Allow index AMs to return either HeapTuple or IndexTuple format during IOS.

Previously, only IndexTuple format was supported for the output data of
an index-only scan.  This is fine for btree, which is just returning a
verbatim index tuple anyway.  It's not so fine for SP-GiST, which can
return reconstructed data that's much larger than a page.

To fix, extend the index AM API so that index-only scan data can be
returned in either HeapTuple or IndexTuple format.  There's other ways
we could have done it, but this way avoids an API break for index AMs
that aren't concerned with the issue, and it costs little except a couple
more fields in IndexScanDescs.

I changed both GiST and SP-GiST to use the HeapTuple method.  I'm not
very clear on whether GiST can reconstruct data that's too large for an
IndexTuple, but that seems possible, and it's not much of a code change to
fix.

Per a complaint from Vik Fearing.  Reviewed by Jason Li.

Discussion: https://postgr.es/m/49527f79-530d-0bfe-3dad-d183596afa92@2ndquadrant.fr

doc/src/sgml/indexam.sgml

@@ -551,15 +551,19 @@ amgettuple (IndexScanDesc scan,
   <para>
    If the index supports <link linkend="indexes-index-only-scans">index-only
    scans</link> (i.e., <function>amcanreturn</function> returns TRUE for it),
-   then on success the AM must also check
-   <literal>scan-&gt;xs_want_itup</>, and if that is true it must return
-   the original indexed data for the index entry, in the form of an
+   then on success the AM must also check <literal>scan-&gt;xs_want_itup</>,
+   and if that is true it must return the originally indexed data for the
+   index entry.  The data can be returned in the form of an
    <structname>IndexTuple</> pointer stored at <literal>scan-&gt;xs_itup</>,
-   with tuple descriptor <literal>scan-&gt;xs_itupdesc</>.
-   (Management of the data referenced by the pointer is the access method's
+   with tuple descriptor <literal>scan-&gt;xs_itupdesc</>; or in the form of
+   a <structname>HeapTuple</> pointer stored at <literal>scan-&gt;xs_hitup</>,
+   with tuple descriptor <literal>scan-&gt;xs_hitupdesc</>.  (The latter
+   format should be used when reconstructing data that might possibly not fit
+   into an IndexTuple.)  In either case,
+   management of the data referenced by the pointer is the access method's
    responsibility.  The data must remain good at least until the next
    <function>amgettuple</>, <function>amrescan</>, or <function>amendscan</>
-   call for the scan.)
+   call for the scan.
   </para>
 
   <para>

src/backend/access/gist/gistget.c

@@ -441,12 +441,13 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem, double *myDistances,
 			so->pageData[so->nPageData].offnum = i;
 
 			/*
-			 * In an index-only scan, also fetch the data from the tuple.
+			 * In an index-only scan, also fetch the data from the tuple.  The
+			 * reconstructed tuples are stored in pageDataCxt.
 			 */
 			if (scan->xs_want_itup)
 			{
 				oldcxt = MemoryContextSwitchTo(so->pageDataCxt);
-				so->pageData[so->nPageData].ftup =
+				so->pageData[so->nPageData].recontup =
 					gistFetchTuple(giststate, r, it);
 				MemoryContextSwitchTo(oldcxt);
 			}
@@ -478,7 +479,7 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem, double *myDistances,
 				 * In an index-only scan, also fetch the data from the tuple.
 				 */
 				if (scan->xs_want_itup)
-					item->data.heap.ftup = gistFetchTuple(giststate, r, it);
+					item->data.heap.recontup = gistFetchTuple(giststate, r, it);
 			}
 			else
 			{
@@ -540,11 +541,11 @@ getNextNearest(IndexScanDesc scan)
 	bool		res = false;
 	int			i;
 
-	if (scan->xs_itup)
+	if (scan->xs_hitup)
 	{
 		/* free previously returned tuple */
-		pfree(scan->xs_itup);
-		scan->xs_itup = NULL;
+		pfree(scan->xs_hitup);
+		scan->xs_hitup = NULL;
 	}
 
 	do
@@ -601,7 +602,7 @@ getNextNearest(IndexScanDesc scan)
 
 			/* in an index-only scan, also return the reconstructed tuple. */
 			if (scan->xs_want_itup)
-				scan->xs_itup = item->data.heap.ftup;
+				scan->xs_hitup = item->data.heap.recontup;
 			res = true;
 		}
 		else
@@ -685,7 +686,7 @@ gistgettuple(IndexScanDesc scan, ScanDirection dir)
 
 				/* in an index-only scan, also return the reconstructed tuple */
 				if (scan->xs_want_itup)
-					scan->xs_itup = so->pageData[so->curPageData].ftup;
+					scan->xs_hitup = so->pageData[so->curPageData].recontup;
 
 				so->curPageData++;
 

src/backend/access/gist/gistscan.c

@@ -155,7 +155,7 @@ gistrescan(IndexScanDesc scan, ScanKey key, int nkeys,
 	 * tuple descriptor to represent the returned index tuples and create a
 	 * memory context to hold them during the scan.
 	 */
-	if (scan->xs_want_itup && !scan->xs_itupdesc)
+	if (scan->xs_want_itup && !scan->xs_hitupdesc)
 	{
 		int			natts;
 		int			attno;
@@ -174,8 +174,9 @@ gistrescan(IndexScanDesc scan, ScanKey key, int nkeys,
 							   scan->indexRelation->rd_opcintype[attno - 1],
 							   -1, 0);
 		}
-		scan->xs_itupdesc = so->giststate->fetchTupdesc;
+		scan->xs_hitupdesc = so->giststate->fetchTupdesc;
 
