presto-main/src/main/java/io/prestosql/operator/scalar/MapSubscriptOperator.java/MapSubscriptOperator/MissingKeyExceptionFactory
class MapSubscriptOperator:        public MissingKeyExceptionFactory(Metadata metadata, Type keyType)
        {
            functionInvoker = new InterpretedFunctionInvoker(metadata);

            ResolvedFunction castFunction = null;
            try {
                castFunction = metadata.getCoercion(keyType, VARCHAR);
            }
            catch (PrestoException ignored) {
            }
            this.castFunction = castFunction;
        }