Buffering GiST index build algorithm.

When building a GiST index that doesn't fit in cache, buffers are attached
to some internal nodes in the index. This speeds up the build by avoiding
random I/O that would otherwise be needed to traverse all the way down the
tree to the find right leaf page for tuple.

Alexander Korotkov

1 files changed, 1068 insertions, 0 deletions

diff --git a/src/backend/access/gist/gistbuild.c b/src/backend/access/gist/gistbuild.c
new file mode 100644
index 00000000000..83192385106
--- /dev/null
+++ b/src/backend/access/gist/gistbuild.c
@@ -0,0 +1,1068 @@
+/*-------------------------------------------------------------------------
+ *
+ * gistbuild.c
+ *	  build algorithm for GiST indexes implementation.
+ *
+ *
+ * Portions Copyright (c) 1996-2011, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/access/gist/gistbuild.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/genam.h"
+#include "access/gist_private.h"
+#include "catalog/index.h"
+#include "miscadmin.h"
+#include "optimizer/cost.h"
+#include "storage/bufmgr.h"
+#include "storage/smgr.h"
+#include "utils/memutils.h"
+#include "utils/rel.h"
+
+/* Step of index tuples for check whether to switch to buffering build mode */
+#define BUFFERING_MODE_SWITCH_CHECK_STEP 256
+
+/*
+ * Number of tuples to process in the slow way before switching to buffering
+ * mode, when buffering is explicitly turned on. Also, the number of tuples
+ * to process between readjusting the buffer size parameter, while in
+ * buffering mode.
+ */
+#define BUFFERING_MODE_TUPLE_SIZE_STATS_TARGET 4096
+
+typedef enum
+{
+	GIST_BUFFERING_DISABLED,	/* in regular build mode and aren't going to
+								 * switch */
+	GIST_BUFFERING_AUTO,		/* in regular build mode, but will switch to
+								 * buffering build mode if the index grows too
+								 * big */
+	GIST_BUFFERING_STATS,		/* gathering statistics of index tuple size
+								 * before switching to the buffering build
+								 * mode */
+	GIST_BUFFERING_ACTIVE		/* in buffering build mode */
+}	GistBufferingMode;
+
+/* Working state for gistbuild and its callback */
+typedef struct
+{
+	Relation	indexrel;
+	GISTSTATE	giststate;
+	GISTBuildBuffers *gfbb;
+
+	int64		indtuples;		/* number of tuples indexed */
+	int64		indtuplesSize;	/* total size of all indexed tuples */
+
+	Size		freespace;		/* amount of free space to leave on pages */
+
+	GistBufferingMode bufferingMode;
+	MemoryContext tmpCtx;
+} GISTBuildState;
+
+static void gistInitBuffering(GISTBuildState *buildstate);
+static int	calculatePagesPerBuffer(GISTBuildState *buildstate, int levelStep);
+static void gistBuildCallback(Relation index,
+				  HeapTuple htup,
+				  Datum *values,
+				  bool *isnull,
+				  bool tupleIsAlive,
+				  void *state);
+static void gistBufferingBuildInsert(GISTBuildState *buildstate,
+						 IndexTuple itup);
+static bool gistProcessItup(GISTBuildState *buildstate, IndexTuple itup,
+				GISTBufferingInsertStack *startparent);
+static void gistbufferinginserttuples(GISTBuildState *buildstate,
+						  Buffer buffer,
+						  IndexTuple *itup, int ntup, OffsetNumber oldoffnum,
+						  GISTBufferingInsertStack *path);
+static void gistBufferingFindCorrectParent(GISTBuildState *buildstate,
+							   GISTBufferingInsertStack *child);
+static void gistProcessEmptyingQueue(GISTBuildState *buildstate);
+static void gistEmptyAllBuffers(GISTBuildState *buildstate);
+static void gistFreeUnreferencedPath(GISTBufferingInsertStack *path);
+static int	gistGetMaxLevel(Relation index);
+
+
+/*
+ * Main entry point to GiST index build. Initially calls insert over and over,
+ * but switches to more efficient buffering build algorithm after a certain
+ * number of tuples (unless buffering mode is disabled).
+ */
+Datum
+gistbuild(PG_FUNCTION_ARGS)
+{
+	Relation	heap = (Relation) PG_GETARG_POINTER(0);
+	Relation	index = (Relation) PG_GETARG_POINTER(1);
+	IndexInfo  *indexInfo = (IndexInfo *) PG_GETARG_POINTER(2);
+	IndexBuildResult *result;
+	double		reltuples;
+	GISTBuildState buildstate;
+	Buffer		buffer;
+	Page		page;
+	MemoryContext oldcxt = CurrentMemoryContext;
+	int			fillfactor;
+
+	buildstate.indexrel = index;
+	if (index->rd_options)
+	{
+		/* Get buffering mode from the options string */
+		GiSTOptions *options = (GiSTOptions *) index->rd_options;
+		char	   *bufferingMode = (char *) options + options->bufferingModeOffset;
+
+		if (strcmp(bufferingMode, "on") == 0)
+			buildstate.bufferingMode = GIST_BUFFERING_STATS;
+		else if (strcmp(bufferingMode, "off") == 0)
+			buildstate.bufferingMode = GIST_BUFFERING_DISABLED;
+		else
+			buildstate.bufferingMode = GIST_BUFFERING_AUTO;
+
+		fillfactor = options->fillfactor;
+	}
+	else
+	{
+		/*
+		 * By default, switch to buffering mode when the index grows too large
+		 * to fit in cache.
