Use fabs [skip ci]

README.md

@@ -312,6 +312,10 @@ Two things you can try are:
 1. use dimensionality reduction
 2. compile Postgres with a larger block size (`./configure --with-blocksize=32`) and edit the limit in `src/ivfflat.h`
 
+#### Why am I seeing less results after adding an index?
+
+The index was likely created with too little data for the number of lists. Drop the index until the table has more data.
+
 ## Reference
 
 ### Vector Type

src/ivfbuild.c

@@ -419,7 +419,7 @@ ComputeCenters(IvfflatBuildState * buildstate)
 
 	/* Target 50 samples per list, with at least 10000 samples */
 	/* The number of samples has a large effect on index build time */
-	numSamples = buildstate->lists * 200;
+	numSamples = buildstate->lists * 50;
 	if (numSamples < 10000)
 		numSamples = 10000;
 

src/ivfflat.h

@@ -206,6 +206,7 @@ typedef struct IvfflatScanOpaqueData
 
 	/* Lists */
 	pairingheap *listQueue;
+	double		minDistance;
 	IvfflatScanList lists[FLEXIBLE_ARRAY_MEMBER];	/* must come last */
 }			IvfflatScanOpaqueData;
 

src/ivfscan.c

@@ -60,6 +60,9 @@ GetScanLists(IndexScanDesc scan, Datum value)
 			/* Use procinfo from the index instead of scan key for performance */
 			distance = DatumGetFloat8(FunctionCall2Coll(so->procinfo, so->collation, PointerGetDatum(&list->center), value));
 
+			if (distance < so->minDistance)
+				so->minDistance = distance;
+
 			if (listCount < so->probes)
 			{
 				scanlist = &so->lists[listCount];
@@ -129,7 +132,13 @@ GetScanItems(IndexScanDesc scan, Datum value)
 	/* Search closest probes lists */
 	while (!pairingheap_is_empty(so->listQueue))
 	{
-		searchPage = ((IvfflatScanList *) pairingheap_remove_first(so->listQueue))->startPage;
+		IvfflatScanList *scanlist = (IvfflatScanList *) pairingheap_remove_first(so->listQueue);
+
+		/* Query-aware dynamic pruning */
+		if (fabs(scanlist->distance) > 1.5 * fabs(so->minDistance))
+			continue;
+
+		searchPage = scanlist->startPage;
 
 		/* Search all entry pages for list */
 		while (BlockNumberIsValid(searchPage))
@@ -252,6 +261,7 @@ ivfflatrescan(IndexScanDesc scan, ScanKey keys, int nkeys, ScanKey orderbys, int
 
 	so->first = true;
 	pairingheap_reset(so->listQueue);
+	so->minDistance = DBL_MAX;
 
 	if (keys && scan->numberOfKeys > 0)
 		memmove(scan->keyData, keys, scan->numberOfKeys * sizeof(ScanKeyData));