In talking to Teodor this week, I have written the attached C comment
patch which improves our description of GiST's NSN and GistBuildLSN
values.
--
Bruce Momjian <br...@momjian.us> https://momjian.us
EDB https://enterprisedb.com
The usefulness of a cup is in its emptiness, Bruce Lee
>From defd16776c8016179a87c3d32ed359570c62727d Mon Sep 17 00:00:00 2001
From: Bruce Momjian <br...@momjian.us>
Date: Tue, 2 Mar 2021 11:38:14 -0500
Subject: [PATCH] gist squash commit
---
src/backend/access/gist/README | 1 +
src/include/access/gist.h | 15 +++++++++++----
2 files changed, 12 insertions(+), 4 deletions(-)
diff --git a/src/backend/access/gist/README b/src/backend/access/gist/README
index 8ca0cf78cc..25cab0047b 100644
--- a/src/backend/access/gist/README
+++ b/src/backend/access/gist/README
@@ -10,6 +10,7 @@ GiST stands for Generalized Search Tree. It was introduced in the seminal paper
Jeffrey F. Naughton, Avi Pfeffer:
http://www.sai.msu.su/~megera/postgres/gist/papers/gist.ps
+ https://dsf.berkeley.edu/papers/sigmod97-gist.pdf
and implemented by J. Hellerstein and P. Aoki in an early version of
PostgreSQL (more details are available from The GiST Indexing Project
diff --git a/src/include/access/gist.h b/src/include/access/gist.h
index aa5f1763dd..4b06575d98 100644
--- a/src/include/access/gist.h
+++ b/src/include/access/gist.h
@@ -51,13 +51,20 @@
#define F_HAS_GARBAGE (1 << 4) /* some tuples on the page are dead,
* but not deleted yet */
-/* NSN - node sequence number, a special-purpose LSN */
+/*
+ * NSN (node sequence number) is a special-purpose LSN which is stored on each
+ * index page in GISTPageOpaqueData and updated only during page splits. By
+ * recording the parent's LSN in GISTSearchItem.parentlsn, it is possible to
+ * detect concurrent child page splits by checking if parentlsn < child's NSN,
+ * and handle them properly. The child page's LSN is insufficient for this
+ * purpose since it is updated for every page change.
+ */
typedef XLogRecPtr GistNSN;
/*
- * A bogus LSN / NSN value used during index build. Must be smaller than any
- * real or fake unlogged LSN, so that after an index build finishes, all the
- * splits are considered completed.
+ * A fake LSN / NSN value used during index builds. Must be smaller than any
+ * real or fake (unlogged) LSN generated after the index build completes so
+ * that all splits are considered complete.
*/
#define GistBuildLSN ((XLogRecPtr) 1)
--
2.20.1
