Avoid simple config when doing FTS in Postgres (#35063) (#35179)

This commit reverts PR #30214, which addressed bug MM-60790 but caused a
performance regression tracked by MM-66782.

This revert has two implications:

1. The performance issue is solved.
2. The original bug is re-introduced.

Re-introducing the original bug seems not to be ideal, but I argue that
the original PR did not actually fix the bug:

- Before that PR, looking for a quoted string would return additional
  results: the UX was slightly confusing, because when the user looked
  for the word "stateful", the results would contain matches like
  "states" (see MM-60790).
- After that PR, looking for a quoted string can timeout, so that the
  list of results becomes empty. The UX here may be less confusing,
  since the user simply doesn't find what they're looking for, and they
  may assume that string is not present in any post, but it's completely
  wrong: the result list is empty because the SQL query timed out and
  thus the endpoint returned 0 results.

The solution to the original issue should be addressed via
Elasticsearch, which should provide a more refined and precise search
results.

For more information on the investigation on this issue and the
motivation behind the revert, see
https://mattermost.atlassian.net/wiki/x/IYAk_w

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/store/searchtest/post_layer.go

@@ -35,11 +35,6 @@ var searchPostStoreTests = []searchTest{
 		Fn:   testSearchANDORQuotesCombinations,
 		Tags: []string{EnginePostgres, EngineMySQL, EngineElasticSearch},
 	},
-	{
-		Name: "Should be able to search without stemming",
-		Fn:   testStemming,
-		Tags: []string{EnginePostgres, EngineMySQL},
-	},
 	{
 		// Postgres supports search with and without quotes
 		Name: "Should be able to search for email addresses with or without quotes",
@@ -472,65 +467,6 @@ func testSearchANDORQuotesCombinations(t *testing.T, th *SearchTestHelper) {
 	}
 }
 
-func testStemming(t *testing.T, th *SearchTestHelper) {
-	p1, err := th.createPost(th.User.Id, th.ChannelBasic.Id, "great minds think", "", model.PostTypeDefault, 0, false)
-	require.NoError(t, err)
-	p2, err := th.createPost(th.User.Id, th.ChannelBasic.Id, "mindful of what you think", "", model.PostTypeDefault, 0, false)
-	require.NoError(t, err)
-
-	defer th.deleteUserPosts(th.User.Id)
-
-	testCases := []struct {
-		name        string
-		terms       string
-		orTerms     bool
-		expectedLen int
-		expectedIDs []string
-	}{
-		{
-			name:        "simple search, no stemming",
-			terms:       `"minds think"`,
-			orTerms:     false,
-			expectedLen: 1,
-			expectedIDs: []string{p1.Id},
-		},
-		{
-			name:        "simple search, single word, no stemming",
-			terms:       `"minds"`,
-			orTerms:     false,
-			expectedLen: 1,
-			expectedIDs: []string{p1.Id},
-		},
-		{
-			name:        "non-simple search, stemming",
-			terms:       `minds think`,
-			orTerms:     true,
-			expectedLen: 2,
-			expectedIDs: []string{p1.Id, p2.Id},
-		},
-		{
-			name:        "simple search, no stemming, no results",
-			terms:       `"mind"`,
-			orTerms:     false,
-			expectedLen: 0,
-			expectedIDs: []string{},
-		},
-	}
-
-	for _, tc := range testCases {
-		t.Run(tc.name, func(t *testing.T) {
-			params := &model.SearchParams{Terms: tc.terms, OrTerms: tc.orTerms}
-			results, err := th.Store.Post().SearchPostsForUser(th.Context, []*model.SearchParams{params}, th.User.Id, th.Team.Id, 0, 20)
-			require.NoError(t, err)
-
-			require.Len(t, results.Posts, tc.expectedLen)
-			for _, id := range tc.expectedIDs {
-				th.checkPostInSearchResults(t, id, results.Posts)
-			}
-		})
-	}
-}
-
 func testSearchEmailAddresses(t *testing.T, th *SearchTestHelper) {
 	p1, err := th.createPost(th.User.Id, th.ChannelBasic.Id, "email test@test.com", "", model.PostTypeDefault, 0, false)
 	require.NoError(t, err)

server/channels/store/sqlstore/post_store.go

@@ -2160,7 +2160,6 @@ func (s *SqlPostStore) search(teamId string, userId string, params *model.Search
 		terms = wildCardRegex.ReplaceAllLiteralString(terms, ":* ")
 		excludedTerms = wildCardRegex.ReplaceAllLiteralString(excludedTerms, ":* ")
 
-		simpleSearch := false
 		// Replace spaces with to_tsquery symbols
 		replaceSpaces := func(input string, excludedInput bool) string {
 			if input == "" {
@@ -2172,11 +2171,6 @@ func (s *SqlPostStore) search(teamId string, userId string, params *model.Search
 
 			// Replace spaces within quoted strings with '<->'
 			input = quotedStringsRegex.ReplaceAllStringFunc(input, func(match string) string {
-				// If the whole search term is a quoted string,
-				// we don't want to do stemming.
-				if input == match {
-					simpleSearch = true
-				}
 				return strings.Replace(match, " ", "<->", -1)
 			})
 
@@ -2197,10 +2191,6 @@ func (s *SqlPostStore) search(teamId string, userId string, params *model.Search
 		}
 
 		textSearchCfg := s.pgDefaultTextSearchConfig
-		if simpleSearch {
-			textSearchCfg = "simple"
-		}
-
 		searchClause := fmt.Sprintf("to_tsvector('%[1]s', %[2]s) @@  to_tsquery('%[1]s', ?)", textSearchCfg, searchType)
 		baseQuery = baseQuery.Where(searchClause, tsQueryClause)
 	} else if s.DriverName() == model.DatabaseDriverMysql {