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[5.0.x] Fixed #35015 -- Updated MySQL notes in migrations topic.

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Backport of d4c583682e7afc2b4e953b8c85af17f1c1514ba9 from main
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Nicolas Lupien 2023-12-06 12:58:59 -05:00 committed by Mariusz Felisiak
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commit bc60b1e292
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docs/topics/migrations.txt
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@ -77,16 +77,17 @@ meaning that if a migration fails to apply you will have to manually unpick
the changes in order to try again (it's impossible to roll back to an
earlier point).
In addition, MySQL will fully rewrite tables for almost every schema operation
and generally takes a time proportional to the number of rows in the table to
add or remove columns. On slower hardware this can be worse than a minute per
million rows - adding a few columns to a table with just a few million rows
could lock your site up for over ten minutes.
MySQL 8.0 introduced significant performance enhancements for
`DDL operations`_, making them more efficient and reducing the need for full
table rebuilds. However, it cannot guarantee a complete absence of locks or
interruptions. In situations where locks are still necessary, the duration of
these operations will be proportionate to the number of rows involved.
Finally, MySQL has relatively small limits on name lengths for columns, tables
and indexes, as well as a limit on the combined size of all columns an index
covers. This means that indexes that are possible on other backends will
fail to be created under MySQL.
Finally, MySQL has a relatively small limit on the combined size of all columns
an index covers. This means that indexes that are possible on other backends
will fail to be created under MySQL.
.. _DDL operations: https://dev.mysql.com/doc/refman/en/innodb-online-ddl-operations.html
SQLite
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