+		 */
+		buildstate.bufferingMode = GIST_BUFFERING_AUTO;
+		fillfactor = GIST_DEFAULT_FILLFACTOR;
+	}
+	/* Calculate target amount of free space to leave on pages */
+	buildstate.freespace = BLCKSZ * (100 - fillfactor) / 100;
+
+	/*
+	 * We expect to be called exactly once for any index relation. If that's
+	 * not the case, big trouble's what we have.
+	 */
+	if (RelationGetNumberOfBlocks(index) != 0)
+		elog(ERROR, "index \"%s\" already contains data",
+			 RelationGetRelationName(index));
+
+	/* no locking is needed */
+	initGISTstate(&buildstate.giststate, index);
+
+	/* initialize the root page */
+	buffer = gistNewBuffer(index);
+	Assert(BufferGetBlockNumber(buffer) == GIST_ROOT_BLKNO);
+	page = BufferGetPage(buffer);
+
+	START_CRIT_SECTION();
+
+	GISTInitBuffer(buffer, F_LEAF);
+
+	MarkBufferDirty(buffer);
+
+	if (RelationNeedsWAL(index))
+	{
+		XLogRecPtr	recptr;
+		XLogRecData rdata;
+
+		rdata.data = (char *) &(index->rd_node);
+		rdata.len = sizeof(RelFileNode);
+		rdata.buffer = InvalidBuffer;
+		rdata.next = NULL;
+
+		recptr = XLogInsert(RM_GIST_ID, XLOG_GIST_CREATE_INDEX, &rdata);
+		PageSetLSN(page, recptr);
+		PageSetTLI(page, ThisTimeLineID);
+	}
+	else
+		PageSetLSN(page, GetXLogRecPtrForTemp());
+
+	UnlockReleaseBuffer(buffer);
+
+	END_CRIT_SECTION();
+
+	/* build the index */
+	buildstate.indtuples = 0;
+	buildstate.indtuplesSize = 0;
+
+	/*
+	 * create a temporary memory context that is reset once for each tuple
+	 * processed.
+	 */
+	buildstate.tmpCtx = createTempGistContext();
+
+	/*
+	 * Do the heap scan.
+	 */
+	reltuples = IndexBuildHeapScan(heap, index, indexInfo, true,
+								   gistBuildCallback, (void *) &buildstate);
+
+	/*
+	 * If buffering was used, flush out all the tuples that are still in the
+	 * buffers.
+	 */
+	if (buildstate.bufferingMode == GIST_BUFFERING_ACTIVE)
+	{
+		elog(DEBUG1, "all tuples processed, emptying buffers");
+		gistEmptyAllBuffers(&buildstate);
+	}
+
+	/* okay, all heap tuples are indexed */
+	MemoryContextSwitchTo(oldcxt);
+	MemoryContextDelete(buildstate.tmpCtx);
+
+	freeGISTstate(&buildstate.giststate);
+
+	/*
+	 * Return statistics
+	 */
+	result = (IndexBuildResult *) palloc(sizeof(IndexBuildResult));
+
+	result->heap_tuples = reltuples;
+	result->index_tuples = (double) buildstate.indtuples;
+
+	PG_RETURN_POINTER(result);
+}
+
+/*
+ * Validator for "buffering" reloption on GiST indexes. Allows "on", "off"
+ * and "auto" values.
+ */
+void
+gistValidateBufferingOption(char *value)
+{
+	if (value == NULL ||
+		(strcmp(value, "on") != 0 &&
+		 strcmp(value, "off") != 0 &&
+		 strcmp(value, "auto") != 0))
+	{
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("invalid value for \"buffering\" option"),
+			   errdetail("Valid values are \"on\", \"off\" and \"auto\".")));
+	}
+}
+
+/*
+ * Attempt to switch to buffering mode.
+ *
+ * If there is not enough memory for buffering build, sets bufferingMode
+ * to GIST_BUFFERING_DISABLED, so that we don't bother to try the switch
+ * anymore. Otherwise initializes the build buffers, and sets bufferingMode to
+ * GIST_BUFFERING_ACTIVE.
+ */
+static void
+gistInitBuffering(GISTBuildState *buildstate)
+{
+	Relation	index = buildstate->indexrel;
+	int			pagesPerBuffer;
+	Size		pageFreeSpace;
+	Size		itupAvgSize,
+				itupMinSize;
+	double		avgIndexTuplesPerPage,
+				maxIndexTuplesPerPage;
+	int			i;
+	int			levelStep;
+
+	/* Calc space of index page which is available for index tuples */
+	pageFreeSpace = BLCKSZ - SizeOfPageHeaderData - sizeof(GISTPageOpaqueData)
+		- sizeof(ItemIdData)
+		- buildstate->freespace;
+
+	/*
+	 * Calculate average size of already inserted index tuples using gathered
+	 * statistics.
