Unknown tags are written to the error log. For example, the following message results if a collation definition contains a <aaa> tag:

[Warning] Buffered warning: Unknown LDML tag:
'charsets/charset/collation/rules/aaa'

If collation initialization is not possible, the server reports an “Unknown collation” error, and also generates warnings explaining the problems, such as in the previous example. In other cases, when a collation description is generally correct but contains some unknown tags, the collation is initialized and is available for use. The unknown parts are ignored, but a warning is generated in the error log.

Problems with collations generate warnings that clients can display with SHOW WARNINGS. Suppose that a reset rule contains an expansion longer than the maximum supported length of 6 characters:

<reset>abcdefghi</reset>
<i>x</i>

An attempt to use the collation produces warnings:

mysql> SELECT _utf8mb4'test' COLLATE utf8mb4_test_ci;
ERROR 1273 (HY000): Unknown collation: 'utf8mb4_test_ci'
mysql> SHOW WARNINGS;
+---------+------+----------------------------------------+
| Level   | Code | Message                                |
+---------+------+----------------------------------------+
| Error   | 1273 | Unknown collation: 'utf8mb4_test_ci'   |
| Warning | 1273 | Expansion is too long at 'abcdefghi=x' |
+---------+------+----------------------------------------+