diff --git a/doc/src/sgml/gist.sgml b/doc/src/sgml/gist.sgml
index 5c0a0c48bab..7a5e664db68 100644
--- a/doc/src/sgml/gist.sgml
+++ b/doc/src/sgml/gist.sgml
@@ -273,13 +273,15 @@ CREATE INDEX ON my_table USING GIST (my_inet_column inet_ops);
    index will depend on the <function>penalty</function> and <function>picksplit</function>
    methods.
    Two optional methods are <function>compress</function> and
-   <function>decompress</function>, which allow an index to have internal tree data of
-   a different type than the data it indexes. The leaves are to be of the
-   indexed data type, while the other tree nodes can be of any C struct (but
+   <function>decompress</function>, which allow an index to store keys that
+   are of a different type than the data it indexes. The index entries can be
+   any valid Datums (but
    you still have to follow <productname>PostgreSQL</productname> data type rules here,
-   see about <literal>varlena</literal> for variable sized data). If the tree's
-   internal data type exists at the SQL level, the <literal>STORAGE</literal> option
-   of the <command>CREATE OPERATOR CLASS</command> command can be used.
+   see about <literal>varlena</literal> for variable sized data).
+   Furthermore, since <function>compress</function> and
+   <function>decompress</function> are told whether they are working on
+   Datums for leaf-level or internal pages, different representations
+   can be used for leaf keys than higher-level pages.
    The optional eighth method is <function>distance</function>, which is needed
    if the operator class wishes to support ordered scans (nearest-neighbor
    searches). The optional ninth method <function>fetch</function> is needed if the
diff --git a/src/backend/access/gist/README b/src/backend/access/gist/README
index 76e0e11f228..75445b07455 100644
--- a/src/backend/access/gist/README
+++ b/src/backend/access/gist/README
@@ -10,9 +10,13 @@ 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
+
+Concurrency support was described in "Concurrency and Recovery in Generalized
+Search Trees", 1997, Marcel Kornacker, C. Mohan, Joseph M. Hellerstein:
+
     https://dsf.berkeley.edu/papers/sigmod97-gist.pdf
 
-and implemented by J. Hellerstein and P. Aoki in an early version of
+GiST was implemented by J. Hellerstein and P. Aoki in an early version of
 PostgreSQL (more details are available from The GiST Indexing Project
 at Berkeley at http://gist.cs.berkeley.edu/). As a "university"
 project it had a limited number of features and was in rare use.
@@ -55,6 +59,9 @@ The original algorithms were modified in several ways:
   it is now a single-pass algorithm.
 * Since the papers were theoretical, some details were omitted and we
   had to find out ourself how to solve some specific problems.
+* The 1997 paper above (but not the 1995 one) states that leaf pages should
+  store the original key.  While that can be done in PostgreSQL, it is
+  also possible to use a compressed representation in leaf pages.
 
 Because of the above reasons, we have revised the interaction of GiST
 core and PostgreSQL WAL system. Moreover, we encountered (and solved)