+	 */
+	itupAvgSize = (double) buildstate->indtuplesSize /
+		(double) buildstate->indtuples;
+
+	/*
+	 * Calculate minimal possible size of index tuple by index metadata.
+	 * Minimal possible size of varlena is VARHDRSZ.
+	 *
+	 * XXX: that's not actually true, as a short varlen can be just 2 bytes.
+	 * And we should take padding into account here.
+	 */
+	itupMinSize = (Size) MAXALIGN(sizeof(IndexTupleData));
+	for (i = 0; i < index->rd_att->natts; i++)
+	{
+		if (index->rd_att->attrs[i]->attlen < 0)
+			itupMinSize += VARHDRSZ;
+		else
+			itupMinSize += index->rd_att->attrs[i]->attlen;
+	}
+
+	/* Calculate average and maximal number of index tuples which fit to page */
+	avgIndexTuplesPerPage = pageFreeSpace / itupAvgSize;
+	maxIndexTuplesPerPage = pageFreeSpace / itupMinSize;
+
+	/*
+	 * We need to calculate two parameters for the buffering algorithm:
+	 * levelStep and pagesPerBuffer.
+	 *
+	 * levelStep determines the size of subtree that we operate on, while
+	 * emptying a buffer. A higher value is better, as you need fewer buffer
+	 * emptying steps to build the index. However, if you set it too high, the
+	 * subtree doesn't fit in cache anymore, and you quickly lose the benefit
+	 * of the buffers.
+	 *
+	 * In Arge et al's paper, levelStep is chosen as logB(M/4B), where B is
+	 * the number of tuples on page (ie. fanout), and M is the amount of
+	 * internal memory available. Curiously, they doesn't explain *why* that
+	 * setting is optimal. We calculate it by taking the highest levelStep so
+	 * that a subtree still fits in cache. For a small B, our way of
+	 * calculating levelStep is very close to Arge et al's formula. For a
+	 * large B, our formula gives a value that is 2x higher.
+	 *
+	 * The average size of a subtree of depth n can be calculated as a
+	 * geometric series:
+	 *
+	 * B^0 + B^1 + B^2 + ... + B^n = (1 - B^(n + 1)) / (1 - B)
+	 *
+	 * where B is the average number of index tuples on page. The subtree is
+	 * cached in the shared buffer cache and the OS cache, so we choose
+	 * levelStep so that the subtree size is comfortably smaller than
+	 * effective_cache_size, with a safety factor of 4.
+	 *
+	 * The estimate on the average number of index tuples on page is based on
+	 * average tuple sizes observed before switching to buffered build, so the
+	 * real subtree size can be somewhat larger. Also, it would selfish to
+	 * gobble the whole cache for our index build. The safety factor of 4
+	 * should account for those effects.
+	 *
+	 * The other limiting factor for setting levelStep is that while
+	 * processing a subtree, we need to hold one page for each buffer at the
+	 * next lower buffered level. The max. number of buffers needed for that
+	 * is maxIndexTuplesPerPage^levelStep. This is very conservative, but
+	 * hopefully maintenance_work_mem is set high enough that you're
+	 * constrained by effective_cache_size rather than maintenance_work_mem.
+	 *
+	 * XXX: the buffer hash table consumes a fair amount of memory too per
+	 * buffer, but that is not currently taken into account. That scales on
+	 * the total number of buffers used, ie. the index size and on levelStep.
+	 * Note that a higher levelStep *reduces* the amount of memory needed for
+	 * the hash table.
+	 */
+	levelStep = 1;
+	while (
+	/* subtree must fit in cache (with safety factor of 4) */
+		   (1 - pow(avgIndexTuplesPerPage, (double) (levelStep + 1))) / (1 - avgIndexTuplesPerPage) < effective_cache_size / 4
+		   &&
+	/* each node in the lowest level of a subtree has one page in memory */
+		   (pow(maxIndexTuplesPerPage, (double) levelStep) < (maintenance_work_mem * 1024) / BLCKSZ)
+		)
+	{
+		levelStep++;
+	}
+
+	/*
+	 * We just reached an unacceptable value of levelStep in previous loop.
+	 * So, decrease levelStep to get last acceptable value.
+	 */
+	levelStep--;
+
+	/*
+	 * If there's not enough cache or maintenance_work_mem, fall back to plain
+	 * inserts.
+	 */
+	if (levelStep <= 0)
+	{
+		elog(DEBUG1, "failed to switch to buffered GiST build");
+		buildstate->bufferingMode = GIST_BUFFERING_DISABLED;
+		return;
+	}
+
+	/*
+	 * The second parameter to set is pagesPerBuffer, which determines the
+	 * size of each buffer. We adjust pagesPerBuffer also during the build,
+	 * which is why this calculation is in a separate function.
