Misc GIN refactoring.

Merge the isEnoughSpace and placeToPage functions in the b-tree interface
into one function that tries to put a tuple on page, and returns false if
it doesn't fit.

Move createPostingTree function to gindatapage.c, and change its contract
so that it can be passed more items than fit on the root page. It's in a
better position than the callers to know how many items fit.

Move ginMergeItemPointers out of gindatapage.c, into a separate file.

These changes make no difference now, but reduce the footprint of Alexander
Korotkov's upcoming patch to pack item pointers more tightly.

src/backend/access/gin/gindatapage.c

@@ -15,47 +15,9 @@
 #include "postgres.h"
 
 #include "access/gin_private.h"
+#include "miscadmin.h"
 #include "utils/rel.h"
 
-/*
- * Merge two ordered arrays of itempointers, eliminating any duplicates.
- * Returns the number of items in the result.
- * Caller is responsible that there is enough space at *dst.
- */
-uint32
-ginMergeItemPointers(ItemPointerData *dst,
-					 ItemPointerData *a, uint32 na,
-					 ItemPointerData *b, uint32 nb)
-{
-	ItemPointerData *dptr = dst;
-	ItemPointerData *aptr = a,
-			   *bptr = b;
-
-	while (aptr - a < na && bptr - b < nb)
-	{
-		int			cmp = ginCompareItemPointers(aptr, bptr);
-
-		if (cmp > 0)
-			*dptr++ = *bptr++;
-		else if (cmp == 0)
-		{
-			/* we want only one copy of the identical items */
-			*dptr++ = *bptr++;
-			aptr++;
-		}
-		else
-			*dptr++ = *aptr++;
-	}
-
-	while (aptr - a < na)
-		*dptr++ = *aptr++;
-
-	while (bptr - b < nb)
-		*dptr++ = *bptr++;
-
-	return dptr - dst;
-}
-
 /*
  * Checks, should we move to right link...
  * Compares inserting itemp pointer with right bound of current page
@@ -387,9 +349,12 @@ dataPrepareData(GinBtree btree, Page page, OffsetNumber off)
 /*
  * Places keys to page and fills WAL record. In case leaf page and
  * build mode puts all ItemPointers to page.
+ *
+ * If none of the keys fit, returns false without modifying the page.
  */
-static void
-dataPlaceToPage(GinBtree btree, Buffer buf, OffsetNumber off, XLogRecData **prdata)
+static bool
+dataPlaceToPage(GinBtree btree, Buffer buf, OffsetNumber off,
+				XLogRecData **prdata)
 {
 	Page		page = BufferGetPage(buf);
 	int			sizeofitem = GinSizeOfDataPageItem(page);
@@ -399,6 +364,10 @@ dataPlaceToPage(GinBtree btree, Buffer buf, OffsetNumber off, XLogRecData **prda
 	static XLogRecData rdata[3];
 	static ginxlogInsert data;
 
+	/* quick exit if it doesn't fit */
+	if (!dataIsEnoughSpace(btree, buf, off))
+		return false;
+
 	*prdata = rdata;
 	Assert(GinPageIsData(page));
 
@@ -464,6 +433,8 @@ dataPlaceToPage(GinBtree btree, Buffer buf, OffsetNumber off, XLogRecData **prda
 	}
 	else
 		GinDataPageAddPostingItem(page, &(btree->pitem), off);
+
+	return true;
 }
 
 /*
@@ -545,8 +516,8 @@ dataSplitPage(GinBtree btree, Buffer lbuf, Buffer rbuf, OffsetNumber off, XLogRe
 	}
 
 	/*
-	 * we suppose that during index creation table scaned from begin to end,
-	 * so ItemPointers are monotonically increased..
+	 * we assume that during index creation the table scanned from beginning
+	 * to end, so ItemPointers are in monotonically increasing order.
 	 */
 	if (btree->isBuild && GinPageRightMost(lpage))
 		separator = freeSpace / sizeofitem;
@@ -575,15 +546,6 @@ dataSplitPage(GinBtree btree, Buffer lbuf, Buffer rbuf, OffsetNumber off, XLogRe
 
 	GinPageGetOpaque(rpage)->maxoff = maxoff - separator;
 
-	PostingItemSetBlockNumber(&(btree->pitem), BufferGetBlockNumber(lbuf));
-	if (GinPageIsLeaf(lpage))
-		btree->pitem.key = *GinDataPageGetItemPointer(lpage,
-											GinPageGetOpaque(lpage)->maxoff);
-	else
-		btree->pitem.key = GinDataPageGetPostingItem(lpage,
-									  GinPageGetOpaque(lpage)->maxoff)->key;
-	btree->rightblkno = BufferGetBlockNumber(rbuf);
-
 	/* set up right bound for left page */
 	bound = GinDataPageGetRightBound(lpage);
 	*bound = btree->pitem.key;
@@ -613,6 +575,16 @@ dataSplitPage(GinBtree btree, Buffer lbuf, Buffer rbuf, OffsetNumber off, XLogRe
 	rdata[1].len = MAXALIGN(maxoff * sizeofitem);
 	rdata[1].next = NULL;
 
