#35015: Update MySQL migration documentation for Mysql 8
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Reporter: Nicolas Lupien | Owner: nobody
Type: Uncategorized | Status: new
Component: Documentation | Version: 5.0
Severity: Normal | Resolution:
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Simon Charette):
> MySQL lacks support for transactions around schema alteration
operations, 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).
This is still true.
> 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.
That's no longer true for most operations and should be adjusted. I guess
[https://dev.mysql.com/doc/refman/8.0/en/innodb-online-ddl-operations.html
we could link to MySQL's Online DDL operations docs].
> 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.
I'm pretty sure [https://dev.mysql.com/doc/refman/8.0/en/innodb-
limits.html this is still true to a certain extent].
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Which parts of the documentation you believe are not accurate? Would you
be interested in submitting a patch that addresses the inaccuracies?
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Ticket URL: <https://code.djangoproject.com/ticket/35015#comment:2>
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