Use a pairing heap for the priority queue in kNN-GiST searches.

This performs slightly better, uses less memory, and needs slightly less
code in GiST, than the Red-Black tree previously used.

Reviewed by Peter Geoghegan

Author: hlinnaka

src/backend/access/gist/gistget.c

@@ -18,6 +18,7 @@
 #include "access/relscan.h"
 #include "miscadmin.h"
 #include "pgstat.h"
+#include "lib/pairingheap.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
@@ -243,8 +244,6 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem, double *myDistances,
 	GISTPageOpaque opaque;
 	OffsetNumber maxoff;
 	OffsetNumber i;
-	GISTSearchTreeItem *tmpItem = so->tmpTreeItem;
-	bool		isNew;
 	MemoryContext oldcxt;
 
 	Assert(!GISTSearchItemIsHeap(*pageItem));
@@ -275,18 +274,15 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem, double *myDistances,
 		oldcxt = MemoryContextSwitchTo(so->queueCxt);
 
 		/* Create new GISTSearchItem for the right sibling index page */
-		item = palloc(sizeof(GISTSearchItem));
-		item->next = NULL;
+		item = palloc(SizeOfGISTSearchItem(scan->numberOfOrderBys));
 		item->blkno = opaque->rightlink;
 		item->data.parentlsn = pageItem->data.parentlsn;
 
 		/* Insert it into the queue using same distances as for this page */
-		tmpItem->head = item;
-		tmpItem->lastHeap = NULL;
-		memcpy(tmpItem->distances, myDistances,
+		memcpy(item->distances, myDistances,
 			   sizeof(double) * scan->numberOfOrderBys);
 
-		(void) rb_insert(so->queue, (RBNode *) tmpItem, &isNew);
+		pairingheap_add(so->queue, &item->phNode);
 
 		MemoryContextSwitchTo(oldcxt);
 	}
@@ -348,8 +344,7 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem, double *myDistances,
 			oldcxt = MemoryContextSwitchTo(so->queueCxt);
 
 			/* Create new GISTSearchItem for this item */
-			item = palloc(sizeof(GISTSearchItem));
-			item->next = NULL;
+			item = palloc(SizeOfGISTSearchItem(scan->numberOfOrderBys));
 
 			if (GistPageIsLeaf(page))
 			{
@@ -372,12 +367,10 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem, double *myDistances,
 			}
 
 			/* Insert it into the queue using new distance data */
-			tmpItem->head = item;
-			tmpItem->lastHeap = GISTSearchItemIsHeap(*item) ? item : NULL;
-			memcpy(tmpItem->distances, so->distances,
+			memcpy(item->distances, so->distances,
 				   sizeof(double) * scan->numberOfOrderBys);
 
-			(void) rb_insert(so->queue, (RBNode *) tmpItem, &isNew);
+			pairingheap_add(so->queue, &item->phNode);
 
 			MemoryContextSwitchTo(oldcxt);
 		}
@@ -390,44 +383,24 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem, double *myDistances,
  * Extract next item (in order) from search queue
  *
  * Returns a GISTSearchItem or NULL.  Caller must pfree item when done with it.
- *
- * NOTE: on successful return, so->curTreeItem is the GISTSearchTreeItem that
- * contained the result item.  Callers can use so->curTreeItem->distances as
- * the distances value for the item.
  */
 static GISTSearchItem *
 getNextGISTSearchItem(GISTScanOpaque so)
 {
-	for (;;)
-	{
-		GISTSearchItem *item;
-
-		/* Update curTreeItem if we don't have one */
-		if (so->curTreeItem == NULL)
-		{
-			so->curTreeItem = (GISTSearchTreeItem *) rb_leftmost(so->queue);
-			/* Done when tree is empty */
-			if (so->curTreeItem == NULL)
-				break;
-		}
+	GISTSearchItem *item;
 
