Refactor checks for deleted GiST pages.

The explicit check in gistScanPage() isn't currently really necessary, as
a deleted page is always empty, so the loop would fall through without
doing anything, anyway. But it's a marginal optimization, and it gives a
nice place to attach a comment to explain how it works.

Backpatch to v12, where GiST page deletion was introduced.

Reviewed-by: Andrey Borodin
Discussion: https://www.postgresql.org/message-id/835A15A5-F1B4-4446-A711-BF48357EB602%40yandex-team.ru

src/backend/access/gist/gist.c

@@ -709,14 +709,15 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace,
 			continue;
 		}
 
-		if (stack->blkno != GIST_ROOT_BLKNO &&
-			stack->parent->lsn < GistPageGetNSN(stack->page))
+		if ((stack->blkno != GIST_ROOT_BLKNO &&
+			 stack->parent->lsn < GistPageGetNSN(stack->page)) ||
+			GistPageIsDeleted(stack->page))
 		{
 			/*
-			 * Concurrent split detected. There's no guarantee that the
-			 * downlink for this page is consistent with the tuple we're
-			 * inserting anymore, so go back to parent and rechoose the best
-			 * child.
+			 * Concurrent split or page deletion detected. There's no
+			 * guarantee that the downlink for this page is consistent with
+			 * the tuple we're inserting anymore, so go back to parent and
+			 * rechoose the best child.
 			 */
 			UnlockReleaseBuffer(stack->buffer);
 			xlocked = false;
@@ -735,9 +736,6 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace,
 			GISTInsertStack *item;
 			OffsetNumber downlinkoffnum;
 
-			/* currently, internal pages are never deleted */
-			Assert(!GistPageIsDeleted(stack->page));
-
 			downlinkoffnum = gistchoose(state.r, stack->page, itup, giststate);
 			iid = PageGetItemId(stack->page, downlinkoffnum);
 			idxtuple = (IndexTuple) PageGetItem(stack->page, iid);
@@ -858,12 +856,13 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace,
 					 * leaf/inner is enough to recognize split for root
 					 */
 				}
-				else if (GistFollowRight(stack->page) ||
-						 stack->parent->lsn < GistPageGetNSN(stack->page))
+				else if ((GistFollowRight(stack->page) ||
+						  stack->parent->lsn < GistPageGetNSN(stack->page)) &&
+						 GistPageIsDeleted(stack->page))
 				{
 					/*
-					 * The page was split while we momentarily unlocked the
-					 * page. Go back to parent.
+					 * The page was split or deleted while we momentarily
+					 * unlocked the page. Go back to parent.
 					 */
 					UnlockReleaseBuffer(stack->buffer);
 					xlocked = false;
@@ -872,18 +871,6 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace,
 				}
 			}
 
-			/*
-			 * The page might have been deleted after we scanned the parent
-			 * and saw the downlink.
-			 */
-			if (GistPageIsDeleted(stack->page))
-			{
-				UnlockReleaseBuffer(stack->buffer);
-				xlocked = false;
-				state.stack = stack = stack->parent;
-				continue;
-			}
-
 			/* now state.stack->(page, buffer and blkno) points to leaf page */
 
 			gistinserttuple(&state, stack, giststate, itup,
@@ -947,6 +934,9 @@ gistFindPath(Relation r, BlockNumber child, OffsetNumber *downlinkoffnum)
 			break;
 		}
 
+		/* currently, internal pages are never deleted */
+		Assert(!GistPageIsDeleted(page));
+
 		top->lsn = BufferGetLSNAtomic(buffer);
 
 		/*

src/backend/access/gist/gistget.c

@@ -377,6 +377,20 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem, double *myDistances,
 		MemoryContextSwitchTo(oldcxt);
 	}
 
+	/*
+	 * Check if the page was deleted after we saw the downlink. There's
+	 * nothing of interest on a deleted page. Note that we must do this
+	 * after checking the NSN for concurrent splits! It's possible that
+	 * the page originally contained some tuples that are visible to us,
+	 * but was split so that all the visible tuples were moved to another
+	 * page, and then this page was deleted.
+	 */
+	if (GistPageIsDeleted(page))
+	{
+		UnlockReleaseBuffer(buffer);
+		return;
+	}
+
 	so->nPageData = so->curPageData = 0;
 	scan->xs_hitup = NULL;		/* might point into pageDataCxt */
 	if (so->pageDataCxt)