MySQL GTID replication: is it possible to make DDL changes to a slave and then catch it up to a master? Here's what I would like to do:
1. hang a slave off my production master.
2. stop replication
3. adjust some data types and create some indexes on the slave (no data mods)
4. restart replication
5. once the slave has caught up, lock the db &amp; swap slave &amp; master

-the goal, obv. is to have no significant downtime while making some changes that will take time (200G table getting modified)
-it should be possible, because the DDL changes I'm making have to do with shrinking data types &amp; adding constraints - nothing that changes the actual values of data.
-since GTID replication is row-based, I don't know if replication will choke trying to insert, for example, 11 digit ids into a 5 digit id column, even if the values are in range.
You have described a common way to make DDL changes.  Yes, it should work.  But...  please describe the datatype change further.

If you are ALTERing a COLUMN that is current INT SIGNED to be a SMALLINT SIGNED, and if none of the values are bigger than 32767, there should be no problem.  If you are changing from SIGNED to UNSIGNED, and none of the numbers are negative, then no problem.  Etc.  In general, if the old value will fit in the new datatype, then it is OK.  If it won't fit, then something will be lost.  1 million into a SMALLINT UNSIGNED will probably become 65535.

As for the application, how does mysql know it is getting an 11-digit id?  It sees a number of some number of digits and shoves it into the target size.

Do SHOW WARNINGS; after the DDL.