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An outdated MySQL client library.
An outdated application (possibly this server was missed when rolling out a new deployment).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

+---------------+---------------+------+------+---------+-------+
| 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;

+-----+
| 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)