pgsql: Avoid pointer chasing in _bt_readpage inner loop.

Avoid pointer chasing in _bt_readpage inner loop.

Make _bt_readpage pass down the current scan direction to various
utility functions within its pstate variable. Also have _bt_readpage
work off of a local copy of scan->ignore_killed_tuples within its
per-tuple loop (rather than using scan->ignore_killed_tuples directly).

Testing has shown that this significantly benefits large range scans,
which are naturally able to take full advantage of the pstate.startikey
optimization added by commit 8a510275. Running a pgbench script with a
"SELECT abalance FROM pgbench_accounts WHERE aid BETWEEN ..." query
shows an increase in transaction throughput of over 5%. There also
appears to be a small performance benefit when running pgbench's
built-in select-only script.

Follow-up to commit 65d6acbc.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Reviewed-By: Victor Yegorov <vyegorov(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAH2-WzmwMwcwKFgaf+mYPwiz3iL4AqpXnwtW_O0vqpWPXRom9Q@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/83a26ba59b1819cbd61705a3ff6aa572081ccc4b

Modified Files
--------------
src/backend/access/nbtree/nbtreadpage.c | 19 ++++++++++---------
1 file changed, 10 insertions(+), 9 deletions(-)