Avoid copying index tuples when building an index.

The previous code, perhaps out of concern for avoid memory leaks, formed
the tuple in one memory context and then copied it to another memory
context.  However, this doesn't appear to be necessary, since
index_form_tuple and the functions it calls take precautions against
leaking memory.  In my testing, building the tuple directly inside the
sort context shaves several percent off the index build time.
Rearrange things so we do that.

Patch by me.  Review by Amit Kapila, Tom Lane, Andres Freund.

src/backend/utils/sort/tuplesort.c

@@ -1134,22 +1134,25 @@ tuplesort_putheaptuple(Tuplesortstate *state, HeapTuple tup)
 }
 
 /*
- * Accept one index tuple while collecting input data for sort.
- *
- * Note that the input tuple is always copied; the caller need not save it.
+ * Collect one index tuple while collecting input data for sort, building
+ * it from caller-supplied values.
  */
 void
-tuplesort_putindextuple(Tuplesortstate *state, IndexTuple tuple)
+tuplesort_putindextuplevalues(Tuplesortstate *state, Relation rel,
+							  ItemPointer self, Datum *values,
+                              bool *isnull)
 {
 	MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
 	SortTuple	stup;
 
-	/*
-	 * Copy the given tuple into memory we control, and decrease availMem.
-	 * Then call the common code.
-	 */
-	COPYTUP(state, &stup, (void *) tuple);
-
+	stup.tuple = index_form_tuple(RelationGetDescr(rel), values, isnull);
+	((IndexTuple) stup.tuple)->t_tid = *self;
+	USEMEM(state, GetMemoryChunkSpace(stup.tuple));
+	/* set up first-column key value */
+	stup.datum1 = index_getattr((IndexTuple) stup.tuple,
+								1,
+								RelationGetDescr(state->indexRel),
+								&stup.isnull1);
 	puttuple_common(state, &stup);
 
 	MemoryContextSwitchTo(oldcontext);