code: protected function getUniqueIndexInformation($table)
      {
          $sql = <<<'SQL'
  SELECT
      i.relname as indexname,
      pg_get_indexdef(idx.indexrelid, k + 1, TRUE) AS columnname
  FROM (
    SELECT *, generate_subscripts(indkey, 1) AS k
    FROM pg_index
  ) idx
  INNER JOIN pg_class i ON i.oid = idx.indexrelid
  INNER JOIN pg_class c ON c.oid = idx.indrelid
  INNER JOIN pg_namespace ns ON c.relnamespace = ns.oid
  WHERE idx.indisprimary = FALSE AND idx.indisunique = TRUE
  AND c.relname = :tableName AND ns.nspname = :schemaName
  ORDER BY i.relname, k
  SQL;
  
          return $this->db->createCommand($sql, [
              ':schemaName' => $table->schemaName,
              ':tableName' => $table->name,
          ])->queryAll();
      }
docs: Gets information about given table unique indexes.
  @param TableSchema $table the table metadata
  @return array with index and column names