Remove assumption about public schema

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/228724

db/gitlab_structure.sql

 -- this file tracks custom GitLab data, such as foreign keys referencing partitioned tables
 -- more details can be found in the issue: https://gitlab.com/gitlab-org/gitlab/-/issues/201872
-SET search_path=public;

lib/gitlab/database/custom_structure.rb

@@ -8,8 +8,7 @@ module Gitlab
       def dump
         File.open(self.class.custom_dump_filepath, 'wb') do |io|
           io << "-- this file tracks custom GitLab data, such as foreign keys referencing partitioned tables\n"
-          io << "-- more details can be found in the issue: https://gitlab.com/gitlab-org/gitlab/-/issues/201872\n"
-          io << "SET search_path=public;\n\n"
+          io << "-- more details can be found in the issue: https://gitlab.com/gitlab-org/gitlab/-/issues/201872\n\n"
           dump_partitioned_foreign_keys(io) if partitioned_foreign_keys_exist?
         end

lib/gitlab/database/schema_cleaner.rb

@@ -18,11 +18,18 @@ module Gitlab
         structure.gsub!(/^SELECT pg_catalog\.set_config\('search_path'.+/, '')
         structure.gsub!(/^--.*/, "\n")
-        structure = "SET search_path=public;\n" + structure
+        # We typically don't assume we're working with the public schema.
+        # pg_dump uses fully qualified object names though, since we have multiple schemas
+        # in the database.
+        #
+        # The intention here is to not introduce an assumption about the standard schema,
+        # unless we have a good reason to do so.
+        structure.gsub!(/public\.(\w+)/, '\1')
+        structure.gsub!(/CREATE EXTENSION IF NOT EXISTS (\w+) WITH SCHEMA public;/, 'CREATE EXTENSION IF NOT EXISTS \1;')
         structure.gsub!(/\n{3,}/, "\n\n")
-        io << structure
+        io << structure.strip
         io << <<~MSG
           -- schema_migrations.version information is no longer stored in this file,
           -- but instead tracked in the db/schema_migrations directory

spec/fixtures/gitlab/database/structure_example_cleaned.sql

-SET search_path=public;
-CREATE EXTENSION IF NOT EXISTS pg_trgm WITH SCHEMA public;
-
-CREATE TABLE public.abuse_reports (
+CREATE EXTENSION IF NOT EXISTS pg_trgm;
+CREATE TABLE abuse_reports (
     id integer NOT NULL,
     reporter_id integer,
     user_id integer,
@@ -13,20 +11,18 @@ CREATE TABLE public.abuse_reports (
     cached_markdown_version integer
 );
-CREATE SEQUENCE public.abuse_reports_id_seq
+CREATE SEQUENCE abuse_reports_id_seq
     START WITH 1
     INCREMENT BY 1
     NO MINVALUE
     NO MAXVALUE
     CACHE 1;
-ALTER TABLE ONLY public.abuse_reports ALTER COLUMN id SET DEFAULT nextval('public.abuse_reports_id_seq'::regclass);
-ALTER TABLE ONLY public.abuse_reports
+ALTER TABLE ONLY abuse_reports ALTER COLUMN id SET DEFAULT nextval('abuse_reports_id_seq'::regclass);
+ALTER TABLE ONLY abuse_reports
     ADD CONSTRAINT abuse_reports_pkey PRIMARY KEY (id);
-CREATE INDEX index_abuse_reports_on_user_id ON public.abuse_reports USING btree (user_id);
-
--- schema_migrations.version information is no longer stored in this file,
+CREATE INDEX index_abuse_reports_on_user_id ON abuse_reports USING btree (user_id);-- schema_migrations.version information is no longer stored in this file,
 -- but instead tracked in the db/schema_migrations directory
 -- see https://gitlab.com/gitlab-org/gitlab/-/issues/218590 for details

spec/lib/gitlab/database/custom_structure_spec.rb

@@ -9,7 +9,6 @@ RSpec.describe Gitlab::Database::CustomStructure do
     <<~DATA
       -- this file tracks custom GitLab data, such as foreign keys referencing partitioned tables
       -- more details can be found in the issue: https://gitlab.com/gitlab-org/gitlab/-/issues/201872
-      SET search_path=public;
     DATA
   end

spec/lib/gitlab/database/schema_cleaner_spec.rb

@@ -15,8 +15,8 @@ RSpec.describe Gitlab::Database::SchemaCleaner do
     expect(subject).not_to include('COMMENT ON EXTENSION')
   end
-  it 'sets the search_path' do
-    expect(subject.split("\n").first).to eq('SET search_path=public;')
+  it 'no assumption about public being the default schema' do
+    expect(subject).not_to match(/public\.\w+/)
   end
   it 'cleans up the full schema as expected (blackbox test with example)' do