-		item = so->curTreeItem->head;
-		if (item != NULL)
-		{
-			/* Delink item from chain */
-			so->curTreeItem->head = item->next;
-			if (item == so->curTreeItem->lastHeap)
-				so->curTreeItem->lastHeap = NULL;
-			/* Return item; caller is responsible to pfree it */
-			return item;
-		}
-
-		/* curTreeItem is exhausted, so remove it from rbtree */
-		rb_delete(so->queue, (RBNode *) so->curTreeItem);
-		so->curTreeItem = NULL;
+	if (!pairingheap_is_empty(so->queue))
+	{
+		item = (GISTSearchItem *) pairingheap_remove_first(so->queue);
+	}
+	else
+	{
+		/* Done when both heaps are empty */
+		item = NULL;
 	}
 
-	return NULL;
+	/* Return item; caller is responsible to pfree it */
+	return item;
 }
 
 /*
@@ -458,7 +431,7 @@ getNextNearest(IndexScanDesc scan)
 			/* visit an index page, extract its items into queue */
 			CHECK_FOR_INTERRUPTS();
 
-			gistScanPage(scan, item, so->curTreeItem->distances, NULL, NULL);
+			gistScanPage(scan, item, item->distances, NULL, NULL);
 		}
 
 		pfree(item);
@@ -491,7 +464,6 @@ gistgettuple(PG_FUNCTION_ARGS)
 		pgstat_count_index_scan(scan->indexRelation);
 
 		so->firstCall = false;
-		so->curTreeItem = NULL;
 		so->curPageData = so->nPageData = 0;
 
 		fakeItem.blkno = GIST_ROOT_BLKNO;
@@ -534,7 +506,7 @@ gistgettuple(PG_FUNCTION_ARGS)
 				 * this page, we fall out of the inner "do" and loop around to
 				 * return them.
 				 */
-				gistScanPage(scan, item, so->curTreeItem->distances, NULL, NULL);
+				gistScanPage(scan, item, item->distances, NULL, NULL);
 
 				pfree(item);
 			} while (so->nPageData == 0);
@@ -560,7 +532,6 @@ gistgetbitmap(PG_FUNCTION_ARGS)
 	pgstat_count_index_scan(scan->indexRelation);
 
 	/* Begin the scan by processing the root page */
-	so->curTreeItem = NULL;
 	so->curPageData = so->nPageData = 0;
 
 	fakeItem.blkno = GIST_ROOT_BLKNO;
@@ -580,7 +551,7 @@ gistgetbitmap(PG_FUNCTION_ARGS)
 
 		CHECK_FOR_INTERRUPTS();
 
-		gistScanPage(scan, item, so->curTreeItem->distances, tbm, &ntids);
+		gistScanPage(scan, item, item->distances, tbm, &ntids);
 
 		pfree(item);
 	}

src/backend/access/gist/gistscan.c

@@ -22,74 +22,30 @@
 
 
 /*
- * RBTree support functions for the GISTSearchTreeItem queue
+ * Pairing heap comparison function for the GISTSearchItem queue
  */
-
 static int
-GISTSearchTreeItemComparator(const RBNode *a, const RBNode *b, void *arg)
+pairingheap_GISTSearchItem_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
 {
-	const GISTSearchTreeItem *sa = (const GISTSearchTreeItem *) a;
-	const GISTSearchTreeItem *sb = (const GISTSearchTreeItem *) b;
+	const GISTSearchItem *sa = (const GISTSearchItem *) a;
+	const GISTSearchItem *sb = (const GISTSearchItem *) b;
 	IndexScanDesc scan = (IndexScanDesc) arg;
 	int			i;
 
 	/* Order according to distance comparison */
 	for (i = 0; i < scan->numberOfOrderBys; i++)
 	{
 		if (sa->distances[i] != sb->distances[i])
-			return (sa->distances[i] > sb->distances[i]) ? 1 : -1;
-	}
-
-	return 0;
-}
-
-static void
-GISTSearchTreeItemCombiner(RBNode *existing, const RBNode *newrb, void *arg)
-{
-	GISTSearchTreeItem *scurrent = (GISTSearchTreeItem *) existing;
-	const GISTSearchTreeItem *snew = (const GISTSearchTreeItem *) newrb;
-	GISTSearchItem *newitem = snew->head;
-
-	/* snew should have just one item in its chain */
-	Assert(newitem && newitem->next == NULL);
-
-	/*
-	 * If new item is heap tuple, it goes to front of chain; otherwise insert
-	 * it before the first index-page item, so that index pages are visited in
-	 * LIFO order, ensuring depth-first search of index pages.  See comments
-	 * in gist_private.h.
-	 */
-	if (GISTSearchItemIsHeap(*newitem))
-	{
-		newitem->next = scurrent->head;
-		scurrent->head = newitem;
-		if (scurrent->lastHeap == NULL)
-			scurrent->lastHeap = newitem;
+			return (sa->distances[i] < sb->distances[i]) ? 1 : -1;
 	}
-	else if (scurrent->lastHeap == NULL)
-	{
-		newitem->next = scurrent->head;
-		scurrent->head = newitem;
-	}
-	else
-	{
-		newitem->next = scurrent->lastHeap->next;
-		scurrent->lastHeap->next = newitem;
-	}
-}
 