+	 */
+	pagesPerBuffer = calculatePagesPerBuffer(buildstate, levelStep);
+
+	/* Initialize GISTBuildBuffers with these parameters */
+	buildstate->gfbb = gistInitBuildBuffers(pagesPerBuffer, levelStep,
+											gistGetMaxLevel(index));
+
+	buildstate->bufferingMode = GIST_BUFFERING_ACTIVE;
+
+	elog(DEBUG1, "switched to buffered GiST build; level step = %d, pagesPerBuffer = %d",
+		 levelStep, pagesPerBuffer);
+}
+
+/*
+ * Calculate pagesPerBuffer parameter for the buffering algorithm.
+ *
+ * Buffer size is chosen so that assuming that tuples are distributed
+ * randomly, emptying half a buffer fills on average one page in every buffer
+ * at the next lower level.
+ */
+static int
+calculatePagesPerBuffer(GISTBuildState *buildstate, int levelStep)
+{
+	double		pagesPerBuffer;
+	double		avgIndexTuplesPerPage;
+	double		itupAvgSize;
+	Size		pageFreeSpace;
+
+	/* Calc space of index page which is available for index tuples */
+	pageFreeSpace = BLCKSZ - SizeOfPageHeaderData - sizeof(GISTPageOpaqueData)
+		- sizeof(ItemIdData)
+		- buildstate->freespace;
+
+	/*
+	 * Calculate average size of already inserted index tuples using gathered
+	 * statistics.
+	 */
+	itupAvgSize = (double) buildstate->indtuplesSize /
+		(double) buildstate->indtuples;
+
+	avgIndexTuplesPerPage = pageFreeSpace / itupAvgSize;
+
+	/*
+	 * Recalculate required size of buffers.
+	 */
+	pagesPerBuffer = 2 * pow(avgIndexTuplesPerPage, levelStep);
+
+	return round(pagesPerBuffer);
+}
+
+/*
+ * Per-tuple callback from IndexBuildHeapScan.
+ */
+static void
+gistBuildCallback(Relation index,
+				  HeapTuple htup,
+				  Datum *values,
+				  bool *isnull,
+				  bool tupleIsAlive,
+				  void *state)
+{
+	GISTBuildState *buildstate = (GISTBuildState *) state;
+	IndexTuple	itup;
+	MemoryContext oldCtx;
+
+	oldCtx = MemoryContextSwitchTo(buildstate->tmpCtx);
+
+	/* form an index tuple and point it at the heap tuple */
+	itup = gistFormTuple(&buildstate->giststate, index, values, isnull, true);
+	itup->t_tid = htup->t_self;
+
+	if (buildstate->bufferingMode == GIST_BUFFERING_ACTIVE)
+	{
+		/* We have buffers, so use them. */
+		gistBufferingBuildInsert(buildstate, itup);
+	}
+	else
+	{
+		/*
+		 * There's no buffers (yet). Since we already have the index relation
+		 * locked, we call gistdoinsert directly.
+		 */
+		gistdoinsert(index, itup, buildstate->freespace,
+					 &buildstate->giststate);
+	}
+
+	/* Update tuple count and total size. */
+	buildstate->indtuples += 1;
+	buildstate->indtuplesSize += IndexTupleSize(itup);
+
+	MemoryContextSwitchTo(oldCtx);
+	MemoryContextReset(buildstate->tmpCtx);
+
+	if (buildstate->bufferingMode == GIST_BUFFERING_ACTIVE &&
+		buildstate->indtuples % BUFFERING_MODE_TUPLE_SIZE_STATS_TARGET == 0)
+	{
+		/* Adjust the target buffer size now */
+		buildstate->gfbb->pagesPerBuffer =
+			calculatePagesPerBuffer(buildstate, buildstate->gfbb->levelStep);
+	}
+
+	/*
+	 * In 'auto' mode, check if the index has grown too large to fit in cache,
+	 * and switch to buffering mode if it has.
+	 *
+	 * To avoid excessive calls to smgrnblocks(), only check this every
+	 * BUFFERING_MODE_SWITCH_CHECK_STEP index tuples
+	 */
+	if ((buildstate->bufferingMode == GIST_BUFFERING_AUTO &&
+		 buildstate->indtuples % BUFFERING_MODE_SWITCH_CHECK_STEP == 0 &&
+		 effective_cache_size < smgrnblocks(index->rd_smgr, MAIN_FORKNUM)) ||
+		(buildstate->bufferingMode == GIST_BUFFERING_STATS &&
+		 buildstate->indtuples >= BUFFERING_MODE_TUPLE_SIZE_STATS_TARGET))
+	{
+		/*
+		 * Index doesn't fit in effective cache anymore. Try to switch to
+		 * buffering build mode.
+		 */
+		gistInitBuffering(buildstate);
+	}
+}
+
+/*
+ * Insert function for buffering index build.
