PERF: avoid regex on uploads table (#12485)

In extreme circumstances when the uploads table is huge, the old version of
this migration could take a very long time.

The rewrite extracts the sha1 directly from the badges table and does an index
based match on the uploads table

db/migrate/20210311070755_add_image_upload_id_to_badges.rb

@@ -8,10 +8,16 @@ class AddImageUploadIdToBadges < ActiveRecord::Migration[6.0]
         DB.exec <<~SQL
           UPDATE badges b1
           SET image_upload_id = u.id
-          FROM badges b2
-          INNER JOIN uploads u
-          ON b2.image ~ CONCAT('/', u.sha1, '\\.\\w')
-          WHERE b1.id = b2.id
+          FROM (
+            SELECT id, (regexp_matches(b.image, '[a-f0-9]{40}'))[1] as sha1
+            FROM badges b
+            WHERE
+              b.image IS NOT NULL AND
+              b.image ~ '[a-f0-9]{40}'
+          ) b2
+          JOIN uploads u ON u.sha1 = b2.sha1
+          WHERE
+            b1.id = b2.id
         SQL
       end
     end