After upgrading from Elasticsearch 6.8 to 7.17 we noticed that highlighting queries started taking several seconds instead of milliseconds.
We have many hundreds of fields in our mapping and the data is sparsely populated.
I have added here a minimal test case that shows the difference (mapping is for 7, insert "_doc" wrapper for 6.8):
# Mapping for ES7, ES8
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# Insert sparse document
PUT my-index/_doc/1
{
"field_086": "The quick brown fox",
"field_498": "jumps over the lazy dog"
}
# Query all fields and highlight
GET my-index/_search
{
"query": {
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{
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"query": "brown fox"
}
}
]
}
},
"highlight": {
"fields": {
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}
}
}
}
With 7.17 this query takes 1399ms.
With 6.8 it takes 19ms.
With 8.0.1 it takes 18ms.
As mentionned in Highlighters are slow with thousands of fields · Issue #36452 · elastic/elasticsearch · GitHub and Bypass highlight query terms extraction on empty fields by jimczi · Pull Request #32090 · elastic/elasticsearch · GitHub there should now be a shortcut to bypass empty fields. From those numbers it looks like maybe the shortcut stopped working in 7?
Unfortunately we are currently unable to upgrade to 8 since there is no .NET client for it yet so we are stuck with 6.8 which is EOL.
)