Revert "Revert "add prefix index on tags (#3858)""

This reverts commit 6210c1554c.

db/structure.sql

@@ -7128,6 +7128,13 @@ CREATE UNIQUE INDEX index_tags_on_name ON public.tags USING btree (name);
 CREATE INDEX index_tags_on_name_pattern ON public.tags USING btree (name text_pattern_ops);
 
 
+--
+-- Name: index_tags_on_name_prefix; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_tags_on_name_prefix ON public.tags USING gin (regexp_replace((name)::text, '([a-z0-9])[a-z0-9'']*($|[^a-z0-9'']+)'::text, '
'::text, 'g'::text) public.gin_trgm_ops) WHERE (post_count > 0);
+
+
 --
 -- Name: index_tags_on_name_trgm; Type: INDEX; Schema: public; Owner: -
 --
@@ -7524,6 +7531,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20180517190048'),
 ('20180518175154'),
 ('20180804203201'),
-('20180816230604');
+('20180816230604'),
+('20180912185624');