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+
+# Release External ID Refactor
+
+Goal is to make the external identifier "namespace" (number of external
+identifiers supported) less scarce, while still allowing fast lookups. Adding
+new identifiers would still require API schema and rust code changes.
+
+This change would also bring the number of columns on `release_rev` back under
+32, which makes diesel happier.
+
+Unclear whether existing extids (like DOI) should get merged and the old
+columns dropped. This could be done consistently by, eg, a rust worker process
+that re-writes tables, probably in multiple stages (eg, first copy, then update
+code to read from new location, then drop old columns and indices). Perhaps for
+very hot/popular columns (like DOI, and maybe ISBN13?) it is better to have
+separate columns/indices.
+
+It would be possible to roll this out as a SQL change first, with identifiers
+still at the top level of the API schema, then later switch the API schema. Not
+sure this is worth it though.
+
+## New API
+
+All identifers as text
+
+    release_entity
+        ext_ids (required)
+            doi
+            pmid
+            pmcid
+            wikidata_qid
+            core
+            isbn13
+            arxiv
+            jstor
+            [...]
+
+## New SQL Schema
+
+Something like:
+
+    release_rev_extid (
+        release_rev_id  UUID foreign key to release_rev.id
+        extid_type      TEXT
+        extid_value     TEXT
+    )
+    PRIMARY KEY (release_rev_id, extid_type)
+    INDEX (extid_type, extid_value)
+
+### Existing prod Column Use
+
+Queries like:
+
+    fatcat_prod=# SELECT COUNT(*) FROM release_rev WHERE wikidata_qid IS NOT NULL;
+    13460723
+
+Results:
+
+    wikidata_qid: 13460723
+    isbn13: 1
+    core_id: 7160477
+    arxiv_id: 3
+    jstor_id: 0
+    pmcid: 3688945
+
+Keep in SQL:
+
+- `doi`
+- `pmid`
+- `pmcid`
+- `wikidata_qid`
+- `core_id`
+
+Drop columns:
+
+- `isbn13`
+- `arxiv_id`
+- `jstor_id`
+
+In new table:
+
+- isbn13
+- arxiv
+- jstor
+- mag
+- ark
+- dblp