+ */
+static void
+gistBufferingBuildInsert(GISTBuildState *buildstate, IndexTuple itup)
+{
+	/* Insert the tuple to buffers. */
+	gistProcessItup(buildstate, itup, NULL);
+
+	/* If we filled up (half of a) buffer, process buffer emptying. */
+	gistProcessEmptyingQueue(buildstate);
+}
+
+/*
+ * Process an index tuple. Runs the tuple down the tree until we reach a leaf
+ * page or node buffer, and inserts the tuple there. Returns true if we have
+ * to stop buffer emptying process (because one of child buffers can't take
+ * index tuples anymore).
+ */
+static bool
+gistProcessItup(GISTBuildState *buildstate, IndexTuple itup,
+				GISTBufferingInsertStack *startparent)
+{
+	GISTSTATE  *giststate = &buildstate->giststate;
+	GISTBuildBuffers *gfbb = buildstate->gfbb;
+	Relation	indexrel = buildstate->indexrel;
+	GISTBufferingInsertStack *path;
+	BlockNumber childblkno;
+	Buffer		buffer;
+	bool		result = false;
+
+	/*
+	 * NULL passed in startparent means that we start index tuple processing
+	 * from the root.
+	 */
+	if (!startparent)
+		path = gfbb->rootitem;
+	else
+		path = startparent;
+
+	/*
+	 * Loop until we reach a leaf page (level == 0) or a level with buffers
+	 * (not including the level we start at, because we would otherwise make
+	 * no progress).
+	 */
+	for (;;)
+	{
+		ItemId		iid;
+		IndexTuple	idxtuple,
+					newtup;
+		Page		page;
+		OffsetNumber childoffnum;
+		GISTBufferingInsertStack *parent;
+
+		/* Have we reached a level with buffers? */
+		if (LEVEL_HAS_BUFFERS(path->level, gfbb) && path != startparent)
+			break;
+
+		/* Have we reached a leaf page? */
+		if (path->level == 0)
+			break;
+
+		/*
+		 * Nope. Descend down to the next level then. Choose a child to
+		 * descend down to.
+		 */
+		buffer = ReadBuffer(indexrel, path->blkno);
+		LockBuffer(buffer, GIST_EXCLUSIVE);
+
+		page = (Page) BufferGetPage(buffer);
+		childoffnum = gistchoose(indexrel, page, itup, giststate);
+		iid = PageGetItemId(page, childoffnum);
+		idxtuple = (IndexTuple) PageGetItem(page, iid);
+		childblkno = ItemPointerGetBlockNumber(&(idxtuple->t_tid));
+
+		/*
+		 * Check that the key representing the target child node is consistent
+		 * with the key we're inserting. Update it if it's not.
+		 */
+		newtup = gistgetadjusted(indexrel, idxtuple, itup, giststate);
+		if (newtup)
+			gistbufferinginserttuples(buildstate, buffer, &newtup, 1,
+									  childoffnum, path);
+		UnlockReleaseBuffer(buffer);
+
+		/* Create new path item representing current page */
+		parent = path;
+		path = (GISTBufferingInsertStack *) MemoryContextAlloc(gfbb->context,
+										   sizeof(GISTBufferingInsertStack));
+		path->parent = parent;
+		path->level = parent->level - 1;
+		path->blkno = childblkno;
+		path->downlinkoffnum = childoffnum;
+		path->refCount = 0;		/* it's unreferenced for now */
+
+		/* Adjust reference count of parent */
+		if (parent)
+			parent->refCount++;
+	}
+
+	if (LEVEL_HAS_BUFFERS(path->level, gfbb))
+	{
+		/*
+		 * We've reached level with buffers. Place the index tuple to the
+		 * buffer, and add the buffer to the emptying queue if it overflows.
+		 */
+		GISTNodeBuffer *childNodeBuffer;
+
+		/* Find the buffer or create a new one */
+		childNodeBuffer = gistGetNodeBuffer(gfbb, giststate, path->blkno,
+										 path->downlinkoffnum, path->parent);
+
+		/* Add index tuple to it */
+		gistPushItupToNodeBuffer(gfbb, childNodeBuffer, itup);
+
+		if (BUFFER_OVERFLOWED(childNodeBuffer, gfbb))
+			result = true;
+	}
+	else
+	{
+		/*
+		 * We've reached a leaf page. Place the tuple here.
+		 */
+		buffer = ReadBuffer(indexrel, path->blkno);
+		LockBuffer(buffer, GIST_EXCLUSIVE);
+		gistbufferinginserttuples(buildstate, buffer, &itup, 1,
+								  InvalidOffsetNumber, path);
+		UnlockReleaseBuffer(buffer);
+	}
+
+	/*
+	 * Free unreferenced path items, if any. Path item may be referenced by
+	 * node buffer.
+	 */
+	gistFreeUnreferencedPath(path);
+
+	return result;
+}
+
+/*
+ * Insert tuples to a given page.
+ *
+ * This is analogous with gistinserttuples() in the regular insertion code.
