diff --git a/plugins/search-backend-module-pg/migrations/20210311110000_init.js b/plugins/search-backend-module-pg/migrations/20210311110000_init.js
index 8eaf86d2b4..70ccf9406b 100644
--- a/plugins/search-backend-module-pg/migrations/20210311110000_init.js
+++ b/plugins/search-backend-module-pg/migrations/20210311110000_init.js
@@ -18,6 +18,8 @@
  * @param {import('knex').Knex} knex
  */
 exports.up = async function up(knex) {
+  // This database schema uses some postgres specific features (like tsvector
+  // and jsonb columns) and can not be used with other database engines.
   await knex.schema.createTable('documents', table => {
     table.comment('The table of documents');
     table
diff --git a/plugins/search-backend-module-pg/migrations/20210727180000_delta_update.js b/plugins/search-backend-module-pg/migrations/20210727180000_delta_update.js
index 9ba4649b65..429c8aa805 100644
--- a/plugins/search-backend-module-pg/migrations/20210727180000_delta_update.js
+++ b/plugins/search-backend-module-pg/migrations/20210727180000_delta_update.js
@@ -18,6 +18,9 @@
  * @param {import('knex').Knex} knex
  */
 exports.up = async function up(knex) {
+  // This database migration uses some postgres specific features (like bytea
+  // columns and build in functions) and can not be used with other database
+  // engines.
   await knex.schema.alterTable('documents', table => {
     // Extend the documents table with a column that allows to check whether a
     // document with the same content is already indexed.