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

1 files changed, 2 insertions, 2 deletions

diff --git a/src/backend/access/index/indexam.c b/src/backend/access/index/indexam.c
index 4e7eca73cce..cc5ac8b8571 100644
--- a/src/backend/access/index/indexam.c
+++ b/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);