+		/* Also create a memory context that will hold the returned tuples */
 		so->pageDataCxt = AllocSetContextCreate(so->giststate->scanCxt,
 												"GiST page data context",
 												ALLOCSET_DEFAULT_SIZES);

src/backend/access/gist/gistutil.c

@@ -624,9 +624,9 @@ gistFetchAtt(GISTSTATE *giststate, int nkey, Datum k, Relation r)
 
 /*
  * Fetch all keys in tuple.
- * returns new IndexTuple that contains GISTENTRY with fetched data
+ * Returns a new HeapTuple containing the originally-indexed data.
  */
-IndexTuple
+HeapTuple
 gistFetchTuple(GISTSTATE *giststate, Relation r, IndexTuple tuple)
 {
 	MemoryContext oldcxt = MemoryContextSwitchTo(giststate->tempCxt);
@@ -660,7 +660,7 @@ gistFetchTuple(GISTSTATE *giststate, Relation r, IndexTuple tuple)
 	}
 	MemoryContextSwitchTo(oldcxt);
 
-	return index_form_tuple(giststate->fetchTupdesc, fetchatt, isnull);
+	return heap_form_tuple(giststate->fetchTupdesc, fetchatt, isnull);
 }
 
 float

src/backend/access/index/genam.c

@@ -119,6 +119,8 @@ RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
 
 	scan->xs_itup = NULL;
 	scan->xs_itupdesc = NULL;
+	scan->xs_hitup = NULL;
+	scan->xs_hitupdesc = NULL;
 
 	ItemPointerSetInvalid(&scan->xs_ctup.t_self);
 	scan->xs_ctup.t_data = NULL;

src/backend/access/index/indexam.c

@@ -535,8 +535,8 @@ index_getnext_tid(IndexScanDesc scan, ScanDirection direction)
 	/*
 	 * The AM's amgettuple proc finds the next index entry matching the scan
 	 * keys, and puts the TID into scan->xs_ctup.t_self.  It should also set
-	 * scan->xs_recheck and possibly scan->xs_itup, though we pay no attention
-	 * to those fields here.
+	 * scan->xs_recheck and possibly scan->xs_itup/scan->xs_hitup, though we
+	 * pay no attention to those fields here.
 	 */
 	found = scan->indexRelation->rd_amroutine->amgettuple(scan, direction);
 

src/backend/access/spgist/spgscan.c

@@ -92,11 +92,11 @@ resetSpGistScanOpaque(SpGistScanOpaque so)
 
 	if (so->want_itup)
 	{
-		/* Must pfree IndexTuples to avoid memory leak */
+		/* Must pfree reconstructed tuples to avoid memory leak */
 		int			i;
 
 		for (i = 0; i < so->nPtrs; i++)
-			pfree(so->indexTups[i]);
+			pfree(so->reconTups[i]);
 	}
 	so->iPtr = so->nPtrs = 0;
 }
@@ -195,8 +195,8 @@ spgbeginscan(Relation rel, int keysz, int orderbysz)
 										"SP-GiST search temporary context",
 										ALLOCSET_DEFAULT_SIZES);
 
-	/* Set up indexTupDesc and xs_itupdesc in case it's an index-only scan */
-	so->indexTupDesc = scan->xs_itupdesc = RelationGetDescr(rel);
+	/* Set up indexTupDesc and xs_hitupdesc in case it's an index-only scan */
+	so->indexTupDesc = scan->xs_hitupdesc = RelationGetDescr(rel);
 
 	scan->opaque = so;
 
@@ -591,12 +591,12 @@ storeGettuple(SpGistScanOpaque so, ItemPointer heapPtr,
 	if (so->want_itup)
 	{
 		/*
-		 * Reconstruct desired IndexTuple.  We have to copy the datum out of
-		 * the temp context anyway, so we may as well create the tuple here.
+		 * Reconstruct index data.  We have to copy the datum out of the temp
+		 * context anyway, so we may as well create the tuple here.
 		 */
-		so->indexTups[so->nPtrs] = index_form_tuple(so->indexTupDesc,
-													&leafValue,
-													&isnull);
+		so->reconTups[so->nPtrs] = heap_form_tuple(so->indexTupDesc,
+												   &leafValue,
+												   &isnull);
 	}
 	so->nPtrs++;
 }
@@ -619,18 +619,18 @@ spggettuple(IndexScanDesc scan, ScanDirection dir)
 			/* continuing to return tuples from a leaf page */
 			scan->xs_ctup.t_self = so->heapPtrs[so->iPtr];
 			scan->xs_recheck = so->recheck[so->iPtr];
-			scan->xs_itup = so->indexTups[so->iPtr];
+			scan->xs_hitup = so->reconTups[so->iPtr];
 			so->iPtr++;
 			return true;
 		}
 
