restore: update the definition of the parameter --load-stats and the usage of pitr id map

br/br-checkpoint-restore.md

@@ -89,7 +89,11 @@ During the initial restore, `br` first enters the snapshot restore phase. BR rec
 
 When entering the log restore phase during the initial restore, `br` creates a `__TiDB_BR_Temporary_Log_Restore_Checkpoint` database in the target cluster. This database records checkpoint data, the upstream cluster ID, and the restore time range (`start-ts` and `restored-ts`). If restore fails during this phase, you need to specify the same `start-ts` and `restored-ts` as recorded in the checkpoint database when retrying. Otherwise, `br` will report an error and prompt that the current specified restore time range or upstream cluster ID is different from the checkpoint record. If the restore cluster has been cleaned, you can manually delete the `__TiDB_BR_Temporary_Log_Restore_Checkpoint` database and retry with a different backup.
 
-Before entering the log restore phase during the initial restore, `br` constructs a mapping of upstream and downstream cluster database and table IDs at the `restored-ts` time point. This mapping is persisted in the system table `mysql.tidb_pitr_id_map` to prevent duplicate allocation of database and table IDs. Deleting data from `mysql.tidb_pitr_id_map` might lead to inconsistent PITR restore data.
+Note that before entering the log restore phase during the initial restore, `br` constructs a mapping of upstream and downstream cluster database and table IDs at the `restored-ts` time point. This mapping is persisted in the system table `mysql.tidb_pitr_id_map` to prevent duplicate allocation of database and table IDs. **Deleting data from `mysql.tidb_pitr_id_map` arbitrarily might lead to inconsistent PITR restore data.**
+
+> **Note:**
+>
+> To ensure compatibility with clusters of earlier versions, starting from v9.0.0, if the system table `mysql.tidb_pitr_id_map` does not exist in the restore cluster, the `pitr_id_map` data will be written to the log backup directory. The file name is `pitr_id_maps/pitr_id_map.cluster_id:{downstream-cluster-ID}.restored_ts:{restored-ts}`.
 
 ## Implementation details: store checkpoint data in the external storage
 
@@ -151,4 +155,4 @@ During the initial restore, `br` first enters the snapshot restore phase. BR rec
 
 When entering the log restore phase during the initial restore, `br` creates a `restore-{downstream-cluster-ID}/log` path in the target cluster. This path records checkpoint data, the upstream cluster ID, and the restore time range (`start-ts` and `restored-ts`). If restore fails during this phase, you need to specify the same `start-ts` and `restored-ts` as recorded in the checkpoint database when retrying. Otherwise, `br` will report an error and prompt that the current specified restore time range or upstream cluster ID is different from the checkpoint record. If the restore cluster has been cleaned, you can manually clean up the checkpoint data in the external storage or specify another external storage path to store checkpoint data, and retry with a different backup.
 
-Before entering the log restore phase during the initial restore, `br` constructs a mapping of the database and table IDs in the upstream and downstream clusters at the `restored-ts` time point. This mapping is persisted in the system table `mysql.tidb_pitr_id_map` to prevent duplicate allocation of database and table IDs. Deleting data from `mysql.tidb_pitr_id_map` might lead to inconsistent PITR restore data.
+Note that before entering the log restore phase during the initial restore, `br` constructs a mapping of the database and table IDs in the upstream and downstream clusters at the `restored-ts` time point. This mapping is persisted in the checkpoint storage with the file name `pitr_id_maps/pitr_id_map.cluster_id:{downstream-cluster-ID}.restored_ts:{restored-ts}` to prevent duplicate allocation of database and table IDs. **Deleting files from the directory `pitr_id_maps` arbitrarily might lead to inconsistent PITR restore data.**

br/br-snapshot-guide.md

@@ -151,6 +151,7 @@ When you perform a snapshot backup, BR backs up system tables as tables with the
 - Starting from BR v5.1.0, when you back up snapshots, BR automatically backs up the **system tables** in the `mysql` schema, but does not restore these system tables by default.
 - Starting from v6.2.0, BR lets you specify `--with-sys-table` to restore **data in some system tables**.
 - Starting from v7.6.0, BR enables `--with-sys-table` by default, which means that BR restores **data in some system tables** by default.
+- Starting from v9.0.0, BR lets you specify `--fast-load-sys-tables` to restore system tables physically. This approach uses the `RENAME TABLE` DDL statement to atomically swap the system tables in the `__TiDB_BR_Temporary_mysql` database with the system tables in the `mysql` database. Unlike the logical restoration of system tables using the `REPLACE INTO` SQL statement, physical restoration completely overwrites the existing data in the system tables.
 
 **BR can restore data in the following system tables:**
 

br/br-snapshot-manual.md

@@ -127,8 +127,21 @@ tiup br restore full \
 --storage local:///br_data/ --pd "${PD_IP}:2379" --log-file restore.log
 ```
 
+> **Note:**
+>
+> Starting from v9.0.0, when the `--load-stats` parameter is set to false, BR no longer writes statistics for the restored tables to the `mysql.stats_meta` table. After the restore is complete, you can manually execute the `ANALYZE TABLE` SQL statement to update the relevant statistics.
+
 When the backup and restore feature backs up data, it stores statistics in JSON format within the `backupmeta` file. When restoring data, it loads statistics in JSON format into the cluster. For more information, see [LOAD STATS](/sql-statements/sql-statement-load-stats.md).
 
+Starting from 9.0.0, BR introduces the `--fast-load-sys-tables` parameter, which is enabled by default. When restoring data to a new cluster using the `br` command-line tool, and the IDs of tables and partitions between the upstream and downstream clusters can be reused (otherwise, BR will automatically fall back to logically load statistics), enabling `--fast-load-sys-tables` lets BR to  first restore the statistics-related system tables to the temporary system database `__TiDB_BR_Temporary_mysql`, and then atomically swap these tables with the corresponding tables in the `mysql` database using the `RENAME TABLE` statement.
+
+The following is an example:
+
+```shell
+tiup br restore full \
+--storage local:///br_data/ --pd "${PD_IP}:2379" --log-file restore.log --load-stats --fast-load-sys-tables
+```
+
 ## Encrypt the backup data
 
 BR supports encrypting backup data at the backup side and [at the storage side when backing up to Amazon S3](/br/backup-and-restore-storages.md#amazon-s3-server-side-encryption). You can choose either encryption method as required.
@@ -181,6 +194,22 @@ Download&Ingest SST <-----------------------------------------------------------
 Restore Pipeline <-------------------------/...............................................> 17.12%
 ```
 
+Starting from TiDB v9.0.0, BR lets you specify `--fast-load-sys-tables` to restore statistics physically in a new cluster:
+
+```shell
+tiup br restore full \
+    --pd "${PD_IP}:2379" \
+    --with-sys-table \
+    --fast-load-sys-tables \
+    --storage "s3://${backup_collection_addr}/snapshot-${date}?access-key=${access-key}&secret-access-key=${secret-access-key}" \
+    --ratelimit 128 \
+    --log-file restorefull.log
+```
+
+> **Note:**
+>
+> Unlike the logical restoration of system tables using the `REPLACE INTO` SQL statement, physical restoration completely overwrites the existing data in the system tables.
+
 ## Restore a database or a table
 
 You can use `br` to restore partial data of a specified database or table from backup data. This feature allows you to filter out data that you do not need during the restore.