Add equalimage B-Tree support functions.

Invent the concept of a B-Tree equalimage ("equality implies image
equality") support function, registered as support function 4.  This
indicates whether it is safe (or not safe) to apply optimizations that
assume that any two datums considered equal by an operator class's order
method must be interchangeable without any loss of semantic information.
This is static information about an operator class and a collation.

Register an equalimage routine for almost all of the existing B-Tree
opclasses.  We only need two trivial routines for all of the opclasses
that are included with the core distribution.  There is one routine for
opclasses that index non-collatable types (which returns 'true'
unconditionally), plus another routine for collatable types (which
returns 'true' when the collation is a deterministic collation).

This patch is infrastructure for an upcoming patch that adds B-Tree
deduplication.

Author: Peter Geoghegan, Anastasia Lubennikova
Discussion: https://postgr.es/m/CAH2-Wzn3Ee49Gmxb7V1VJ3-AC8fWn-Fr8pfWQebHe8rYRxt5OQ@mail.gmail.com

1 files changed, 26 insertions, 0 deletions

diff --git a/src/backend/utils/adt/datum.c b/src/backend/utils/adt/datum.c
index 4e819473520..34cdde1bb91 100644
--- a/src/backend/utils/adt/datum.c
+++ b/src/backend/utils/adt/datum.c
@@ -44,6 +44,7 @@
 
 #include "access/detoast.h"
 #include "fmgr.h"
+#include "utils/builtins.h"
 #include "utils/datum.h"
 #include "utils/expandeddatum.h"
 
@@ -324,6 +325,31 @@ datum_image_eq(Datum value1, Datum value2, bool typByVal, int typLen)
 }
 
 /*-------------------------------------------------------------------------
+ * btequalimage
+ *
+ * Generic "equalimage" support function.
+ *
+ * B-Tree operator classes whose equality function could safely be replaced by
+ * datum_image_eq() in all cases can use this as their "equalimage" support
+ * function.
+ *
+ * Currently, we unconditionally assume that any B-Tree operator class that
+ * registers btequalimage as its support function 4 must be able to safely use
+ * optimizations like deduplication (i.e. we return true unconditionally).  If
+ * it ever proved necessary to rescind support for an operator class, we could
+ * do that in a targeted fashion by doing something with the opcintype
+ * argument.
+ *-------------------------------------------------------------------------
+ */
+Datum
+btequalimage(PG_FUNCTION_ARGS)
+{
+	/* Oid		opcintype = PG_GETARG_OID(0); */
+
+	PG_RETURN_BOOL(true);
+}
+
+/*-------------------------------------------------------------------------
  * datumEstimateSpace
  *
  * Compute the amount of space that datumSerialize will require for a