Elasticsearch单索引能否设置多文档类型？多类型是否影响性能？

Hey there! Let me break down these two Elasticsearch questions for you clearly, based on my hands-on experience and knowledge of the platform:

This depends entirely on which Elasticsearch version you're using:

• Versions 6.x and earlier: Yes, you could define multiple document types (like tweet and user) within a single index. Each type could have its own mappings, though Elasticsearch would share the underlying field data across types if field names overlapped.
• Versions 7.x and later: No, document types were completely removed. The only allowed type is the default _doc, and you can't create custom types anymore. Elasticsearch made this change because multiple types in one index often led to confusion and performance issues (which ties into your second question).

If you're on a version that supports multiple types (pre-7.x), the short answer is yes, it can negatively impact performance, and here's why:

• Field conflicts: If two types have the same field name but different data types (e.g., text vs integer), Elasticsearch will throw an error or silently coerce values, which leads to unexpected results and extra processing overhead during indexing/querying.
• Increased memory usage: Each document type adds extra mapping metadata that Elasticsearch needs to store and manage. This can bloat the cluster's memory footprint, especially if you have many types or complex mappings.
• Slower queries: When you run a query without specifying a document type, Elasticsearch has to scan all types in the index. Even if you specify a type, the shared underlying index structure means the engine still has to account for all mapped fields, leading to slightly slower query execution compared to using a single type.
• Future-proofing issues: Since Elasticsearch removed types in 7.x, using multiple types now means you'll have more work to migrate later (you'd need to split types into separate indices or use a field to distinguish document categories instead).

As a best practice, even in older versions, it's better to use separate indices for different document structures instead of multiple types in one index. This keeps your mappings clean, avoids performance pitfalls, and aligns with Elasticsearch's modern design recommendations.

内容的提问来源于stack exchange，提问作者Etherealm