[MM-39157] Thread participant removal with just one query using DB's JSON functions (#18725)

* feat: removes thread participant with just one query using DB's native JSON functions

* fix: corrects syntax post test failure

store/sqlstore/post_store.go

@@ -6,7 +6,6 @@ package sqlstore
 import (
 	"context"
 	"database/sql"
-	"encoding/json"
 	"fmt"
 	"reflect"
 	"regexp"
@@ -2411,27 +2410,15 @@ func (s *SqlPostStore) cleanupThreads(postId, rootId string, permanent bool, use
 		updateQuery := s.getQueryBuilder().Update("Threads")
 
 		if count == 0 {
-			var participants model.StringArray
-			err = s.getQueryBuilder().
-				Select("Participants").
-				From("Threads").
-				Where(sq.Eq{"PostId": rootId}).
-				RunWith(s.GetReplica()).
-				QueryRow().
-				Scan(&participants)
-
-			if err != nil {
-				return errors.Wrap(err, "failed getting thread participants")
-			}
-
-			if participants.Contains(userId) {
-				participants = participants.Remove(userId)
-				var participantsJSON []byte
-				participantsJSON, err = json.Marshal(participants)
-				if err != nil {
-					return errors.Wrap(err, "failed marshalling thread participants")
-				}
-				updateQuery = updateQuery.Set("Participants", string(participantsJSON))
+			if s.DriverName() == model.DatabaseDriverPostgres {
+				updateQuery = updateQuery.Set("Participants", sq.Expr("Participants - ?", userId))
+			} else {
+				// The .Where is because JSON_REMOVE returns null if the element to remove wasn't present
+				updateQuery = updateQuery.
+					Set("Participants", sq.Expr(
+						`JSON_REMOVE(Participants, JSON_UNQUOTE(JSON_SEARCH(Participants, 'one', ?)))`, userId,
+					)).
+					Where(sq.Expr(`JSON_CONTAINS(Participants, ?)`, strconv.Quote(userId)))
 			}
 		}