Add index to improve speed of vulnerability_reads

Add index on (project_id, state, report_type, severity,
vulnerability_id) to speed up commonly executed queries.

Changelog: changed

db/post_migrate/20220315211043_add_index_to_vulnerability_reads.rb

+# frozen_string_literal: true
+
+class AddIndexToVulnerabilityReads < Gitlab::Database::Migration[1.0]
+  INDEX_NAME = 'index_vulnerability_reads_common_finder_query'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index(
+      :vulnerability_reads,
+      %i[project_id state report_type severity vulnerability_id],
+      name: INDEX_NAME,
+      order: { vulnerability_id: :desc }
+    )
+  end
+
+  def down
+    remove_concurrent_index_by_name(
+      :vulnerability_reads,
+      INDEX_NAME
+    )
+  end
+end

db/schema_migrations/20220315211043

+ae734a1ae3555a237e427dbcc0ace6c461c50cf98bc1076ca59c90b603b88c29
\ No newline at end of file

db/structure.sql

@@ -29437,6 +29437,8 @@ CREATE UNIQUE INDEX index_vulnerability_occurrences_on_uuid ON vulnerability_occ
 
 CREATE INDEX index_vulnerability_occurrences_on_vulnerability_id ON vulnerability_occurrences USING btree (vulnerability_id);
 
+CREATE INDEX index_vulnerability_reads_common_finder_query ON vulnerability_reads USING btree (project_id, state, report_type, severity, vulnerability_id DESC);
+
 CREATE INDEX index_vulnerability_reads_on_cluster_agent_id ON vulnerability_reads USING btree (cluster_agent_id) WHERE (report_type = 7);
 
 CREATE INDEX index_vulnerability_reads_on_location_image ON vulnerability_reads USING btree (location_image) WHERE (report_type = ANY (ARRAY[2, 7]));