code: func (b *builtinDatabaseSig) evalString(row chunk.Row) (string, bool, error) {
  	currentDB := b.ctx.GetSessionVars().CurrentDB
  	return currentDB, currentDB == "", nil
  }
docs: // evalString evals a builtinDatabaseSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/information-functions.html