why can't see the updated value after SPI_execute("update ....", false, 1);

First I use SPI_execute("update ....
bool succ;
SPI_connect();
int ret = SPI_execute("update ....where uid = 1", false, 1);//later will
add error processing
if (ret == SPI_OK_UPDATE && SPI_processed == 1) {
succ = true;
}
SPI_finish();

Then I use SPI_execute("select .....where uid = 1", ....

SPI_connect();
int ret = SPI_execute("select .....where uid = 1", true, 1);//later will
add error processing
if (ret == SPI_OK_SELECT && SPI_processed == 1) {
HeapTuple tuple;
tuple = SPI_tuptable->vals[0];
...
datum = heap_getattr(tuple,attrno,SPI_tuptable->tupdesc,&isnull);
....
}
the datum doesn't change, why ?