Primary key for elasticsearch_indexed_namespaces

Add a new migration that promotes the unique index on
`elasticsearch_indexed_namespaces.namespace_id` to be the primary key of
the table.

changelogs/unreleased/270090-add-pk-to-elasticsearch-indexed-namespaces.yml

+---
+title: Add primary key to elasticsearch_indexed_namespaces
+merge_request: 48944
+author:
+type: changed

db/migrate/20201201192112_add_primary_key_to_elastic_search_indexed_namespaces.rb

+# frozen_string_literal: true
+
+class AddPrimaryKeyToElasticSearchIndexedNamespaces < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  UNIQUE_INDEX_NAME = 'index_elasticsearch_indexed_namespaces_on_namespace_id'
+  PRIMARY_KEY_NAME = 'elasticsearch_indexed_namespaces_pkey'
+
+  disable_ddl_transaction!
+
+  def up
+    with_lock_retries do
+      execute(<<~SQL)
+        LOCK TABLE elasticsearch_indexed_namespaces IN ACCESS EXCLUSIVE MODE;
+
+        DELETE FROM elasticsearch_indexed_namespaces
+        WHERE namespace_id IS NULL;
+
+        ALTER TABLE elasticsearch_indexed_namespaces
+        ADD CONSTRAINT #{PRIMARY_KEY_NAME} PRIMARY KEY USING INDEX #{UNIQUE_INDEX_NAME};
+      SQL
+    end
+  end
+
+  def down
+    add_concurrent_index :elasticsearch_indexed_namespaces, :namespace_id, unique: true, name: UNIQUE_INDEX_NAME
+
+    with_lock_retries do
+      execute(<<~SQL)
+        ALTER TABLE elasticsearch_indexed_namespaces
+        DROP CONSTRAINT #{PRIMARY_KEY_NAME},
+        ALTER COLUMN namespace_id DROP NOT NULL
+      SQL
+    end
+  end
+end

db/schema_migrations/20201201192112

+bd10ddc5e78ff2d878f3ce6a45a3e808b89e0885dcc710b87f44ede15ace372b
\ No newline at end of file

db/structure.sql

@@ -11950,7 +11950,7 @@ ALTER SEQUENCE elastic_reindexing_tasks_id_seq OWNED BY elastic_reindexing_tasks
 CREATE TABLE elasticsearch_indexed_namespaces (
     created_at timestamp with time zone NOT NULL,
     updated_at timestamp with time zone NOT NULL,
-    namespace_id integer
+    namespace_id integer NOT NULL
 );
 
 CREATE TABLE elasticsearch_indexed_projects (
@@ -19350,6 +19350,9 @@ ALTER TABLE ONLY draft_notes
 ALTER TABLE ONLY elastic_reindexing_tasks
     ADD CONSTRAINT elastic_reindexing_tasks_pkey PRIMARY KEY (id);
 
+ALTER TABLE ONLY elasticsearch_indexed_namespaces
+    ADD CONSTRAINT elasticsearch_indexed_namespaces_pkey PRIMARY KEY (namespace_id);
+
 ALTER TABLE ONLY elasticsearch_indexed_projects
     ADD CONSTRAINT elasticsearch_indexed_projects_pkey PRIMARY KEY (project_id);
 
@@ -21174,8 +21177,6 @@ CREATE INDEX index_elastic_reindexing_tasks_on_state ON elastic_reindexing_tasks
 
 CREATE INDEX index_elasticsearch_indexed_namespaces_on_created_at ON elasticsearch_indexed_namespaces USING btree (created_at);
 
-CREATE UNIQUE INDEX index_elasticsearch_indexed_namespaces_on_namespace_id ON elasticsearch_indexed_namespaces USING btree (namespace_id);
-
 CREATE UNIQUE INDEX index_emails_on_confirmation_token ON emails USING btree (confirmation_token);
 
 CREATE UNIQUE INDEX index_emails_on_email ON emails USING btree (email);