Set pd_lower on internal GIN posting tree pages.

This allows squeezing out the unused space in full-page writes. And more
importantly, it can be a useful debugging aid.

In hindsight we should've done this back when GIN was added - we wouldn't
need the 'maxoff' field in the page opaque struct if we had used pd_lower
and pd_upper like on normal pages. But as long as there can be pages in the
index that have been binary-upgraded from pre-9.4 versions, we can't rely
on that, and have to continue using 'maxoff'.

Most of the code churn comes from renaming some macros, now that they're
used on internal pages, too.

This change is completely backwards-compatible, no effect on pg_upgrade.

1 files changed, 8 insertions, 1 deletions

diff --git a/src/backend/access/gin/ginvacuum.c b/src/backend/access/gin/ginvacuum.c
index 7dcc5cae476..72f734caf8d 100644
--- a/src/backend/access/gin/ginvacuum.c
+++ b/src/backend/access/gin/ginvacuum.c
@@ -301,6 +301,13 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 		data.leftBlkno = leftBlkno;
 		data.rightLink = GinPageGetOpaque(page)->rightlink;
 
+		/*
+		 * We can't pass buffer_std = TRUE, because we didn't set pd_lower
+		 * on pre-9.4 versions. The page might've been binary-upgraded from
+		 * an older version, and hence not have pd_lower set correctly.
+		 * Ditto for the left page, but removing the item from the parent
+		 * updated its pd_lower, so we know that's OK at this point.
+		 */
 		rdata[0].buffer = dBuffer;
 		rdata[0].buffer_std = FALSE;
 		rdata[0].data = NULL;
@@ -308,7 +315,7 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 		rdata[0].next = rdata + 1;
 
 		rdata[1].buffer = pBuffer;
-		rdata[1].buffer_std = FALSE;
+		rdata[1].buffer_std = TRUE;
 		rdata[1].data = NULL;
 		rdata[1].len = 0;
 		rdata[1].next = rdata + 2;