GiST comment improvement

From defd16776c8016179a87c3d32ed359570c62727d Mon Sep 17 00:00:00 2001
From: Bruce Momjian <bruce@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