+ */
+static void
+gistbufferinginserttuples(GISTBuildState *buildstate, Buffer buffer,
+						  IndexTuple *itup, int ntup, OffsetNumber oldoffnum,
+						  GISTBufferingInsertStack *path)
+{
+	GISTBuildBuffers *gfbb = buildstate->gfbb;
+	List	   *splitinfo;
+	bool		is_split;
+
+	is_split = gistplacetopage(buildstate->indexrel,
+							   buildstate->freespace,
+							   &buildstate->giststate,
+							   buffer,
+							   itup, ntup, oldoffnum,
+							   InvalidBuffer,
+							   &splitinfo,
+							   false);
+
+	/*
+	 * If this is a root split, update the root path item kept in memory. This
+	 * ensures that all path stacks are always complete, including all parent
+	 * nodes up to the root. That simplifies the algorithm to re-find correct
+	 * parent.
+	 */
+	if (is_split && BufferGetBlockNumber(buffer) == GIST_ROOT_BLKNO)
+	{
+		GISTBufferingInsertStack *oldroot = gfbb->rootitem;
+		Page		page = BufferGetPage(buffer);
+		ItemId		iid;
+		IndexTuple	idxtuple;
+		BlockNumber leftmostchild;
+
+		gfbb->rootitem = (GISTBufferingInsertStack *) MemoryContextAlloc(
+							gfbb->context, sizeof(GISTBufferingInsertStack));
+		gfbb->rootitem->parent = NULL;
+		gfbb->rootitem->blkno = GIST_ROOT_BLKNO;
+		gfbb->rootitem->downlinkoffnum = InvalidOffsetNumber;
+		gfbb->rootitem->level = oldroot->level + 1;
+		gfbb->rootitem->refCount = 1;
+
+		/*
+		 * All the downlinks on the old root page are now on one of the child
+		 * pages. Change the block number of the old root entry in the stack
+		 * to point to the leftmost child. The other child pages will be
+		 * accessible from there by walking right.
+		 */
+		iid = PageGetItemId(page, FirstOffsetNumber);
+		idxtuple = (IndexTuple) PageGetItem(page, iid);
+		leftmostchild = ItemPointerGetBlockNumber(&(idxtuple->t_tid));
+
+		oldroot->parent = gfbb->rootitem;
+		oldroot->blkno = leftmostchild;
+		oldroot->downlinkoffnum = InvalidOffsetNumber;
+	}
+
+	if (splitinfo)
+	{
+		/*
+		 * Insert the downlinks to the parent. This is analogous with
+		 * gistfinishsplit() in the regular insertion code, but the locking is
+		 * simpler, and we have to maintain the buffers.
+		 */
+		IndexTuple *downlinks;
+		int			ndownlinks,
+					i;
+		Buffer		parentBuffer;
+		ListCell   *lc;
+
+		/* Parent may have changed since we memorized this path. */
+		gistBufferingFindCorrectParent(buildstate, path);
+
+		/*
+		 * If there's a buffer associated with this page, that needs to be
+		 * split too. gistRelocateBuildBuffersOnSplit() will also adjust the
+		 * downlinks in 'splitinfo', to make sure they're consistent not only
+		 * with the tuples already on the pages, but also the tuples in the
+		 * buffers that will eventually be inserted to them.
+		 */
+		gistRelocateBuildBuffersOnSplit(gfbb,
+										&buildstate->giststate,
+										buildstate->indexrel,
+										path, buffer, splitinfo);
+
+		/* Create an array of all the downlink tuples */
+		ndownlinks = list_length(splitinfo);
+		downlinks = (IndexTuple *) palloc(sizeof(IndexTuple) * ndownlinks);
+		i = 0;
+		foreach(lc, splitinfo)
+		{
+			GISTPageSplitInfo *splitinfo = lfirst(lc);
+
+			/*
+			 * Since there's no concurrent access, we can release the lower
+			 * level buffers immediately. Don't release the buffer for the
+			 * original page, though, because the caller will release that.
+			 */
+			if (splitinfo->buf != buffer)
+				UnlockReleaseBuffer(splitinfo->buf);
+			downlinks[i++] = splitinfo->downlink;
+		}
+
+		/* Insert them into parent. */
+		parentBuffer = ReadBuffer(buildstate->indexrel, path->parent->blkno);
+		LockBuffer(parentBuffer, GIST_EXCLUSIVE);
+		gistbufferinginserttuples(buildstate, parentBuffer,
+								  downlinks, ndownlinks,
+								  path->downlinkoffnum, path->parent);
+		UnlockReleaseBuffer(parentBuffer);
+
+		list_free_deep(splitinfo);		/* we don't need this anymore */
+	}
+}
+
+/*
+ * Find correct parent by following rightlinks in buffering index build. This
+ * method of parent searching is possible because no concurrent activity is
+ * possible while index builds.
