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, 27 insertions, 3 deletions

diff --git a/src/backend/commands/opclasscmds.c b/src/backend/commands/opclasscmds.c
index e2c6de457cf..743511bdf21 100644
--- a/src/backend/commands/opclasscmds.c
+++ b/src/backend/commands/opclasscmds.c
@@ -1143,9 +1143,10 @@ assignProcTypes(OpFamilyMember *member, Oid amoid, Oid typeoid)
 	/*
 	 * btree comparison procs must be 2-arg procs returning int4.  btree
 	 * sortsupport procs must take internal and return void.  btree in_range
-	 * procs must be 5-arg procs returning bool.  hash support proc 1 must be
-	 * a 1-arg proc returning int4, while proc 2 must be a 2-arg proc
-	 * returning int8.  Otherwise we don't know.
+	 * procs must be 5-arg procs returning bool.  btree equalimage procs must
+	 * take 1 arg and return bool.  hash support proc 1 must be a 1-arg proc
+	 * returning int4, while proc 2 must be a 2-arg proc returning int8.
+	 * Otherwise we don't know.
 	 */
 	if (amoid == BTREE_AM_OID)
 	{
@@ -1205,6 +1206,29 @@ assignProcTypes(OpFamilyMember *member, Oid amoid, Oid typeoid)
 			if (!OidIsValid(member->righttype))
 				member->righttype = procform->proargtypes.values[2];
 		}
+		else if (member->number == BTEQUALIMAGE_PROC)
+		{
+			if (procform->pronargs != 1)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						 errmsg("btree equal image functions must have one argument")));
+			if (procform->prorettype != BOOLOID)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						 errmsg("btree equal image functions must return boolean")));
+			/*
+			 * pg_amproc functions are indexed by (lefttype, righttype), but
+			 * an equalimage function can only be called at CREATE INDEX time.
+			 * The same opclass opcintype OID is always used for leftype and
+			 * righttype.  Providing a cross-type routine isn't sensible.
+			 * Reject cross-type ALTER OPERATOR FAMILY ...  ADD FUNCTION 4
+			 * statements here.
+			 */
+			if (member->lefttype != member->righttype)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						 errmsg("btree equal image functions must not be cross-type")));
+		}
 	}
 	else if (amoid == HASH_AM_OID)
 	{