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  • An outdated MySQL client library.
  • An outdated application (possibly this server was missed when rolling out a new deployment).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.

USE INFORMATION_SCHEMA; DESC CLIENT_ERRORS_SUMMARY_BY_HOST;

The output is as follows:

+---------------+---------------+------+------+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------------+---------------+------+------+---------+-------+ | HOST | varchar(255) | NO | | NULL | | | ERROR_NUMBER | bigint(64) | NO | | NULL | | | ERROR_MESSAGE | varchar(1024) | NO | | NULL | | | ERROR_COUNT | bigint(64) | NO | | NULL | | | WARNING_COUNT | bigint(64) | NO | | NULL | | | FIRST_SEEN | timestamp | YES | | NULL | | | LAST_SEEN | timestamp | YES | | NULL | | +---------------+---------------+------+------+---------+-------+ 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:

SELECT 0/0; SELECT * FROM CLIENT_ERRORS_SUMMARY_BY_HOST; FLUSH CLIENT_ERRORS_SUMMARY; SELECT * FROM CLIENT_ERRORS_SUMMARY_BY_HOST;

The output is as follows:

+-----+ | 0/0 | +-----+ | NULL | +-----+ 1 row in set, 1 warning (0.00 sec) +-----------+--------------+---------------+-------------+---------------+---------------------+---------------------+ | HOST | ERROR_NUMBER | ERROR_MESSAGE | ERROR_COUNT | WARNING_COUNT | FIRST_SEEN | LAST_SEEN | +-----------+--------------+---------------+-------------+---------------+---------------------+---------------------+ | 127.0.0.1 | 1365 | Division by 0 | 0 | 1 | 2021-03-18 12:51:54 | 2021-03-18 12:51:54 | +-----------+--------------+---------------+-------------+---------------+---------------------+---------------------+ 1 row in set (0.00 sec) Query OK, 0 rows affected (0.00 sec) Empty set (0.00 sec)

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