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FAQ: update slow log, DDL and Syncer related questions #854

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FAQ: update slow log, DDL and Syncer related questions #854

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FAQ: update slow log, DDL and Syncer related questions
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15 changes: 15 additions & 0 deletions FAQ.md
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Expand Up @@ -257,6 +257,8 @@ You can use Docker Compose to build a TiDB cluster locally, including the cluste

2. If a slow query occurs, you can locate the `tidb-server` instance where the slow query is and the slow query time point using Grafana and find the SQL statement information recorded in the log on the corresponding node.

3. In addition to the log, you can also view the slow query using the `admin show slow` command. For details, see [`admin show slow` command](sql/slow-query.md#admin-show-slow-command).

#### How to add the `label` configuration if `label` of TiKV was not configured when I deployed the TiDB cluster for the first time?

The configuration of TiDB `label` is related to the cluster deployment architecture. It is important and is the basis for PD to execute global management and scheduling. If you did not configure `label` when deploying the cluster previously, you should adjust the deployment structure by manually adding the `location-labels` information using the PD management tool `pd-ctl`, for example, `config set location-labels "zone, rack, host"` (you should configure it based on the practical `label` level name).
Expand Down Expand Up @@ -749,6 +751,10 @@ CREATE TABLE if not exists mysql.user (
INSERT INTO mysql.user VALUES ("%", "root", "", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y", "Y");
```

#### Can TiDB provide services while Loader is running?

TiDB can provide services while Loader is running because Loader inserts the data logically. But do not perform the related DDL operations.

#### How to export the data in TiDB?

Currently, TiDB does not support `select into outfile`. You can use the following methods to export the data in TiDB:
Expand Down Expand Up @@ -788,6 +794,10 @@ Two solutions:

- You can also increase the limited number of statements in a single TiDB transaction, but this will consume more memory.

#### Does TiDB have a function like the Flashback Query in Oracle? Does it support DDL?

Yes, it does. And it supports DDL as well. For details, see [how TiDB reads data from history versions](op-guide/history-read.md).

### Migrate the data online

#### Syncer
Expand Down Expand Up @@ -829,6 +839,11 @@ Two solutions:
- Put the `syncer.meta` file in a relatively secure disk. For example, use disks with RAID 1.
- Restore the location information of history synchronization according to the monitoring data that Syncer reports to Prometheus regularly. But the location information might be inaccurate due to the delay when a large amount of data is synchronized.

##### If the downstream TiDB data is not consistent with the MySQL data during the synchronization process of Syncer, will DML operations cause exits?

- If the data exists in the upstream MySQL but does not exist in the downstream TiDB, when the upstream MySQL performs the `UPDATE` or `DELETE` operation on this row of data, Syncer will not report an error and the synchronization process will not exit, and this row of data does not exist in the downstream.
- If a conflict exists in the primary key indexes or the unique indexes in the downstream, preforming the `UPDATE` operation will cause an exit and performing the `INSERT` operation will not cause an exit.

### Migrate the traffic

#### How to migrate the traffic quickly?
Expand Down