Why does exec_simple_query requires 2 snapshots

Hi,

When I run "SELECT * FROM pg_class LIMIT 1"; then postgresql
run GetSnapshotData twice, one is

/*
* Set up a snapshot if parse analysis/planning will need one.
*/
if (analyze_requires_snapshot(parsetree))
{
PushActiveSnapshot(GetTransactionSnapshot());
snapshot_set = true;
}

the other one is in PortalStart.

My question is why can't we share the same snapshot for the 2 cases?
parser & planner requires Catalog Snapshot which should be the
latest one, but in the above case, looks the executor can reuse it as
well. Is there anything I missed?

Thanks

--
Best Regards
Andy Fan