diff --git a/src/flex-lua-table.cpp b/src/flex-lua-table.cpp
index 98888fdd3..242463d09 100644
--- a/src/flex-lua-table.cpp
+++ b/src/flex-lua-table.cpp
@@ -108,6 +108,9 @@ static void parse_create_index(lua_State *lua_state, flex_table_t *table)
     lua_pop(lua_state, 1); // "create_index"
     if (create_index == "always") {
         table->set_always_build_id_index();
+    } else if (create_index == "unique") {
+        table->set_always_build_id_index();
+        table->set_build_unique_id_index();
     } else if (create_index != "auto") {
         throw fmt_error("Unknown value '{}' for 'create_index' field of ids",
                         create_index);
diff --git a/src/flex-table.cpp b/src/flex-table.cpp
index 6dff56a4e..76224a6b7 100644
--- a/src/flex-table.cpp
+++ b/src/flex-table.cpp
@@ -226,7 +226,8 @@ std::string flex_table_t::build_sql_column_list() const
 
 std::string flex_table_t::build_sql_create_id_index() const
 {
-    return fmt::format("CREATE INDEX ON {} USING BTREE ({}) {}", full_name(),
+    return fmt::format("CREATE {}INDEX ON {} USING BTREE ({}) {}",
+                       m_build_unique_id_index ? "UNIQUE " : "", full_name(),
                        id_column_names(),
                        tablespace_clause(index_tablespace()));
 }
diff --git a/src/flex-table.hpp b/src/flex-table.hpp
index c1278f9f7..5fef823e1 100644
--- a/src/flex-table.hpp
+++ b/src/flex-table.hpp
@@ -195,6 +195,16 @@ class flex_table_t
         return m_always_build_id_index;
     }
 
+    void set_build_unique_id_index() noexcept
+    {
+        m_build_unique_id_index = true;
+    }
+
+    bool build_unique_id_index() const noexcept
+    {
+        return m_build_unique_id_index;
+    }
+
     bool has_columns_with_expire() const noexcept;
 
     std::size_t num() const noexcept { return m_table_num; }
@@ -253,6 +263,9 @@ class flex_table_t
     /// Always build the id index, not only when it is needed for updates?
     bool m_always_build_id_index = false;
 
+    /// Build the index as a unique index.
+    bool m_build_unique_id_index = false;
+
 }; // class flex_table_t
 
 class table_connection_t