On the need for a snapshot in exec_bind_message()

In particular:
exec_bind_message()
PushActiveSnapshot(GetTransactionSnapshot());

Suppressing this I've achieved over 1.9 M TXN's a second on select only pgbench on a 48 core box. It is about 50% faster with this change. The cpu usage of GetSnapshotData drops from about 22% to 4.5%.

If there were no input functions, that needed this, nor reparsing or reanalyzing needed, and we knew this up front, it'd be a huge win. We could test for a number of conditions on the first parse/optimization of the query and set a flag to suppress this for subsequent executions.

NOTE:

In GetSnapshotData because pgxact, is declared volatile, the compiler will not reduce the following two IF tests into a single test:

if (pgxact->vacuumFlags & PROC_IN_LOGICAL_DECODING)
continue;

if (pgxact->vacuumFlags & PROC_IN_VACUUM)
continue;

You can reduce the code path in the inner loop by coding this as:

if (pgxact->vacuumFlags & (PROC_IN_LOGICAL_DECODING|PROC_IN_VACUUM))
continue;

I'm still working on quantifying any gain. Note it isn't just one L1 cache

fetch and one conditional branch eliminated. Due to the update frequency of the pgxact cache line, for single statement TXN's, there are a certain number of full cache misses, due to invalidation, that occurs when given pgxact is updated between the first fetch of vacuumFlags and the 2nd fetch.