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-rw-r--r--doc/src/sgml/gist.sgml2
1 files changed, 2 insertions, 0 deletions
diff --git a/doc/src/sgml/gist.sgml b/doc/src/sgml/gist.sgml
index 587517da1d1..b0ac8e52b6a 100644
--- a/doc/src/sgml/gist.sgml
+++ b/doc/src/sgml/gist.sgml
@@ -147,6 +147,8 @@
Returns a value indicating the <quote>cost</quote> of inserting the new
entry into a particular branch of the tree. items will be inserted
down the path of least <function>penalty</function> in the tree.
+ Values returned by <function>penalty</function> should be non-negative.
+ If a negative value is returned, it will be treated as zero.
</para>
</listitem>
</varlistentry>