Change the unique index on `security_findings` table

db/migrate/20201215132151_change_unique_index_on_security_findings.rb

+# frozen_string_literal: true
+
+class ChangeUniqueIndexOnSecurityFindings < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  OLD_INDEX_NAME = 'index_security_findings_on_uuid'
+  NEW_INDEX_NAME = 'index_security_findings_on_uuid_and_scan_id'
+
+  disable_ddl_transaction!
+
+  class SecurityFinding < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'security_findings'
+  end
+
+  def up
+    add_concurrent_index :security_findings, [:uuid, :scan_id], unique: true, name: NEW_INDEX_NAME
+
+    remove_concurrent_index_by_name :security_findings, OLD_INDEX_NAME
+  end
+
+  def down
+    # It is very unlikely that we rollback this migration but just in case if we have to,
+    # we have to clear the table because there can be multiple records with the same UUID
+    # which would break the creation of unique index on the `uuid` column.
+    # We choose clearing the table because just removing the duplicated records would
+    # cause data inconsistencies.
+    SecurityFinding.each_batch(of: 10000) { |relation| relation.delete_all }
+
+    add_concurrent_index :security_findings, :uuid, unique: true, name: OLD_INDEX_NAME
+
+    remove_concurrent_index_by_name :security_findings, NEW_INDEX_NAME
+  end
+end

db/schema_migrations/20201215132151

+916f29e6ab89551fd785c3a8584c24b72d9002ada30d159e9ff826cb247199b5
\ No newline at end of file

db/structure.sql

@@ -22529,7 +22529,7 @@ CREATE INDEX index_security_findings_on_scanner_id ON security_findings USING bt
 
 CREATE INDEX index_security_findings_on_severity ON security_findings USING btree (severity);
 
-CREATE UNIQUE INDEX index_security_findings_on_uuid ON security_findings USING btree (uuid);
+CREATE UNIQUE INDEX index_security_findings_on_uuid_and_scan_id ON security_findings USING btree (uuid, scan_id);
 
 CREATE INDEX index_self_managed_prometheus_alert_events_on_environment_id ON self_managed_prometheus_alert_events USING btree (environment_id);