pgsql: Don't duplicate parallel seqscan shmem sizing logic in nbtree.

Don't duplicate parallel seqscan shmem sizing logic in nbtree.

This is architecturally mildly problematic, which becomes more
pronounced with the upcoming introduction of pluggable storage.

To fix, teach heap_parallelscan_estimate() to deal with SnapshotAny
snapshots, and then use it from _bt_parallel_estimate_shared().

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/90525d7b4e0fe5ebd53960cd7ef59ee11ff06516

Modified Files
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src/backend/access/heap/heapam.c    | 10 ++++++++--
src/backend/access/nbtree/nbtsort.c | 13 +++----------
2 files changed, 11 insertions(+), 12 deletions(-)