FEATURE: Roll out new search optimisations (#20364)

- Reduce duplication of terms in post index from unlimited to 6. This will
result in reduced index size and reduced weighting for posts containing
a huge amount of duplicate terms. (Eg: a post containing "sam sam sam sam
sam sam sam sam", will index as "sam sam sam sam sam sam", only including
the word up to 6 times.) This corrects a flaw where title weighting could
be ignored.

- Prioritize exact matches of words in titles. Our search always performs
a prefix match. However we want to give special weight to exact title matches
meaning that a search for "sum" will find topics such as "the sum of us" vs
"summer in spring".

- Pick up fixes to our search algorithm which are missing from old indexes.
Specifically pick up the fix that indexes URLs properly. (`https://happy.com`
was stemmed to `happi` in keywords and then was not searchable)

see also:

https://meta.discourse.org/t/refinements-to-search-being-tested-on-meta/254158

Indexing will take a while and work in batches, in the background.

app/services/search_indexer.rb

@@ -1,12 +1,15 @@
 # frozen_string_literal: true
 
 class SearchIndexer
-  POST_INDEX_VERSION = 4
-  MIN_POST_REINDEX_VERSION = 3
-  TOPIC_INDEX_VERSION = 3
+  MIN_POST_BLURB_INDEX_VERSION = 4
+
+  POST_INDEX_VERSION = 5
+  TOPIC_INDEX_VERSION = 4
   CATEGORY_INDEX_VERSION = 3
   USER_INDEX_VERSION = 3
   TAG_INDEX_VERSION = 3
+
+  # version to apply when issuing a background reindex
   REINDEX_VERSION = 0
   TS_VECTOR_PARSE_REGEX = /('([^']*|'')*'\:)(([0-9]+[A-D]?,?)+)/
 

config/site_settings.yml

@@ -2191,10 +2191,10 @@ search:
     default: false
     hidden: true
   prioritize_exact_search_title_match:
-    default: false
+    default: true
     hidden: true
   max_duplicate_search_index_terms:
-    default: -1
+    default: 6
     hidden: true
   use_pg_headlines_for_excerpt:
     default: false

lib/search/grouped_search_results.rb

@@ -71,7 +71,7 @@ class Search
     def blurb(post)
       opts = { term: @blurb_term, blurb_length: @blurb_length }
 
-      if post.post_search_data.version > SearchIndexer::MIN_POST_REINDEX_VERSION &&
+      if post.post_search_data.version >= SearchIndexer::MIN_POST_BLURB_INDEX_VERSION &&
            !Search.segment_chinese? && !Search.segment_japanese?
         if SiteSetting.use_pg_headlines_for_excerpt
           scrubbed_headline = post.headline.gsub(SCRUB_HEADLINE_REGEXP, '\1')

spec/jobs/reindex_search_spec.rb

@@ -70,7 +70,7 @@ RSpec.describe Jobs::ReindexSearch do
     end
 
     it "should not reindex posts with a developmental version" do
-      post = Fabricate(:post, version: SearchIndexer::MIN_POST_REINDEX_VERSION + 1)
+      Fabricate(:post, version: SearchIndexer::POST_INDEX_VERSION + 1)
 
       subject.rebuild_posts(indexer: FakeIndexer)
 

spec/lib/search_spec.rb

@@ -122,6 +122,8 @@ RSpec.describe Search do
 
       before do
         SearchIndexer.enable
+        SiteSetting.max_duplicate_search_index_terms = -1
+        SiteSetting.prioritize_exact_search_title_match = false
         [post1, post2].each { |post| SearchIndexer.index(post, force: true) }
       end