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Use PLAN REPLAYER to Save and Restore the On-Site Information of a Cluster

When you locate and troubleshoot the issues of a TiDB cluster, you often need to provide information on the system and the execution plan. To help you get the information and troubleshoot cluster issues in a more convenient and efficient way, the PLAN REPLAYER command is introduced in TiDB v5.3.0. This command enables you to easily save and restore the on-site information of a cluster, improves the efficiency of troubleshooting, and helps you more easily archive the issue for management.

The features of PLAN REPLAYER are as follows:

  • Exports the information of a TiDB cluster at an on-site troubleshooting to a ZIP-formatted file for storage.
  • Imports into a cluster the ZIP-formatted file exported from another TiDB cluster. This file contains the information of the latter TiDB cluster at an on-site troubleshooting.

Use PLAN REPLAYER to export cluster information

You can use PLAN REPLAYER to save the on-site information of a TiDB cluster. The export interface is as follows:

PLAN REPLAYER DUMP EXPLAIN [ANALYZE] [WITH STATS AS OF TIMESTAMP expression] sql-statement;

Based on sql-statement, TiDB sorts out and exports the following on-site information:

  • TiDB version
  • TiDB configuration
  • TiDB session variables
  • TiDB SQL bindings
  • The table schema in sql-statement
  • The statistics of the table in sql-statement
  • The result of EXPLAIN [ANALYZE] sql-statement
  • Some internal procudures of query optimization

If historical statistics are enabled, you can specify a time in the PLAN REPLAYER statement to get the historical statistics for the corresponding time. You can directly specify a time and date or specify a timestamp. TiDB looks for the historical statistics before the specified time and exports the latest one among them.

If there are no historical statistics before the specified time, TiDB exports the latest statistics, which is consistent with the behavior when no time is specified. In addition, TiDB prints the error messages in the errors.txt file within the exported ZIP file.

Examples of exporting cluster information

use test;
create table t(a int, b int);
insert into t values(1,1), (2, 2), (3, 3);
analyze table t;

plan replayer dump explain select * from t;
plan replayer dump with stats as of timestamp '2023-07-17 12:00:00' explain select * from t;
plan replayer dump with stats as of timestamp '442012134592479233' explain select * from t;

PLAN REPLAYER DUMP packages the table information above into a ZIP file and returns the file identifier as the execution result.

MySQL [test]> plan replayer dump explain select * from t;

+------------------------------------------------------------------+
| Dump_link                                                        |
+------------------------------------------------------------------+
| replayer_JOGvpu4t7dssySqJfTtS4A==_1635750890568691080.zip |
+------------------------------------------------------------------+
1 row in set (0.015 sec)

Alternatively, you can use the session variable tidb_last_plan_replayer_token to obtain the result of the last PLAN REPLAYER DUMP execution.

SELECT @@tidb_last_plan_replayer_token;

+-----------------------------------------------------------+
| @@tidb_last_plan_replayer_token                           |
+-----------------------------------------------------------+
| replayer_Fdamsm3C7ZiPJ-LQqgVjkA==_1663304195885090000.zip |
+-----------------------------------------------------------+
1 row in set (0.00 sec)

When there are multiple SQL statements, you can obtain the result of the PLAN REPLAYER DUMP execution using a file. The results of multiple SQL statements are separated by ; in this file.

plan replayer dump explain 'sqls.txt';

Query OK, 0 rows affected (0.03 sec)

SELECT @@tidb_last_plan_replayer_token;

+-----------------------------------------------------------+
| @@tidb_last_plan_replayer_token                           |
+-----------------------------------------------------------+
| replayer_LEDKg8sb-K0u24QesiH8ig==_1663226556509182000.zip |
+-----------------------------------------------------------+
1 row in set (0.00 sec)

Because the file cannot be downloaded on MySQL Client, you need to use the TiDB HTTP interface and the file identifier to download the file:

http://${tidb-server-ip}:${tidb-server-status-port}/plan_replayer/dump/${file_token}

${tidb-server-ip}:${tidb-server-status-port} is the address of any TiDB server in the cluster. For example:

curl http://127.0.0.1:10080/plan_replayer/dump/replayer_JOGvpu4t7dssySqJfTtS4A==_1635750890568691080.zip > plan_replayer.zip

Use PLAN REPLAYER to import cluster information

With an existing ZIP file exported using PLAN REPLAYER, you can use the PLAN REPLAYER import interface to restore the on-site information of a cluster to any other TiDB cluster. The syntax is as follows:

PLAN REPLAYER LOAD 'file_name';

In the statement above, file_name is the name of the ZIP file to be exported.

For example:

PLAN REPLAYER LOAD 'plan_replayer.zip';

After the cluster information is imported, the TiDB cluster is loaded with the required table schema, statistics and other information that affects the construction of the execution plan. You can view the execution plan and verify statistics in the following way:

mysql> desc t;
+-------+---------+------+------+---------+-------+
| Field | Type    | Null | Key  | Default | Extra |
+-------+---------+------+------+---------+-------+
| a     | int(11) | YES  |      | NULL    |       |
| b     | int(11) | YES  |      | NULL    |       |
+-------+---------+------+------+---------+-------+
2 rows in set (0.01 sec)

mysql> explain select * from t where a = 1 or b =1;
+-------------------------+---------+-----------+---------------+--------------------------------------+
| id                      | estRows | task      | access object | operator info                        |
+-------------------------+---------+-----------+---------------+--------------------------------------+
| TableReader_7           | 0.01    | root      |               | data:Selection_6                     |
| └─Selection_6           | 0.01    | cop[tikv] |               | or(eq(test.t.a, 1), eq(test.t.b, 1)) |
|   └─TableFullScan_5     | 6.00    | cop[tikv] | table:t       | keep order:false, stats:pseudo       |
+-------------------------+---------+-----------+---------------+--------------------------------------+
3 rows in set (0.00 sec)

mysql> show stats_meta;
+---------+------------+----------------+---------------------+--------------+-----------+
| Db_name | Table_name | Partition_name | Update_time         | Modify_count | Row_count |
+---------+------------+----------------+---------------------+--------------+-----------+
| test    | t          |                | 2022-08-26 15:52:07 |            3 |         6 |
+---------+------------+----------------+---------------------+--------------+-----------+
1 row in set (0.04 sec)

After the scene is loaded and restored, you can diagnose and improve the execution plan for the cluster.