+	/* Prepare a downlink tuple for insertion to the parent */
+	PostingItemSetBlockNumber(&(btree->pitem), BufferGetBlockNumber(lbuf));
+	if (GinPageIsLeaf(lpage))
+		btree->pitem.key = *GinDataPageGetItemPointer(lpage,
+											GinPageGetOpaque(lpage)->maxoff);
+	else
+		btree->pitem.key = GinDataPageGetPostingItem(lpage,
+									  GinPageGetOpaque(lpage)->maxoff)->key;
+	btree->rightblkno = BufferGetBlockNumber(rbuf);
+
 	return lpage;
 }
 
@@ -638,6 +610,92 @@ ginDataFillRoot(GinBtree btree, Buffer root, Buffer lbuf, Buffer rbuf)
 	GinDataPageAddPostingItem(page, &ri, InvalidOffsetNumber);
 }
 
+/*
+ * Creates new posting tree containing the given TIDs. Returns the page
+ * number of the root of the new posting tree.
+ *
+ * items[] must be in sorted order with no duplicates.
+ */
+BlockNumber
+createPostingTree(Relation index, ItemPointerData *items, uint32 nitems,
+				  GinStatsData *buildStats)
+{
+	BlockNumber blkno;
+	Buffer		buffer;
+	Page		page;
+	int			itemsCount;
+
+	/* Calculate how many TIDs will fit on first page. */
+	itemsCount = Min(nitems, GinMaxLeafDataItems);
+
+	/*
+	 * Create the root page.
+	 */
+	buffer = GinNewBuffer(index);
+	page = BufferGetPage(buffer);
+	blkno = BufferGetBlockNumber(buffer);
+
+	START_CRIT_SECTION();
+
+	GinInitBuffer(buffer, GIN_DATA | GIN_LEAF);
+	memcpy(GinDataPageGetData(page), items, sizeof(ItemPointerData) * nitems);
+	GinPageGetOpaque(page)->maxoff = nitems;
+
+	MarkBufferDirty(buffer);
+
+	if (RelationNeedsWAL(index))
+	{
+		XLogRecPtr	recptr;
+		XLogRecData rdata[2];
+		ginxlogCreatePostingTree data;
+
+		data.node = index->rd_node;
+		data.blkno = blkno;
+		data.nitem = nitems;
+
+		rdata[0].buffer = InvalidBuffer;
+		rdata[0].data = (char *) &data;
+		rdata[0].len = sizeof(ginxlogCreatePostingTree);
+		rdata[0].next = &rdata[1];
+
+		rdata[1].buffer = InvalidBuffer;
+		rdata[1].data = (char *) items;
+		rdata[1].len = sizeof(ItemPointerData) * itemsCount;
+		rdata[1].next = NULL;
+
+		recptr = XLogInsert(RM_GIN_ID, XLOG_GIN_CREATE_PTREE, rdata);
+		PageSetLSN(page, recptr);
+	}
+
+	UnlockReleaseBuffer(buffer);
+
+	END_CRIT_SECTION();
+
+	/* During index build, count the newly-added data page */
+	if (buildStats)
+		buildStats->nDataPages++;
+
+	/*
+	 * Add any remaining TIDs to the newly-created posting tree.
+	 */
+	if (itemsCount < nitems)
+	{
+		GinPostingTreeScan *gdi;
+
+		gdi = ginPrepareScanPostingTree(index, blkno, FALSE);
+		gdi->btree.isBuild = (buildStats != NULL);
+
+		ginInsertItemPointers(gdi,
+							  items + itemsCount,
+							  nitems - itemsCount,
+							  buildStats);
+
+		pfree(gdi);
+	}
+
+	return blkno;
+}
+
 void
 ginPrepareDataScan(GinBtree btree, Relation index)
 {
@@ -650,7 +708,6 @@ ginPrepareDataScan(GinBtree btree, Relation index)
 	btree->findItem = dataLocateLeafItem;
 	btree->findChildPtr = dataFindChildPtr;
 	btree->getLeftMostPage = dataGetLeftMostPage;
-	btree->isEnoughSpace = dataIsEnoughSpace;
 	btree->placeToPage = dataPlaceToPage;
 	btree->splitPage = dataSplitPage;
 	btree->fillRoot = ginDataFillRoot;