 		if (so->want_itup)
 		{
-			/* Must pfree IndexTuples to avoid memory leak */
+			/* Must pfree reconstructed tuples to avoid memory leak */
 			int			i;
 
 			for (i = 0; i < so->nPtrs; i++)
-				pfree(so->indexTups[i]);
+				pfree(so->reconTups[i]);
 		}
 		so->iPtr = so->nPtrs = 0;
 

src/backend/executor/nodeIndexonlyscan.c

@@ -149,9 +149,26 @@ IndexOnlyNext(IndexOnlyScanState *node)
 		}
 
 		/*
-		 * Fill the scan tuple slot with data from the index.
+		 * Fill the scan tuple slot with data from the index.  This might be
+		 * provided in either HeapTuple or IndexTuple format.  Conceivably an
+		 * index AM might fill both fields, in which case we prefer the heap
+		 * format, since it's probably a bit cheaper to fill a slot from.
 		 */
-		StoreIndexTuple(slot, scandesc->xs_itup, scandesc->xs_itupdesc);
+		if (scandesc->xs_hitup)
+		{
+			/*
+			 * We don't take the trouble to verify that the provided tuple has
+			 * exactly the slot's format, but it seems worth doing a quick
+			 * check on the number of fields.
+			 */
+			Assert(slot->tts_tupleDescriptor->natts ==
+				   scandesc->xs_hitupdesc->natts);
+			ExecStoreTuple(scandesc->xs_hitup, slot, InvalidBuffer, false);
+		}
+		else if (scandesc->xs_itup)
+			StoreIndexTuple(slot, scandesc->xs_itup, scandesc->xs_itupdesc);
+		else
+			elog(ERROR, "no data returned for index-only scan");
 
 		/*
 		 * If the index was lossy, we have to recheck the index quals.

src/include/access/gist_private.h

@@ -119,7 +119,7 @@ typedef struct GISTSearchHeapItem
 	ItemPointerData heapPtr;
 	bool		recheck;		/* T if quals must be rechecked */
 	bool		recheckDistances;		/* T if distances must be rechecked */
-	IndexTuple	ftup;			/* data fetched back from the index, used in
+	HeapTuple	recontup;		/* data reconstructed from the index, used in
 								 * index-only scans */
 	OffsetNumber offnum;		/* track offset in page to mark tuple as
 								 * LP_DEAD */
@@ -477,7 +477,7 @@ extern void gistMakeUnionItVec(GISTSTATE *giststate, IndexTuple *itvec, int len,
 extern bool gistKeyIsEQ(GISTSTATE *giststate, int attno, Datum a, Datum b);
 extern void gistDeCompressAtt(GISTSTATE *giststate, Relation r, IndexTuple tuple, Page p,
 				  OffsetNumber o, GISTENTRY *attdata, bool *isnull);
-extern IndexTuple gistFetchTuple(GISTSTATE *giststate, Relation r,
+extern HeapTuple gistFetchTuple(GISTSTATE *giststate, Relation r,
 			   IndexTuple tuple);
 extern void gistMakeUnionKey(GISTSTATE *giststate, int attno,
 				 GISTENTRY *entry1, bool isnull1,

src/include/access/relscan.h

@@ -104,9 +104,16 @@ typedef struct IndexScanDescData
 	/* index access method's private state */
 	void	   *opaque;			/* access-method-specific info */
 
-	/* in an index-only scan, this is valid after a successful amgettuple */
+	/*
+	 * In an index-only scan, a successful amgettuple call must fill either
+	 * xs_itup (and xs_itupdesc) or xs_hitup (and xs_hitupdesc) to provide the
+	 * data returned by the scan.  It can fill both, in which case the heap
+	 * format will be used.
+	 */
 	IndexTuple	xs_itup;		/* index tuple returned by AM */
 	TupleDesc	xs_itupdesc;	/* rowtype descriptor of xs_itup */
+	HeapTuple	xs_hitup;		/* index data returned by AM, as HeapTuple */
+	TupleDesc	xs_hitupdesc;	/* rowtype descriptor of xs_hitup */
 
 	/* xs_ctup/xs_cbuf/xs_recheck are valid after a successful index_getnext */
 	HeapTupleData xs_ctup;		/* current heap tuple, if any */

src/include/access/spgist_private.h

@@ -159,7 +159,7 @@ typedef struct SpGistScanOpaqueData
 	int			iPtr;			/* index for scanning through same */
 	ItemPointerData heapPtrs[MaxIndexTuplesPerPage];	/* TIDs from cur page */
 	bool		recheck[MaxIndexTuplesPerPage]; /* their recheck flags */
-	IndexTuple	indexTups[MaxIndexTuplesPerPage];		/* reconstructed tuples */
+	HeapTuple	reconTups[MaxIndexTuplesPerPage];		/* reconstructed tuples */
 
 	/*
 	 * Note: using MaxIndexTuplesPerPage above is a bit hokey since