introduces a faster query to tuple GC

we've noticed that even after #1550
was introduced and made the nested GC SELECT query faster,
deletions are still causing spikes in postgres
and Aurora instances.

The proposed query seemed 1 order of magnitude less cost
that the current query on a test PG 15.6 with 5M dead
tuples and 5M alive tuples.

internal/datastore/postgres/gc.go

@@ -15,16 +15,7 @@ import (
 var (
 	_ common.GarbageCollector = (*pgDatastore)(nil)
 
-	relationTuplePKCols = []string{
-		colNamespace,
-		colObjectID,
-		colRelation,
-		colUsersetNamespace,
-		colUsersetObjectID,
-		colUsersetRelation,
-		colCreatedXid,
-		colDeletedXid,
-	}
+	gcPKCols = []string{"ctid"}
 
 	namespacePKCols = []string{colNamespace, colCreatedXid, colDeletedXid}
 
@@ -88,7 +79,7 @@ func (pgd *pgDatastore) DeleteBeforeTx(ctx context.Context, txID datastore.Revis
 	removed.Relationships, err = pgd.batchDelete(
 		ctx,
 		tableTuple,
-		relationTuplePKCols,
+		gcPKCols,
 		sq.Lt{colDeletedXid: minTxAlive},
 	)
 	if err != nil {
@@ -135,7 +126,6 @@ func (pgd *pgDatastore) batchDelete(
 	}
 
 	pkColsExpression := strings.Join(pkCols, ", ")
-
 	query := fmt.Sprintf(`WITH rows AS (%[1]s)
 		  DELETE FROM %[2]s
 		  WHERE (%[3]s) IN (SELECT %[3]s FROM rows);