Improve hash index bucket split behavior

Enterprise / PostgreSQL - Robert Haas [postgresql.org] - 30 November 2016 14:39 UTC

Previously, the right to split a bucket was represented by a heavyweight lock on the page number of the primary bucket page. Unfortunately, this meant that every scan needed to take a heavyweight lock on that bucket also, which was bad for concurrency. Instead, use a cleanup lock on the primary bucket page to indicate the right to begin a split, so that scans only need to retain a pin on that page, which is they would have to acquire anyway, and which is also much cheaper.

In addition to reducing the locking cost, this also avoids locking out scans and inserts for the entire lifetime of the split: while the new bucket is being populated with copies of the appropriate tuples from the old bucket, scans and inserts can happen in parallel. There are minor concurrency improvements for vacuum operations as well, though the situation there is still far from ideal.

This patch also removes the unworldly assumption that a split will never be interrupted. With the new code, a split is done in a series of small steps and the system can pick up where it left off if it is interrupted prior to completion. While this patch does not itself add write-ahead logging for hash indexes, it is clearly a necessary first step, since one of the things that could interrupt a split is the removal of electrical power from the machine performing it.

Amit Kapila. I wrote the original design on which this patch is based, and did a good bit of work on the comments and README through multiple rounds of review, but all of the code is Amit's. Also reviewed by Jesper Pedersen, Jeff Janes, and others.

Discussion: http://postgr.es/m/CAA4eK1LfzcZYxLoXS874Ad0+S-ZM60U9bwcyiUZx9mHZ-KCWhw@mail.gmail.com

6d46f47 Improve hash index bucket split behavior.
src/backend/access/hash/Makefile | 4 +-
src/backend/access/hash/README | 311 ++++++++++---------
src/backend/access/hash/hash.c | 350 ++++++++++++++++------
src/backend/access/hash/hashinsert.c | 86 ++++--
src/backend/access/hash/hashovfl.c | 154 ++++++----
src/backend/access/hash/hashpage.c | 527 ++++++++++++++++++++++++++-------
src/backend/access/hash/hashscan.c | 153 ----------
src/backend/access/hash/hashsearch.c | 225 ++++++++++++--
src/backend/access/hash/hashutil.c | 94 ++++++
src/backend/utils/resowner/resowner.c | 3 -
src/include/access/hash.h | 82 +++--
src/include/access/itup.h | 2 +-
12 files changed, 1365 insertions(+), 626 deletions(-)

Upstream: git.postgresql.org

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