Add combined index for issue_user.uid and issue_id (#28080)

fixes #27877

---------

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/migrations/v1_22/v283.go

@@ -0,0 +1,34 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_22 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddCombinedIndexToIssueUser(x *xorm.Engine) error {
+	type OldIssueUser struct {
+		IssueID int64
+		UID     int64
+		Cnt     int64
+	}
+
+	var duplicatedIssueUsers []OldIssueUser
+	if err := x.SQL("select * from (select issue_id, uid, count(1) as cnt from issue_user group by issue_id, uid) a where a.cnt > 1").
+		Find(&duplicatedIssueUsers); err != nil {
+		return err
+	}
+	for _, issueUser := range duplicatedIssueUsers {
+		if _, err := x.Exec("delete from issue_user where id in (SELECT id FROM issue_user WHERE issue_id = ? and uid = ? limit ?)", issueUser.IssueID, issueUser.UID, issueUser.Cnt-1); err != nil {
+			return err
+		}
+	}
+
+	type IssueUser struct {
+		UID     int64 `xorm:"INDEX unique(uid_to_issue)"` // User ID.
+		IssueID int64 `xorm:"INDEX unique(uid_to_issue)"`
+	}
+
+	return x.Sync(&IssueUser{})
+}