From dc73e25a5ed1980a49a8ff4655ff623fea9fd92f Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 25 Jul 2000 05:26:40 +0000
Subject: [PATCH] Add commentary about varying usage of scankeys in btree code.
---
src/backend/access/nbtree/README | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/src/backend/access/nbtree/README b/src/backend/access/nbtree/README
index e771798dc58..cff7ff0d655 100644
--- a/src/backend/access/nbtree/README
+++ b/src/backend/access/nbtree/README
@@ -1,4 +1,4 @@
-$Header: /cvsroot/pgsql/src/backend/access/nbtree/README,v 1.3 2000/07/21 22:14:09 tgl Exp $
+$Header: /cvsroot/pgsql/src/backend/access/nbtree/README,v 1.4 2000/07/25 05:26:40 tgl Exp $
This directory contains a correct implementation of Lehman and Yao's
high-concurrency B-tree management algorithm (P. Lehman and S. Yao,
@@ -119,6 +119,17 @@ In addition, the following things are handy to know:
exists only to support VACUUM and allow it to delete items while
it's scanning the index.
++ "ScanKey" data structures are used in two fundamentally different ways
+ in this code. Searches for the initial position for a scan, as well as
+ insertions, use scankeys in which the comparison function is a 3-way
+ comparator (<0, =0, >0 result). These scankeys are built within the
+ btree code (eg, by _bt_mkscankey()) and used by _bt_compare(). Once we
+ are positioned, sequential examination of tuples in a scan is done by
+ _bt_checkkeys() using scankeys in which the comparison functions return
+ booleans --- for example, int4lt might be used. These scankeys are the
+ ones originally passed in from outside the btree code. Same
+ representation, but different comparison functions!
+
Notes about data representation:
+ The right-sibling link required by L&Y is kept in the page "opaque
--
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