Allow no-op GiST support functions to be omitted.

There are common use-cases in which the compress and/or decompress
functions can be omitted, with the result being that we make no
data transformation when storing or retrieving index values.
Previously, you had to provide a no-op function anyway, but this
patch allows such opclass support functions to be omitted.

Furthermore, if the compress function is omitted, then the core code
knows that the stored representation is the same as the original data.
This means we can allow index-only scans without requiring a fetch
function to be provided either.  Previously you had to provide a
no-op fetch function if you wanted IOS to work.

This reportedly provides a small performance benefit in such cases,
but IMO the real reason for doing it is just to reduce the amount of
useless boilerplate code that has to be written for GiST opclasses.

Andrey Borodin, reviewed by Dmitriy Sarafannikov

Discussion: https://postgr.es/m/CAJEAwVELVx9gYscpE=Be6iJxvdW5unZ_LkcAaVNSeOwvdwtD=A@mail.gmail.com

src/backend/access/gist/gist.c

@@ -1453,12 +1453,23 @@ initGISTstate(Relation index)
 		fmgr_info_copy(&(giststate->unionFn[i]),
 					   index_getprocinfo(index, i + 1, GIST_UNION_PROC),
 					   scanCxt);
-		fmgr_info_copy(&(giststate->compressFn[i]),
-					   index_getprocinfo(index, i + 1, GIST_COMPRESS_PROC),
-					   scanCxt);
-		fmgr_info_copy(&(giststate->decompressFn[i]),
-					   index_getprocinfo(index, i + 1, GIST_DECOMPRESS_PROC),
-					   scanCxt);
+
+		/* opclasses are not required to provide a Compress method */
+		if (OidIsValid(index_getprocid(index, i + 1, GIST_COMPRESS_PROC)))
+			fmgr_info_copy(&(giststate->compressFn[i]),
+						   index_getprocinfo(index, i + 1, GIST_COMPRESS_PROC),
+						   scanCxt);
+		else
+			giststate->compressFn[i].fn_oid = InvalidOid;
+
+		/* opclasses are not required to provide a Decompress method */
+		if (OidIsValid(index_getprocid(index, i + 1, GIST_DECOMPRESS_PROC)))
+			fmgr_info_copy(&(giststate->decompressFn[i]),
+						   index_getprocinfo(index, i + 1, GIST_DECOMPRESS_PROC),
+						   scanCxt);
+		else
+			giststate->decompressFn[i].fn_oid = InvalidOid;
+
 		fmgr_info_copy(&(giststate->penaltyFn[i]),
 					   index_getprocinfo(index, i + 1, GIST_PENALTY_PROC),
 					   scanCxt);
@@ -1468,6 +1479,7 @@ initGISTstate(Relation index)
 		fmgr_info_copy(&(giststate->equalFn[i]),
 					   index_getprocinfo(index, i + 1, GIST_EQUAL_PROC),
 					   scanCxt);
+
 		/* opclasses are not required to provide a Distance method */
 		if (OidIsValid(index_getprocid(index, i + 1, GIST_DISTANCE_PROC)))
 			fmgr_info_copy(&(giststate->distanceFn[i]),

src/backend/access/gist/gistget.c

@@ -801,11 +801,13 @@ gistgetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
  * Can we do index-only scans on the given index column?
  *
  * Opclasses that implement a fetch function support index-only scans.
+ * Opclasses without compression functions also support index-only scans.
  */
 bool
 gistcanreturn(Relation index, int attno)
 {
-	if (OidIsValid(index_getprocid(index, attno, GIST_FETCH_PROC)))
+	if (OidIsValid(index_getprocid(index, attno, GIST_FETCH_PROC)) ||
+		!OidIsValid(index_getprocid(index, attno, GIST_COMPRESS_PROC)))
 		return true;
 	else
 		return false;

src/backend/access/gist/gistutil.c

@@ -550,6 +550,11 @@ gistdentryinit(GISTSTATE *giststate, int nkey, GISTENTRY *e,
 		GISTENTRY  *dep;
 
 		gistentryinit(*e, k, r, pg, o, l);
+
+		/* there may not be a decompress function in opclass */
+		if (!OidIsValid(giststate->decompressFn[nkey].fn_oid))
+			return;
+
 		dep = (GISTENTRY *)
 			DatumGetPointer(FunctionCall1Coll(&giststate->decompressFn[nkey],
 											  giststate->supportCollation[nkey],
@@ -585,10 +590,14 @@ gistFormTuple(GISTSTATE *giststate, Relation r,
 
 			gistentryinit(centry, attdata[i], r, NULL, (OffsetNumber) 0,
 						  isleaf);
-			cep = (GISTENTRY *)
-				DatumGetPointer(FunctionCall1Coll(&giststate->compressFn[i],
-												  giststate->supportCollation[i],
-												  PointerGetDatum(&centry)));
+			/* there may not be a compress function in opclass */
+			if (OidIsValid(giststate->compressFn[i].fn_oid))
+				cep = (GISTENTRY *)
+					DatumGetPointer(FunctionCall1Coll(&giststate->compressFn[i],
+													  giststate->supportCollation[i],
+													  PointerGetDatum(&centry)));
+			else
+				cep = &centry;
 			compatt[i] = cep->key;
 		}
 	}
@@ -648,6 +657,17 @@ gistFetchTuple(GISTSTATE *giststate, Relation r, IndexTuple tuple)
 			else
 				fetchatt[i] = (Datum) 0;
 		}
+		else if (giststate->compressFn[i].fn_oid == InvalidOid)
+		{
+			/*
+			 * If opclass does not provide compress method that could change
+			 * original value, att is necessarily stored in original form.
+			 */
+			if (!isnull[i])
+				fetchatt[i] = datum;
+			else
+				fetchatt[i] = (Datum) 0;
+		}
 		else
 		{
 			/*
@@ -934,6 +954,20 @@ gistproperty(Oid index_oid, int attno,
 								 ObjectIdGetDatum(opcintype),
 								 ObjectIdGetDatum(opcintype),
 								 Int16GetDatum(procno));
+
+	/*
+	 * Special case: even without a fetch function, AMPROP_RETURNABLE is true
+	 * if the opclass has no compress function.
+	 */
+	if (prop == AMPROP_RETURNABLE && !*res)
+	{
+		*res = !SearchSysCacheExists4(AMPROCNUM,
+									  ObjectIdGetDatum(opfamily),
+									  ObjectIdGetDatum(opcintype),
+									  ObjectIdGetDatum(opcintype),
+									  Int16GetDatum(GIST_COMPRESS_PROC));
+	}
+
 	return true;
 }
 

src/backend/access/gist/gistvalidate.c

@@ -258,7 +258,8 @@ gistvalidate(Oid opclassoid)
 		if (opclassgroup &&
 			(opclassgroup->functionset & (((uint64) 1) << i)) != 0)
 			continue;			/* got it */
-		if (i == GIST_DISTANCE_PROC || i == GIST_FETCH_PROC)
+		if (i == GIST_DISTANCE_PROC || i == GIST_FETCH_PROC ||
+			i == GIST_COMPRESS_PROC || i == GIST_DECOMPRESS_PROC)
 			continue;			/* optional methods */
 		ereport(INFO,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),