Add support for index-only scans in GiST.

This adds a new GiST opclass method, 'fetch', which is used to reconstruct
the original Datum from the value stored in the index. Also, the 'canreturn'
index AM interface function gains a new 'attno' argument. That makes it
possible to use index-only scans on a multi-column index where some of the
opclasses support index-only scans but some do not.

This patch adds support in the box and point opclasses. Other opclasses
can added later as follow-on patches (btree_gist would be particularly
interesting).

Anastasia Lubennikova, with additional fixes and modifications by me.

1 files changed, 63 insertions, 1 deletions

diff --git a/src/backend/access/gist/gistutil.c b/src/backend/access/gist/gistutil.c
index 824c40eb20..1680251a18 100644
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -294,8 +294,9 @@ gistDeCompressAtt(GISTSTATE *giststate, Relation r, IndexTuple tuple, Page p,
 
 	for (i = 0; i < r->rd_att->natts; i++)
 	{
-		Datum		datum = index_getattr(tuple, i + 1, giststate->tupdesc, &isnull[i]);
+		Datum		datum;
 
+		datum = index_getattr(tuple, i + 1, giststate->tupdesc, &isnull[i]);
 		gistdentryinit(giststate, i, &attdata[i],
 					   datum, r, p, o,
 					   FALSE, isnull[i]);
@@ -598,6 +599,67 @@ gistFormTuple(GISTSTATE *giststate, Relation r,
 	return res;
 }
 
+/*
+ * initialize a GiST entry with fetched value in key field
+ */
+static Datum
+gistFetchAtt(GISTSTATE *giststate, int nkey, Datum k, Relation r)
+{
+	GISTENTRY	fentry;
+	GISTENTRY  *fep;
+
+	gistentryinit(fentry, k, r, NULL, (OffsetNumber) 0, false);
+
+	fep = (GISTENTRY *)
+		DatumGetPointer(FunctionCall1Coll(&giststate->fetchFn[nkey],
+										  giststate->supportCollation[nkey],
+										  PointerGetDatum(&fentry)));
+
+	/* fetchFn set 'key', return it to the caller */
+	return fep->key;
+}
+
+/*
+ * Fetch all keys in tuple.
+ * returns new IndexTuple that contains GISTENTRY with fetched data
+ */
+IndexTuple
+gistFetchTuple(GISTSTATE *giststate, Relation r, IndexTuple tuple)
+{
+	MemoryContext oldcxt = MemoryContextSwitchTo(giststate->tempCxt);
+	Datum		fetchatt[INDEX_MAX_KEYS];
+	bool		isnull[INDEX_MAX_KEYS];
+	int			i;
+
+	for (i = 0; i < r->rd_att->natts; i++)
+	{
+		Datum		datum;
+
+		datum = index_getattr(tuple, i + 1, giststate->tupdesc, &isnull[i]);
+
+		if (giststate->fetchFn[i].fn_oid != InvalidOid)
+		{
+			if (!isnull[i])
+				fetchatt[i] = gistFetchAtt(giststate, i, datum, r);
+			else
+				fetchatt[i] = (Datum) 0;
+		}
+		else
+		{
+			/*
+			 * Index-only scans not supported for this column. Since the
+			 * planner chose an index-only scan anyway, it is not interested
+			 * in this column, and we can replace it with a NULL.
+			 */
+			isnull[i] = true;
+			fetchatt[i] = (Datum) 0;
+		}
+	}
+	MemoryContextSwitchTo(oldcxt);
+
+	return index_form_tuple(giststate->tupdesc, fetchatt, isnull);
+}
+
 float
 gistpenalty(GISTSTATE *giststate, int attno,
 			GISTENTRY *orig, bool isNullOrig,