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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 f6e31092aa1..78171cfa3fd 100644
--- a/doc/src/sgml/gist.sgml
+++ b/doc/src/sgml/gist.sgml
@@ -378,6 +378,8 @@ my_decompress(PG_FUNCTION_ARGS)
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>
<para>