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.
**Background:**
General plan for the whole schema change is:
1. Add term_entity_id_s VARBINARY(32) to wb_terms. All values NULL
2. Populate term_entity_id_s based on existing data in wb_terms
3. Create index on term_entity_s
4. Create index on term_language, term_entity_id_s, term_type, term_search_key(16)
5. Drop old column term_entity_id from wb_terms.
6. Rename term_entity_id_s to term_entity_id