This is a part of schema change of wb_terms table (custom table of Wikibase extension).
We would like to know if it would be possible to add empty (all NULLs) column to wb_terms without needing to make Wikidata DB read-only.
https://gerrit.wikimedia.org/r/#/c/341322/1/repo/sql/AddTermsFullEntityId.sql is a draft of the change we want to do.
There will be no code writing to and reading from the new column until the process of adding the column is complete.
Current schema change plan (as suggested by Jaime)
- Create a new column with new indexes
- Populate new column at the same time than the current one + populate old values with maintenance
- Deploy the new code using only the new column
- At some point in the future, drop the old column