Further optimize multi-key GIN searches.

If we're skipping past a certain TID, avoid decoding posting list segments
that only contain smaller TIDs.

Extracted from Alexander Korotkov's fast scan patch, heavily modified.

src/backend/access/gin/gindatapage.c

@@ -96,19 +96,49 @@ static void dataPlaceToPageLeafSplit(Buffer buf,
 						 XLogRecData **prdata, Page lpage, Page rpage);
 
 /*
- * Read all TIDs from leaf data page to single uncompressed array.
+ * Read TIDs from leaf data page to single uncompressed array. The TIDs are
+ * returned in ascending order.
+ *
+ * advancePast is a hint, indicating that the caller is only interested in
+ * TIDs > advancePast. To return all items, use ItemPointerSetMin.
+ *
+ * Note: This function can still return items smaller than advancePast that
+ * are in the same posting list as the items of interest, so the caller must
+ * still check all the returned items. But passing it allows this function to
+ * skip whole posting lists.
  */
 ItemPointer
-GinDataLeafPageGetItems(Page page, int *nitems)
+GinDataLeafPageGetItems(Page page, int *nitems, ItemPointerData advancePast)
 {
 	ItemPointer result;
 
 	if (GinPageIsCompressed(page))
 	{
-		GinPostingList *ptr = GinDataLeafPageGetPostingList(page);
+		GinPostingList *seg = GinDataLeafPageGetPostingList(page);
 		Size		len = GinDataLeafPageGetPostingListSize(page);
+		Pointer		endptr = ((Pointer) seg) + len;
+		GinPostingList *next;
 
-		result = ginPostingListDecodeAllSegments(ptr, len, nitems);
+		/* Skip to the segment containing advancePast+1 */
+		if (ItemPointerIsValid(&advancePast))
+		{
+			next = GinNextPostingListSegment(seg);
+			while ((Pointer) next < endptr &&
+				   ginCompareItemPointers(&next->first, &advancePast) <= 0)
+			{
+				seg = next;
+				next = GinNextPostingListSegment(seg);
+			}
+			len = endptr - (Pointer) seg;
+		}
+
+		if (len > 0)
+			result = ginPostingListDecodeAllSegments(seg, len, nitems);
+		else
+		{
+			result = NULL;
+			*nitems = 0;
+		}
 	}
 	else
 	{

src/backend/access/gin/ginget.c

@@ -400,6 +400,7 @@ restartScanEntry:
 			BlockNumber rootPostingTree = GinGetPostingTree(itup);
 			GinBtreeStack *stack;
 			Page		page;
+			ItemPointerData minItem;
 
 			/*
 			 * We should unlock entry page before touching posting tree to
@@ -426,7 +427,8 @@ restartScanEntry:
 			/*
 			 * Load the first page into memory.
 			 */
-			entry->list = GinDataLeafPageGetItems(page, &entry->nlist);
+			ItemPointerSetMin(&minItem);
+			entry->list = GinDataLeafPageGetItems(page, &entry->nlist, minItem);
 
 			entry->predictNumberResult = stack->predictNumber * entry->nlist;
 
@@ -558,7 +560,7 @@ entryLoadMoreItems(GinState *ginstate, GinScanEntry entry, ItemPointerData advan
 			continue;
 		}
 
-		entry->list = GinDataLeafPageGetItems(page, &entry->nlist);
+		entry->list = GinDataLeafPageGetItems(page, &entry->nlist, advancePast);
 
 		for (i = 0; i < entry->nlist; i++)
 		{

src/include/access/gin_private.h

@@ -692,7 +692,7 @@ extern ItemPointer ginReadTuple(GinState *ginstate, OffsetNumber attnum,
 			 IndexTuple itup, int *nitems);
 
 /* gindatapage.c */
-extern ItemPointer GinDataLeafPageGetItems(Page page, int *nitems);
+extern ItemPointer GinDataLeafPageGetItems(Page page, int *nitems, ItemPointerData advancePast);
 extern int GinDataLeafPageGetItemsToTbm(Page page, TIDBitmap *tbm);
 extern BlockNumber createPostingTree(Relation index,
 				  ItemPointerData *items, uint32 nitems,