CLIENT_ERRORS_SUMMARY_BY_HOST

The table CLIENT_ERRORS_SUMMARY_BY_HOST provides a summary of SQL errors and warnings that have been returned to clients that connect to a TiDB server. These include:

  • Malformed SQL statements.
  • Division by zero errors.
  • The attempt to insert out-of-range or duplicate key values.
  • Permission errors.
  • A table that does not exist.

These errors are returned to the client via the MySQL server protocol, where applications are expected to take appropriate action. The INFORMATION_SCHEMA.CLIENT_ERRORS_SUMMARY_BY_HOST table provides a useful method to inspect errors in the scenario where applications are not correctly handling (or logging) errors returned by the TiDB server.

Because CLIENT_ERRORS_SUMMARY_BY_HOST summarizes the errors on a per-remote-host basis, it can be useful to diagnose scenarios where one application server is generating more errors than other servers. Possible scenarios include:

  • An outdated MySQL client library.
  • An outdated application (possibly this server was missed when rolling out a new deployment).
  • Incorrect usage of the “host” portion of user permissions.
  • Unreliable network connectivity generating more timeouts or disconnected connections.

The summarized counts can be reset using the statement FLUSH CLIENT_ERRORS_SUMMARY. The summary is local to each TiDB server and is only retained in memory. Summaries will be lost if the TiDB server restarts.

  1. USE INFORMATION_SCHEMA;
  2. DESC CLIENT_ERRORS_SUMMARY_BY_HOST;

The output is as follows:

  1. +---------------+---------------+------+------+---------+-------+
  2. | Field         | Type          | Null | Key  | Default | Extra |
  3. +---------------+---------------+------+------+---------+-------+
  4. | HOST          | varchar(255)  | NO   |      | NULL    |       |
  5. | ERROR_NUMBER  | bigint(64)    | NO   |      | NULL    |       |
  6. | ERROR_MESSAGE | varchar(1024) | NO   |      | NULL    |       |
  7. | ERROR_COUNT   | bigint(64)    | NO   |      | NULL    |       |
  8. | WARNING_COUNT | bigint(64)    | NO   |      | NULL    |       |
  9. | FIRST_SEEN    | timestamp     | YES  |      | NULL    |       |
  10. | LAST_SEEN     | timestamp     | YES  |      | NULL    |       |
  11. +---------------+---------------+------+------+---------+-------+
  12. 7 rows in set (0.00 sec)

Field description:

  • HOST: The remote host of the client.
  • ERROR_NUMBER: The MySQL-compatible error number that was returned.
  • ERROR_MESSAGE: The error message which matches the error number (in prepared statement form).
  • ERROR_COUNT: The number of times this error was returned to the client host.
  • WARNING_COUNT: The number of times this warning was returned to the client host.
  • FIRST_SEEN: The first time this error (or warning) was seen from the client host.
  • LAST_SEEN: The most recent time this error (or warning) was seen from the client host.

The following example shows a warning being generated when the client connects to a local TiDB server. The summary is reset after executing FLUSH CLIENT_ERRORS_SUMMARY:

  1. SELECT 0/0;
  2. SELECT * FROM CLIENT_ERRORS_SUMMARY_BY_HOST;
  3. FLUSH CLIENT_ERRORS_SUMMARY;
  4. SELECT * FROM CLIENT_ERRORS_SUMMARY_BY_HOST;

The output is as follows:

  1. +-----+
  2. | 0/0 |
  3. +-----+
  4. | NULL |
  5. +-----+
  6. 1 row in set, 1 warning (0.00 sec)
  8. +-----------+--------------+---------------+-------------+---------------+---------------------+---------------------+
  9. | HOST      | ERROR_NUMBER | ERROR_MESSAGE | ERROR_COUNT | WARNING_COUNT | FIRST_SEEN          | LAST_SEEN           |
  10. +-----------+--------------+---------------+-------------+---------------+---------------------+---------------------+
  11. | 127.0.0.1 |         1365 | Division by 0 |           0 |             1 | 2021-03-18 12:51:54 | 2021-03-18 12:51:54 |
  12. +-----------+--------------+---------------+-------------+---------------+---------------------+---------------------+
  13. 1 row in set (0.00 sec)
  15. Query OK, 0 rows affected (0.00 sec)
  17. Empty set (0.00 sec)