Online DDL: enforce ALGORITHM=COPY on shadow table

[shlomi-noach]

Description

Similarly to #12436, this PR uses explicit ALGORITHM=COPY when altering Online DDL shadow tables. The shadow table is empty anyway, so there is no performance impact to using COPY. The advantage of using COPY is that Xtrabackup (at least on some versions?) has problems backing up tables created with INSTANT algorithm.

Related Issue(s)

Fixes #12517

Checklist

• "Backport to:" labels have been added if this change should be back-ported
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• Documentation was added or is not required

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[vitess-bot]

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[deepthi]

Is there a way to test this change in CI? Ideally a unit test..

[shlomi-noach]

Existing unit tests actually failed due to the change, and now adapted!

[deepthi]

We probably need to write a website FAQ / doc telling people how to work themselves out if they run into this situation. Not blocking this PR on that.

[shlomi-noach]

Per https://docs.percona.com/percona-xtrabackup/8.0/em/instant.html, the issue is resolved in MySQL 8.0.32 + Percona xtrabackup 8.0.32.

Copy from said doc:

To find tables with INSTANT ADD/DROP COLUMNS run the following command:

mysql> SELECT NAME FROM INFORMATION_SCHEMA.INNODB_TABLES WHERE TOTAL_ROW_VERSIONS > 0;
+---------+
| NAME    |
+---------+
| test/t1 |
| test/t2 |
| test/t3 |
+---------+
3 rows in set (0.02 sec)