Add IssueLinks migration helper index

This index is needed to back the data integrity fixing process outlined
in https://gitlab.com/gitlab-org/gitlab/-/issues/224457

changelogs/unreleased/224457-fix-data-integrity-on-issue_feedback-issue_links.yml

+---
+title: Add migration helper index for Vulnerabilities::Finding table
+merge_request: 38898
+author:
+type: changed

db/post_migrate/20200807110237_add_migration_index_to_vulnerabilities_occurrences.rb

+# frozen_string_literal: true
+
+class AddMigrationIndexToVulnerabilitiesOccurrences < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :vulnerability_occurrences,
+      "project_id, report_type, encode(project_fingerprint, 'hex'::text)",
+      name: 'index_vulnerability_occurrences_for_issue_links_migration'
+  end
+
+  def down
+    remove_concurrent_index_by_name :vulnerability_occurrences,
+      :index_vulnerability_occurrences_for_issue_links_migration
+  end
+end

db/schema_migrations/20200807110237

+5aeb0e74859df14e460b39b9f070624a02278c13b1da9393e1671b4c79635f52
\ No newline at end of file

db/structure.sql

@@ -20849,6 +20849,8 @@ CREATE UNIQUE INDEX index_vulnerability_occurrence_identifiers_on_unique_keys ON
 
 CREATE INDEX index_vulnerability_occurrence_pipelines_on_pipeline_id ON public.vulnerability_occurrence_pipelines USING btree (pipeline_id);
 
+CREATE INDEX index_vulnerability_occurrences_for_issue_links_migration ON public.vulnerability_occurrences USING btree (project_id, report_type, encode(project_fingerprint, 'hex'::text));
+
 CREATE INDEX index_vulnerability_occurrences_on_primary_identifier_id ON public.vulnerability_occurrences USING btree (primary_identifier_id);
 
 CREATE INDEX index_vulnerability_occurrences_on_scanner_id ON public.vulnerability_occurrences USING btree (scanner_id);