perf: extend action c_u index to include created_unix for faster dashboard feeds (#38076)

Adds `created_unix` as the third column of the `c_u` composite index on
the `action` table, changing it from `(user_id, is_deleted)` to
`(user_id, is_deleted, created_unix)`.

Migration 337 drops and recreates the index. No data is touched.

## Root causes

#32333 introduced the `c_u` index to speed up dashboard queries, but
defined it as `(user_id, is_deleted)` — without `created_unix`.

#3368 The simple query is now efficient enough for the database to
actually use `c_u`, but because `created_unix` is absent from the index,
the database must load and sort **every** matching row before returning
the first page of 20.

The existing `c_u_d` index `(created_unix, user_id, is_deleted)` does
not help because its leading column is `created_unix`, which can't be
used for an equality seek on `user_id`.

Those two caused this issue:
https://github.com/go-gitea/gitea/issues/38075

With the fix, the database seeks directly to `(user_id=X,
is_deleted=false)` and walks `created_unix` in descending order,
stopping after 20 rows.

Fixes https://github.com/go-gitea/gitea/issues/38075

models/activities/action.go

@@ -166,7 +166,7 @@ func (a *Action) TableIndices() []*schemas.Index {
 	cudIndex.AddColumn("created_unix", "user_id", "is_deleted")
 
 	cuIndex := schemas.NewIndex("c_u", schemas.IndexType)
-	cuIndex.AddColumn("user_id", "is_deleted")
+	cuIndex.AddColumn("user_id", "is_deleted", "created_unix")
 
 	actUserUserIndex := schemas.NewIndex("au_c_u", schemas.IndexType)
 	actUserUserIndex.AddColumn("act_user_id", "created_unix", "user_id")

models/migrations/migrations.go

@@ -416,6 +416,7 @@ func prepareMigrationTasks() []*migration {
 		newMigration(336, "Add ActionRunJobSummary table", v1_27.AddActionRunJobSummaryTable),
 		newMigration(337, "Add visibility to team", v1_27.AddVisibilityToTeam),
 		newMigration(338, "Expand legacy MSSQL issue/comment long-text columns", v1_27.ExpandIssueAndCommentLongTextFieldsForMSSQL),
+		newMigration(339, "Extend action c_u index to include created_unix for faster dashboard feed queries", v1_27.AddCreatedUnixToActionUserIsDeletedIndex),
 	}
 	return preparedMigrations
 }

models/migrations/v1_27/v339.go

@@ -0,0 +1,40 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_27
+
+import (
+	"context"
+
+	"gitea.dev/models/db"
+
+	"xorm.io/xorm/schemas"
+)
+
+// AddCreatedUnixToActionUserIsDeletedIndex extends the c_u composite index on
+// the action table to include created_unix, enabling efficient ORDER BY on the
+// dashboard feed query without a full sort of all matching rows.
+func AddCreatedUnixToActionUserIsDeletedIndex(x db.EngineMigration) error {
+	// xorm Sync cannot reliably update an index when another index already
+	// covers the same columns in a different order (Equal() is order-insensitive).
+	// Drop the old c_u index explicitly, then recreate it with the new column set.
+	indexes, err := x.Dialect().GetIndexes(x.DB(), context.Background(), "action")
+	if err != nil {
+		return err
+	}
+	for _, idx := range indexes {
+		if idx.Name == "c_u" {
+			if _, err := x.Exec(x.Dialect().DropIndexSQL("action", idx)); err != nil {
+				return err
+			}
+			break
+		}
+	}
+
+	newIndex := schemas.NewIndex("c_u", schemas.IndexType)
+	newIndex.AddColumn("user_id", "is_deleted", "created_unix")
+	if _, err := x.Exec(x.Dialect().CreateIndexSQL("action", newIndex)); err != nil {
+		return err
+	}
+	return nil
+}

models/migrations/v1_27/v339_test.go

@@ -0,0 +1,72 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_27
+
+import (
+	"context"
+	"testing"
+
+	"gitea.dev/models/migrations/migrationtest"
+	"gitea.dev/modules/timeutil"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"xorm.io/xorm/schemas"
+)
+
+type actionBeforeV339 struct {
+	ID          int64 `xorm:"pk autoincr"`
+	UserID      int64 `xorm:"INDEX"`
+	OpType      int
+	ActUserID   int64
+	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"`
+}
+
+func (actionBeforeV339) TableName() string { return "action" }
+
+func (actionBeforeV339) 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")
+
+	// old 2-column index, before the migration
+	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")
+
+	return []*schemas.Index{actUserIndex, repoIndex, cudIndex, cuIndex, actUserUserIndex}
+}
+
+func Test_AddCreatedUnixToActionUserIsDeletedIndex(t *testing.T) {
+	x, deferable := migrationtest.PrepareTestEnv(t, 0, new(actionBeforeV339))
+	defer deferable()
+	if x == nil || t.Failed() {
+		return
+	}
+
+	indexes, err := x.Dialect().GetIndexes(x.DB(), context.Background(), "action")
+	require.NoError(t, err)
+	assert.True(t, hasIndexWithColumns(indexes, []string{"user_id", "is_deleted"}, false), "old c_u index should exist before migration")
+	assert.False(t, hasIndexWithColumns(indexes, []string{"user_id", "is_deleted", "created_unix"}, false), "new c_u index should not exist before migration")
+
+	require.NoError(t, AddCreatedUnixToActionUserIsDeletedIndex(x))
+
+	indexes, err = x.Dialect().GetIndexes(x.DB(), context.Background(), "action")
+	require.NoError(t, err)
+	assert.False(t, hasIndexWithColumns(indexes, []string{"user_id", "is_deleted"}, false), "old 2-column c_u index should be gone after migration")
+	assert.True(t, hasIndexWithColumns(indexes, []string{"user_id", "is_deleted", "created_unix"}, false), "new 3-column c_u index must exist after migration")
+}