KQL searches case-insensitively, so every keyword field gets its lowercased
search sibling unless it opts out: ids are opaque, paths are POSIX, the mime
type is normalized already. That takes the facets along, artist or camera
model match regardless of case, while the case-preserved base still answers
and aggregates. Which siblings a field carries is decided once, from the
struct and the overrides, and the renderers, the document writer and the
query lowering all read it from there.
A single word finds the names and titles that contain it, `report` finds
Report.txt, on bleve as well as on OpenSearch, which so far only did it by
accident of its dynamic mapping. Modelled like SharePoint's NoWordBreaker:
a keyword field is one whole value unless the override switches that off,
which adds a search-only _words sibling next to _lowercase, analyzed into
lowercased words (a dot is a word boundary, no stemming). The base stays the
whole value for returning and aggregating, wildcards and whole values keep
using _lowercase. Quotes do not change the meaning, a phrase is a phrase
either way, and there is no exact-match operator yet.
Keyword and path fields always index their case-preserved base and, when CaseInsensitive is set, an additional <field>_lowercase sibling used only for matching. The KQL lowering marks a restriction case-insensitive; each backend searches the sibling and lowercases the query value the same way the sibling is precomputed at index time (Go strings.ToLower on both sides, so non-ASCII stays consistent).
Search always returns the case-preserved base, so the sibling never has to be read back. In bleve it is indexed but not stored, kept out of _all, and without doc values. In OpenSearch it deliberately stays in _source: excluding it would make every update-by-query script rebuild all siblings from the document via painless toLowerCase, which lowercases differently than Go and would drift from the query side. Keeping it in _source avoids that, and a lowercased copy of a name or path is negligible disk in a cluster.
The OpenSearch move script keeps the base and its sibling in sync by swapping the moved prefix in Path_lowercase and setting Name_lowercase from Go-lowercased params, so case-insensitive search still finds a file after it moves (previously the sibling went stale). bleve re-indexes the whole document on move/delete/restore, so its siblings stay fresh for free.
This also repairs OpenSearch path search (the query value was no longer folded to lowercase, so path:<Foo> returned nothing) and makes bleve path queries match a folder and its descendants like OpenSearch's path_hierarchy. The Path base stays case-preserved so the move/delete descendant update (an exact TermQuery on Path) matches mixed-case folders.
Dominik Schmidt