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Reduce pinning and buffer content locking for btree scans.

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Even though the main benefit of the Lehman and Yao algorithm for
btrees is that no locks need be held between page reads in an
index search, we were holding a buffer pin on each leaf page after
it was read until we were ready to read the next one.  The reason
was so that we could treat this as a weak lock to create an
"interlock" with vacuum's deletion of heap line pointers, even
though our README file pointed out that this was not necessary for
a scan using an MVCC snapshot.

The main goal of this patch is to reduce the blocking of vacuum
processes by in-progress btree index scans (including a cursor
which is idle), but the code rearrangement also allows for one
less buffer content lock to be taken when a forward scan steps from
one page to the next, which results in a small but consistent
performance improvement in many workloads.

This patch leaves behavior unchanged for some cases, which can be
addressed separately so that each case can be evaluated on its own
merits.  These unchanged cases are when a scan uses a non-MVCC
snapshot, an index-only scan, and a scan of a btree index for which
modifications are not WAL-logged.  If later patches allow  all of
these cases to drop the buffer pin after reading a leaf page, then
the btree vacuum process can be simplified; it will no longer need
the "super-exclusive" lock to delete tuples from a page.

Reviewed by Heikki Linnakangas and Kyotaro Horiguchi
This commit is contained in:
Kevin Grittner
2015-03-25 14:24:43 -05:00
parent 8217fb1441
commit 2ed5b87f96
8 changed files with 326 additions and 132 deletions
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4
src/backend/access/nbtree/nbtinsert.c
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@@ -498,9 +498,9 @@ _bt_check_unique(Relation rel, IndexTuple itup, Relation heapRel,
* If there's not enough room in the space, we try to make room by
* removing any LP_DEAD tuples.
*
* On entry, *buf and *offsetptr point to the first legal position
* On entry, *bufptr and *offsetptr point to the first legal position
* where the new tuple could be inserted. The caller should hold an
* exclusive lock on *buf. *offsetptr can also be set to
* exclusive lock on *bufptr. *offsetptr can also be set to
* InvalidOffsetNumber, in which case the function will search for the
* right location within the page if needed. On exit, they point to the
* chosen insert location. If _bt_findinsertloc decides to move right,