+ */
+static void
+gistBufferingFindCorrectParent(GISTBuildState *buildstate,
+							   GISTBufferingInsertStack *child)
+{
+	GISTBuildBuffers *gfbb = buildstate->gfbb;
+	Relation	indexrel = buildstate->indexrel;
+	GISTBufferingInsertStack *parent = child->parent;
+	OffsetNumber i,
+				maxoff;
+	ItemId		iid;
+	IndexTuple	idxtuple;
+	Buffer		buffer;
+	Page		page;
+	bool		copied = false;
+
+	buffer = ReadBuffer(indexrel, parent->blkno);
+	page = BufferGetPage(buffer);
+	LockBuffer(buffer, GIST_EXCLUSIVE);
+	gistcheckpage(indexrel, buffer);
+
+	/* Check if it was not moved */
+	if (child->downlinkoffnum != InvalidOffsetNumber &&
+		child->downlinkoffnum <= PageGetMaxOffsetNumber(page))
+	{
+		iid = PageGetItemId(page, child->downlinkoffnum);
+		idxtuple = (IndexTuple) PageGetItem(page, iid);
+		if (ItemPointerGetBlockNumber(&(idxtuple->t_tid)) == child->blkno)
+		{
+			/* Still there */
+			UnlockReleaseBuffer(buffer);
+			return;
+		}
+	}
+
+	/* parent has changed, look child in right links until found */
+	while (true)
+	{
+		/* Search for relevant downlink in the current page */
+		maxoff = PageGetMaxOffsetNumber(page);
+		for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
+		{
+			iid = PageGetItemId(page, i);
+			idxtuple = (IndexTuple) PageGetItem(page, iid);
+			if (ItemPointerGetBlockNumber(&(idxtuple->t_tid)) == child->blkno)
+			{
+				/* yes!!, found */
+				child->downlinkoffnum = i;
+				UnlockReleaseBuffer(buffer);
+				return;
+			}
+		}
+
+		/*
+		 * We should copy parent path item because some other path items can
+		 * refer to it.
+		 */
+		if (!copied)
+		{
+			parent = (GISTBufferingInsertStack *) MemoryContextAlloc(gfbb->context,
+										   sizeof(GISTBufferingInsertStack));
+			memcpy(parent, child->parent, sizeof(GISTBufferingInsertStack));
+			if (parent->parent)
+				parent->parent->refCount++;
+			gistDecreasePathRefcount(child->parent);
+			child->parent = parent;
+			parent->refCount = 1;
+			copied = true;
+		}
+
+		/*
+		 * Not found in current page. Move towards rightlink.
+		 */
+		parent->blkno = GistPageGetOpaque(page)->rightlink;
+		UnlockReleaseBuffer(buffer);
+
+		if (parent->blkno == InvalidBlockNumber)
+		{
+			/*
+			 * End of chain and still didn't find parent. Should not happen
+			 * during index build.
+			 */
+			break;
+		}
+
+		/* Get the next page */
+		buffer = ReadBuffer(indexrel, parent->blkno);
+		page = BufferGetPage(buffer);
+		LockBuffer(buffer, GIST_EXCLUSIVE);
+		gistcheckpage(indexrel, buffer);
+	}
+
+	elog(ERROR, "failed to re-find parent for block %u", child->blkno);
+}
+
+/*
+ * Process buffers emptying stack. Emptying of one buffer can cause emptying
+ * of other buffers. This function iterates until this cascading emptying
+ * process finished, e.g. until buffers emptying stack is empty.
+ */
+static void
+gistProcessEmptyingQueue(GISTBuildState *buildstate)
+{
+	GISTBuildBuffers *gfbb = buildstate->gfbb;
+
+	/* Iterate while we have elements in buffers emptying stack. */
+	while (gfbb->bufferEmptyingQueue != NIL)
+	{
+		GISTNodeBuffer *emptyingNodeBuffer;
+
+		/* Get node buffer from emptying stack. */
+		emptyingNodeBuffer = (GISTNodeBuffer *) linitial(gfbb->bufferEmptyingQueue);
+		gfbb->bufferEmptyingQueue = list_delete_first(gfbb->bufferEmptyingQueue);
+		emptyingNodeBuffer->queuedForEmptying = false;
+
+		/*
+		 * We are going to load last pages of buffers where emptying will be
+		 * to. So let's unload any previously loaded buffers.
+		 */
+		gistUnloadNodeBuffers(gfbb);
+
+		/*
+		 * Pop tuples from the buffer and run them down to the buffers at
+		 * lower level, or leaf pages. We continue until one of the lower
+		 * level buffers fills up, or this buffer runs empty.
+		 *
+		 * In Arge et al's paper, the buffer emptying is stopped after
+		 * processing 1/2 node buffer worth of tuples, to avoid overfilling
+		 * any of the lower level buffers. However, it's more efficient to
+		 * keep going until one of the lower level buffers actually fills up,
+		 * so that's what we do. This doesn't need to be exact, if a buffer
+		 * overfills by a few tuples, there's no harm done.
+		 */
+		while (true)
+		{
+			IndexTuple	itup;
+
+			/* Get next index tuple from the buffer */
+			if (!gistPopItupFromNodeBuffer(gfbb, emptyingNodeBuffer, &itup))
+				break;
+
+			/*
+			 * Run it down to the underlying node buffer or leaf page.
+			 *
+			 * Note: it's possible that the buffer we're emptying splits as a
+			 * result of this call. If that happens, our emptyingNodeBuffer
+			 * points to the left half of the split. After split, it's very
+			 * likely that the new left buffer is no longer over the half-full
+			 * threshold, but we might as well keep flushing tuples from it
+			 * until we fill a lower-level buffer.
