Filter stop words when searching posts in mysql (#14509)

store/sqlstore/post_store.go

@@ -6,6 +6,7 @@ package sqlstore
 import (
 	"database/sql"
 	"fmt"
+	"github.com/mattermost/mattermost-server/v5/store/searchlayer"
 	"net/http"
 	"regexp"
 	"strconv"
@@ -1273,6 +1274,18 @@ func (s *SqlPostStore) search(teamId string, userId string, params *model.Search
 		searchClause := fmt.Sprintf("AND to_tsvector('english', %s) @@  to_tsquery('english', :Terms)", searchType)
 		searchQuery = strings.Replace(searchQuery, "SEARCH_CLAUSE", searchClause, 1)
 	} else if s.DriverName() == model.DATABASE_DRIVER_MYSQL {
+		if searchType == "Message" {
+			var err error
+			terms, err = removeMysqlStopWordsFromTerms(terms)
+			if err != nil {
+				return nil, model.NewAppError("SqlPostStore.search", "store.sql_post.search.app_error", nil, err.Error(), http.StatusInternalServerError)
+			}
+
+			if terms == "" {
+				return list, nil
+			}
+		}
+
 		searchClause := fmt.Sprintf("AND MATCH (%s) AGAINST (:Terms IN BOOLEAN MODE)", searchType)
 		searchQuery = strings.Replace(searchQuery, "SEARCH_CLAUSE", searchClause, 1)
 
@@ -1318,6 +1331,25 @@ func (s *SqlPostStore) search(teamId string, userId string, params *model.Search
 	return list, nil
 }
 
+func removeMysqlStopWordsFromTerms(terms string) (string, error) {
+	stopWords := make([]string, len(searchlayer.MYSQL_STOP_WORDS))
+	copy(stopWords, searchlayer.MYSQL_STOP_WORDS)
+	re, err := regexp.Compile(fmt.Sprintf(`^(%s)$`, strings.Join(stopWords, "|")))
+	if err != nil {
+		return "", err
+	}
+
+	newTerms := make([]string, 0)
+	separatedTerms := strings.Fields(terms)
+	for _, term := range separatedTerms {
+		term = strings.TrimSpace(term)
+		if term = re.ReplaceAllString(term, ""); term != "" {
+			newTerms = append(newTerms, term)
+		}
+	}
+	return strings.Join(newTerms, " "), nil
+}
+
 func (s *SqlPostStore) AnalyticsUserCountsWithPostsByDay(teamId string) (model.AnalyticsRows, *model.AppError) {
 	query :=
 		`SELECT DISTINCT