Optimize heatmap query (#33853)

When there are over 5M records on `action` table, the heatmap on
dashboard is very slow as below SQL.
```
database duration=1.8881s db.sql="SELECT created_unix DIV 900 * 900 AS timestamp, count(user_id) as contributions FROM `action` WHERE user_id=? AND act_user_id=? AND (created_unix > ?) GROUP BY timestamp ORDER BY timestamp"
```

This PR add a new index for `action` table with columns `user_id`,
`act_user_id` and `created_unix` so that this query will become about 6
times faster than before.

Author: lunny

Diff for: models/activities/action.go

@@ -172,7 +172,10 @@ func (a *Action) TableIndices() []*schemas.Index {
 	cuIndex := schemas.NewIndex("c_u", schemas.IndexType)
 	cuIndex.AddColumn("user_id", "is_deleted")
 
-	indices := []*schemas.Index{actUserIndex, repoIndex, cudIndex, cuIndex}
+	actUserUserIndex := schemas.NewIndex("au_c_u", schemas.IndexType)
+	actUserUserIndex.AddColumn("act_user_id", "created_unix", "user_id")
+
+	indices := []*schemas.Index{actUserIndex, repoIndex, cudIndex, cuIndex, actUserUserIndex}
 
 	return indices
 }

Diff for: models/migrations/migrations.go

@@ -377,6 +377,7 @@ func prepareMigrationTasks() []*migration {
 		newMigration(314, "Update OwnerID as zero for repository level action tables", v1_24.UpdateOwnerIDOfRepoLevelActionsTables),
 		newMigration(315, "Add Ephemeral to ActionRunner", v1_24.AddEphemeralToActionRunner),
 		newMigration(316, "Add description for secrets and variables", v1_24.AddDescriptionForSecretsAndVariables),
+		newMigration(317, "Add new index for action for heatmap", v1_24.AddNewIndexForUserDashboard),
 	}
 	return preparedMigrations
 }

Diff for: models/migrations/v1_24/v317.go

@@ -0,0 +1,56 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_24 //nolint
+
+import (
+	"code.gitea.io/gitea/modules/timeutil"
+
+	"xorm.io/xorm"
+	"xorm.io/xorm/schemas"
+)
+
+type improveActionTableIndicesAction struct {
+	ID          int64 `xorm:"pk autoincr"`
+	UserID      int64 `xorm:"INDEX"` // Receiver user id.
+	OpType      int
+	ActUserID   int64 // Action user id.
+	RepoID      int64
+	CommentID   int64 `xorm:"INDEX"`
+	IsDeleted   bool  `xorm:"NOT NULL DEFAULT false"`
+	RefName     string
+	IsPrivate   bool               `xorm:"NOT NULL DEFAULT false"`
+	Content     string             `xorm:"TEXT"`
+	CreatedUnix timeutil.TimeStamp `xorm:"created"`
+}
+
+// TableName sets the name of this table
+func (*improveActionTableIndicesAction) TableName() string {
+	return "action"
+}
+
+// TableIndices implements xorm's TableIndices interface
+func (a *improveActionTableIndicesAction) TableIndices() []*schemas.Index {
+	repoIndex := schemas.NewIndex("r_u_d", schemas.IndexType)
+	repoIndex.AddColumn("repo_id", "user_id", "is_deleted")
+
+	actUserIndex := schemas.NewIndex("au_r_c_u_d", schemas.IndexType)
+	actUserIndex.AddColumn("act_user_id", "repo_id", "created_unix", "user_id", "is_deleted")
+
+	cudIndex := schemas.NewIndex("c_u_d", schemas.IndexType)
+	cudIndex.AddColumn("created_unix", "user_id", "is_deleted")
+
+	cuIndex := schemas.NewIndex("c_u", schemas.IndexType)
+	cuIndex.AddColumn("user_id", "is_deleted")
+
+	actUserUserIndex := schemas.NewIndex("au_c_u", schemas.IndexType)
+	actUserUserIndex.AddColumn("act_user_id", "created_unix", "user_id")
+
+	indices := []*schemas.Index{actUserIndex, repoIndex, cudIndex, cuIndex, actUserUserIndex}
+
+	return indices
+}
+
+func AddNewIndexForUserDashboard(x *xorm.Engine) error {
+	return x.Sync(new(improveActionTableIndicesAction))
+}