+			 */
+			if (gistProcessItup(buildstate, itup, emptyingNodeBuffer->path))
+			{
+				/*
+				 * A lower level buffer filled up. Stop emptying this buffer,
+				 * to avoid overflowing the lower level buffer.
+				 */
+				break;
+			}
+
+			/* Free all the memory allocated during index tuple processing */
+			MemoryContextReset(CurrentMemoryContext);
+		}
+	}
+}
+
+/*
+ * Empty all node buffers, from top to bottom. This is done at the end of
+ * index build to flush all remaining tuples to the index.
+ *
+ * Note: This destroys the buffersOnLevels lists, so the buffers should not
+ * be inserted to after this call.
+ */
+static void
+gistEmptyAllBuffers(GISTBuildState *buildstate)
+{
+	GISTBuildBuffers *gfbb = buildstate->gfbb;
+	MemoryContext oldCtx;
+	int			i;
+
+	oldCtx = MemoryContextSwitchTo(buildstate->tmpCtx);
+
+	/*
+	 * Iterate through the levels from top to bottom.
+	 */
+	for (i = gfbb->buffersOnLevelsLen - 1; i >= 0; i--)
+	{
+		/*
+		 * Empty all buffers on this level. Note that new buffers can pop up
+		 * in the list during the processing, as a result of page splits, so a
+		 * simple walk through the list won't work. We remove buffers from the
+		 * list when we see them empty; a buffer can't become non-empty once
+		 * it's been fully emptied.
+		 */
+		while (gfbb->buffersOnLevels[i] != NIL)
+		{
+			GISTNodeBuffer *nodeBuffer;
+
+			nodeBuffer = (GISTNodeBuffer *) linitial(gfbb->buffersOnLevels[i]);
+
+			if (nodeBuffer->blocksCount != 0)
+			{
+				/*
+				 * Add this buffer to the emptying queue, and proceed to empty
+				 * the queue.
+				 */
+				MemoryContextSwitchTo(gfbb->context);
+				gfbb->bufferEmptyingQueue =
+					lcons(nodeBuffer, gfbb->bufferEmptyingQueue);
+				MemoryContextSwitchTo(buildstate->tmpCtx);
+				gistProcessEmptyingQueue(buildstate);
+			}
+			else
+				gfbb->buffersOnLevels[i] =
+					list_delete_first(gfbb->buffersOnLevels[i]);
+		}
+	}
+	MemoryContextSwitchTo(oldCtx);
+}
+
+/*
+ * Free unreferenced parts of a path stack.
+ */
+static void
+gistFreeUnreferencedPath(GISTBufferingInsertStack *path)
+{
+	while (path->refCount == 0)
+	{
+		/*
+		 * Path part is unreferenced. We can free it and decrease reference
+		 * count of parent. If parent becomes unreferenced too procedure
+		 * should be repeated for it.
+		 */
+		GISTBufferingInsertStack *tmp = path->parent;
+
+		pfree(path);
+		path = tmp;
+		if (path)
+			path->refCount--;
+		else
+			break;
+	}
+}
+
+/*
+ * Decrease reference count of a path part, and free any unreferenced parts of
+ * the path stack.
+ */
+void
+gistDecreasePathRefcount(GISTBufferingInsertStack *path)
+{
+	path->refCount--;
+	gistFreeUnreferencedPath(path);
+}
+
+/*
+ * Get the depth of the GiST index.
+ */
+static int
+gistGetMaxLevel(Relation index)
+{
+	int			maxLevel;
+	BlockNumber blkno;
+
+	/*
+	 * Traverse down the tree, starting from the root, until we hit the leaf
+	 * level.
+	 */
+	maxLevel = 0;
+	blkno = GIST_ROOT_BLKNO;
+	while (true)
+	{
+		Buffer		buffer;
+		Page		page;
+		IndexTuple	itup;
+
+		buffer = ReadBuffer(index, blkno);
+
+		/*
+		 * There's no concurrent access during index build, so locking is just
+		 * pro forma.
+		 */
+		LockBuffer(buffer, GIST_SHARE);
+		page = (Page) BufferGetPage(buffer);
+
+		if (GistPageIsLeaf(page))
+		{
+			/* We hit the bottom, so we're done. */
+			UnlockReleaseBuffer(buffer);
+			break;
+		}
+
+		/*
+		 * Pick the first downlink on the page, and follow it. It doesn't
+		 * matter which downlink we choose, the tree has the same depth
+		 * everywhere, so we just pick the first one.
+		 */
+		itup = (IndexTuple) PageGetItem(page,
+									 PageGetItemId(page, FirstOffsetNumber));
+		blkno = ItemPointerGetBlockNumber(&(itup->t_tid));
+		UnlockReleaseBuffer(buffer);
+
+		/*
+		 * We're going down on the tree. It means that there is yet one more
+		 * level is the tree.
+		 */
+		maxLevel++;
+	}
+	return maxLevel;
+}