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MySQL 8.0 Reference Manual
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Excerpts from this Manual Diagnostics During Index.xml Parsing

The MySQL server generates diagnostics when it finds problems while parsing the Index.xml file:

  • 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:
  • 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:


    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' |