Fixes #34954: keep the parent of every glossary search match in the hierarchy

[sonika-shah]

Describe your changes:

Fixes #34954

Glossary term search with getHierarchy=true (used by the glossary term picker) can return a glossary with no terms under it, even though terms matched.

Why: the page (e.g. 25) holds the matches and the parent terms needed to draw the tree, ranked together. With more matches than the page holds, the parents rank below the cut. buildGlossaryTermSearchHierarchy only attaches a term whose parent is in the response, so every match is dropped and the glossary comes back as children: [].

What changed:

• Missing parents are fetched after the cut. One exact-FQN lookup for the parents of the page's matches (Approved only, same as the search). It runs only when a parent is missing; if it fails, it logs and falls back to the previous behaviour.
• Parents no longer take page slots from matches. For glossary terms the parent-term clauses score 0, so the page is filled by the best matches by relevance; parents fill leftover slots and the rest are fetched as above. The fqnParts aggregation flow is unchanged. Domain search is unchanged.
• Same change in ElasticSearchSearchManager and OpenSearchSearchManager.

The parent lookup builds on the backfill from #34955 (2.0). That PR keeps the picker's UI changes; this PR carries the backend fix for main, to be backported to 2.0.

Type of change:

• Bug fix

Tests:

• GlossaryTermResourceIT#test_hierarchySearchKeepsParentOfEveryMatchOnThePage: one parent that doesn't match + 30 matching children, searched with size=25&getHierarchy=true. Asserts the parent is returned with all 25 matches under it.
  • Without the fix: fails (Parent of the matches should be returned even though it ranks last, glossary has no children).
  • With the fix: passes (MySQL + Elasticsearch 9.3).

Details

Scoring. Each match also matches its own exact-FQN clause, so normally matches outrank pure parents. But the fqnParts aggregation is capped at 1000 buckets ordered by doc count; with many matches in deep glossaries, a leaf's own-FQN bucket (count 1) is cut first, the match loses that clause, and a parent can outrank it. Wrapping the parent clauses in constant_score with boost: 0 removes parents from ranking entirely. Checked on Elasticsearch 9.3 and OpenSearch 3.4: matches come back ordered by relevance, the parent last with score 0.

Parent lookup. Parents are derived from each hit's FQN (term level only; the glossary root still comes from each hit's glossary field) and fetched with a terms query on fullyQualifiedName (lowercase-normalized keyword). One query, bounded by page size × depth.

Not in scope. Matches ranked below the page are still not returned; that needs paging in the picker's search mode.

[gitar-bot]

Code Review ✅ Approved

🟡 Medium risk · Zero-scoring glossary parent clauses and fetching missing ancestors change search results; failed lookups could leave returned matches detached from the hierarchy.

Fixes glossary term search with getHierarchy=true returning glossaries with no terms under them when matches overflow the page. Missing parent terms are now fetched after the page cut, and parent-term clauses are scored at 0 so parents no longer take slots from matches, in both the Elasticsearch and OpenSearch search managers. No issues found.

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