Use PLAN REPLAYER CAPTURE to capture target plans

When you locate the execution plan of TiDB in some scenarios, the target SQL statement and the target execution plan might only appear occasionally in the query, so you cannot directly capture the statement and the plan using PLAN REPLAYER. In such cases, you can use PLAN REPLAYER CAPTURE to help you capture the optimizer information of the target SQL statement and the target plan.

PLAN REPLAYER CAPTURE has the following main features:

  • Registers the target SQL statement and the digest of the target execution plan in the TiDB cluster in advance, and starts matching the target query.
  • When the target query is matched successfully, directly captures its optimizer-related information and exports it as a ZIP file.
  • For each matched SQL and execution plan, the information is only captured once.
  • Displays the ongoing matching tasks and generated files through the system table.
  • Periodically cleans up historical files.

Enable PLAN REPLAYER CAPTURE

PLAN REPLAYER CAPTURE is controlled by the system variable tidb_enable_plan_replayer_capture. To enable PLAN REPLAYER CAPTURE, set the value of the system variable to ON.

Use PLAN REPLAYER CAPTURE

You can register the digest of the target SQL statement and execution plan in the TiDB cluster using the following statement:

PLAN REPLAYER CAPTURE 'sql_digest' 'plan_digest';

If the target SQL statement has multiple execution plans and you want to capture all execution plans, you can register all the execution plans at once using the following statement:

PLAN REPLAYER CAPTURE 'sql_digest' '*';

View the capture tasks

You can view the ongoing capture tasks of PLAN REPLAYER CAPTURE in the TiDB cluster using the following statement:

mysql> PLAN PLAYER CAPTURE 'example_sql' 'example_plan';
Query OK, 1 row affected (0.01 sec)

mysql> SELECT * FROM mysql.plan_replayer_task;
+-------------+--------------+---------------------+
| sql_digest  | plan_digest  | update_time         |
+-------------+--------------+---------------------+
| example_sql | example_plan | 2023-01-28 11:58:22 |
+-------------+--------------+---------------------+
1 row in set (0.01 sec)

View the capture results

After PLAN REPLAYER CAPTURE successfully captures the result, you can view the token used for file download using the following SQL statement:

mysql> SELECT * FROM mysql.plan_replayer_status;
+------------------------------------------------------------------+------------------------------------------------------------------+------------+-----------------------------------------------------------+---------------------+-------------+-----------------+
| sql_digest                                                       | plan_digest                                                      | origin_sql | token                                                     | update_time         | fail_reason | instance        |
+------------------------------------------------------------------+------------------------------------------------------------------+------------+-----------------------------------------------------------+---------------------+-------------+-----------------+
| 086e3fbd2732f7671c17f299d4320689deeeb87ba031240e1e598a0ca14f808c | 042de2a6652a6d20afc629ff90b8507b7587a1c7e1eb122c3e0b808b1d80cc02 |            | replayer_Utah4nkz2sIEzkks7tIRog==_1668746293523179156.zip | 2022-11-18 12:38:13 | NULL        | 172.16.4.4:4022 |
| b5b38322b7be560edb04f33f15b15a885e7c6209a22b56b0804622e397199b54 | 1770efeb3f91936e095f0344b629562bf1b204f6e46439b7d8f842319297c3b5 |            | replayer_Z2mUXNHDjU_WBmGdWQqifw==_1668746293560115314.zip | 2022-11-18 12:38:13 | NULL        | 172.16.4.4:4022 |
| 96d00c0b3f08795fe94e2d712fa1078ab7809faf4e81d198f276c0dede818cf9 | 8892f74ac2a42c2c6b6152352bc491b5c07c73ac3ed66487b2c990909bae83e8 |            | replayer_RZcRHJB7BaCccxFfOIAhWg==_1668746293578282450.zip | 2022-11-18 12:38:13 | NULL        | 172.16.4.4:4022 |
+------------------------------------------------------------------+------------------------------------------------------------------+------------+-----------------------------------------------------------+---------------------+-------------+-----------------+
3 rows in set (0.00 sec)

The method of downloading the file of PLAN REPLAYER CAPTURE is the same as that of PLAN REPLAYER. For details, see Examples of exporting cluster information.

Use PLAN REPLAYER CONTINUOUS CAPTURE

After PLAN REPLAYER CONTINUOUS CAPTURE is enabled, TiDB asynchronously records the applications' SQL statements with the PLAN REPLAYER method according to their SQL DIGEST and PLAN DIGEST. For SQL statements and execution plans that share the same DIGEST, PLAN REPLAYER CONTINUOUS CAPTURE does not record them repeatedly.

Enable PLAN REPLAYER CONTINUOUS CAPTURE

PLAN REPLAYER CONTINUOUS CAPTURE is controlled by the system variable tidb_enable_plan_replayer_continuous_capture. To enable PLAN REPLAYER CONTINUOUS CAPTURE, set the value of the system variable to ON.

View the capture results

The method of viewing the capture results of PLAN REPLAYER CONTINUOUS CAPTURE is the same as that of Viewing the capture results of PLAN REPLAYER CAPTURE.

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