Which one is the best to retrieve selected fields from a search: `_source` or `fields`?

Elastic Stack Elasticsearch
1

_source vs fields: which is fastest to retrieve selected fields?

The ES 8.10.x official doc mentioned that ES will load whole _source object while using _source and the doc preferred to use fields.

  • You can use both of these methods, though the fields option is preferred because it consults both the document data and index mappings.
  • A document’s _source is stored as a single field in Lucene. This structure means that the whole _source object must be loaded and parsed even if you’re only requesting part of it.
  • Elasticsearch always attempts to load values from _source. This behavior has the same implications of source filtering where Elasticsearch needs to load and parse the entire _source to retrieve just one field.

My simple test using same query: the time_in_nanos of FetchFieldsPhase is much higher than that of FetchSourcePhase!

Please help me find which one is fastest to retrieve fields from a search only considering retrieval speed???

2

Is your test using realistic document sizes and structures? The amont of work required to parse the source document will increase with size.

3

Thanks for your reply! The test ran in the real production environment.
The test used the same query but different approach to retrieve selected fields (_source and no fields vs fields and _source=false). Before executing those 2 tests, I had also warmed up both queries.

4

What is the average size of your documents?

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