-static RBNode *
-GISTSearchTreeItemAllocator(void *arg)
-{
-	IndexScanDesc scan = (IndexScanDesc) arg;
-
-	return palloc(GSTIHDRSZ + sizeof(double) * scan->numberOfOrderBys);
-}
+	/* Heap items go before inner pages, to ensure a depth-first search */
+	if (GISTSearchItemIsHeap(*sa) && !GISTSearchItemIsHeap(*sb))
+		return -1;
+	if (!GISTSearchItemIsHeap(*sa) && GISTSearchItemIsHeap(*sb))
+		return 1;
 
-static void
-GISTSearchTreeItemDeleter(RBNode *rb, void *arg)
-{
-	pfree(rb);
+	return 0;
 }
 
 
@@ -127,7 +83,6 @@ gistbeginscan(PG_FUNCTION_ARGS)
 	so->queueCxt = giststate->scanCxt;	/* see gistrescan */
 
 	/* workspaces with size dependent on numberOfOrderBys: */
-	so->tmpTreeItem = palloc(GSTIHDRSZ + sizeof(double) * scan->numberOfOrderBys);
 	so->distances = palloc(sizeof(double) * scan->numberOfOrderBys);
 	so->qual_ok = true;			/* in case there are zero keys */
 
@@ -188,15 +143,9 @@ gistrescan(PG_FUNCTION_ARGS)
 
 	/* create new, empty RBTree for search queue */
 	oldCxt = MemoryContextSwitchTo(so->queueCxt);
-	so->queue = rb_create(GSTIHDRSZ + sizeof(double) * scan->numberOfOrderBys,
-						  GISTSearchTreeItemComparator,
-						  GISTSearchTreeItemCombiner,
-						  GISTSearchTreeItemAllocator,
-						  GISTSearchTreeItemDeleter,
-						  scan);
+	so->queue = pairingheap_allocate(pairingheap_GISTSearchItem_cmp, scan);
 	MemoryContextSwitchTo(oldCxt);
 
-	so->curTreeItem = NULL;
 	so->firstCall = true;
 
 	/* Update scan key, if a new one is given */

src/backend/lib/Makefile

@@ -12,6 +12,6 @@ subdir = src/backend/lib
 top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = ilist.o binaryheap.o stringinfo.o
+OBJS = ilist.o binaryheap.o pairingheap.o stringinfo.o
 
 include $(top_srcdir)/src/backend/common.mk

src/backend/lib/README

@@ -0,0 +1,24 @@
+This directory contains a general purpose data structures, for use anywhere
+in the backend:
+
+binaryheap.c - a binary heap
+
+pairingheap.c - a pairing heap
+
+ilist.c - single and double-linked lists.
+
+stringinfo.c - an extensible string type
+
+
+Aside from the inherent characteristics of the data structures, there are a
+few practical differences between the binary heap and the pairing heap. The
+binary heap is fully allocated at creation, and cannot be expanded beyond the
+allocated size. The pairing heap on the other hand has no inherent maximum
+size, but the caller needs to allocate each element being stored in the heap,
+while the binary heap works with plain Datums or pointers.
+
+The linked-lists in ilist.c can be embedded directly into other structs, as
+opposed to the List interface in nodes/pg_list.h.
+
+In addition to these, there is an implementation of a Red-Black tree in
+src/backend/utils/adt/rbtree.c.