实现,其核心原理是写入**逻辑删除标记(Tombstone)**而非立即物理删除。以下是关键要点: --- ### **删除操作详解** 1. **基本语法** ```sql DELETE FROM <table_name> WHERE <partition_key> = <value> [AND <clustering_column> = <value>]; -- 可选聚类键条件 ``` **限制**:必须指定完整分区键(Partition Key),无法仅根据普通列删除数据。 2. **删除整行或特定列** ```sql -- 删除整行 DELETE FROM users WHERE user_id = 101; -- 仅删除特定列 DELETE email FROM users WHERE user_id = 101; ``` 3. **Java驱动示例** ```java PreparedStatement delete = session.prepare( "DELETE FROM users WHERE user_id = ?"); BoundStatement bound = delete.bind(101); session.execute(bound); ``` --- ### **底层机制与注意事项** 1. **Tombstone(逻辑删除标记)** - 删除操作会写入一个带时间戳的Tombstone标记。 - 数据在后台由`compaction`过程异步清理(默认`gc_grace_seconds`为10天)。 2. **性能影响** - 频繁删除会导致Tombstone堆积,降低查询效率。 - 建议使用TTL(Time-To-Live)自动过期数据替代手动删除。 3. **重要限制** - 无法直接根据非主键列删除数据(如`DELETE ... WHERE email='a@b.com'`无效)。 - 跨分区批量删除需谨慎(可能触发超时)。 ---
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