elasticsearch 关联查询对比
两种方式 嵌套和父子关联Nested
[*]Nested docs are stored in the same Lucene block as each other, which helps read/query performance. Reading a nested doc is faster than the equivalent parent/child.
[*]Updating a single field in a nested document (parent or nested children) forces ES to reindex the entire nested document. This can be very expensive for large nested docs
[*]"Cross referencing" nested documents is impossible
[*]Best suited for data that does not change frequently
Parent/Child
[*]Children are stored separately from the parent, but are routed to the same shard. So parent/children are slightly less performance on read/query than nested
[*]Parent/child mappings have a bit extra memory overhead, since ES maintains a "join" list in memory
[*]Updating a child doc does not affect the parent or any other children, which can potentially save a lot of indexing on large docs
[*]Sorting/scoring can be difficult with Parent/Child since the Has Child/Has Parent operations can be opaque at times
综上所述，两种方式均有利弊，官方建议，自己处理关联关系，减轻ES的压力。
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