Optimizing Read-Only Scalability

In a thread on -perform it has been observed that our Read-Only
scalability is not as good as it could be. One problem being that we
need to scan the whole of the ProcArray to derive a snapshot, which
becomes the dominant task with many users.

If we think about a situation where write transactions happen
infrequently, then the likelihood is that we end up with xmin==xmax most
of the time.

If our last snapshot had xmin=xmax and the xmax hasn't changed since our
last snapshot then we don't need to scan the procarray at all, just look
at the header.

So we can optimize away the scan through the procarray by doing two "if"
tests, one outside of the lock, one inside. In normal running, both will
be optimized away, though in read-only periods we would avoid much work.

We don't need to change the API to GetSnapshotData since the snapshot is
statically allocated and unless newly created will contain the "last"
snapshot's data.

Interesting?

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support