pgsql: Reduce path length for locking leaf B-tree pages during insertio

Reduce path length for locking leaf B-tree pages during insertion

In our B-tree implementation appropriate leaf page for new tuple
insertion is acquired using _bt_search() function. This function always
returns leaf page locked in shared mode. In order to obtain exclusive
lock, caller have to relock the page.

This commit makes _bt_search() function lock leaf page immediately in
exclusive mode when needed. That removes unnecessary relock and, in
turn reduces lock contention for B-tree leaf pages. Our experiments
on multi-core systems showed acceleration up to 4.5 times in corner
case.

Discussion: https://postgr.es/m/CAPpHfduAMDFMNYTCN7VMBsFg_hsf0GqiqXnt%2BbSeaJworwFoig%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Yoshikazu Imai, Simon Riggs, Peter Geoghegan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d2086b08b023c0749a53d617ff3fe0f052646254

Modified Files
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src/backend/access/nbtree/nbtinsert.c | 19 ++++-----------
src/backend/access/nbtree/nbtsearch.c | 44 +++++++++++++++++++++++++++++------
2 files changed, 41 insertions(+), 22 deletions(-)