Fix tuple counting in SP-GiST index build.

Count the number of tuples in the index honestly, instead of assuming
that it's the same as the number of tuples in the heap.  (It might be
different if the index is partial.)

Back-patch to all supported versions.

Tomas Vondra

Discussion: https://postgr.es/m/3b3d8eac-c709-0d25-088e-b98339a1b28a@2ndquadrant.com

src/backend/access/spgist/spginsert.c

@@ -32,6 +32,7 @@
 typedef struct
 {
 	SpGistState spgstate;		/* SPGiST's working state */
+	int64		indtuples;		/* total number of tuples indexed */
 	MemoryContext tmpCtx;		/* per-tuple temporary context */
 } SpGistBuildState;
 
@@ -59,6 +60,9 @@ spgistBuildCallback(Relation index, HeapTuple htup, Datum *values,
 		MemoryContextReset(buildstate->tmpCtx);
 	}
 
+	/* Update total tuple count */
+	buildstate->indtuples += 1;
+
 	MemoryContextSwitchTo(oldCtx);
 	MemoryContextReset(buildstate->tmpCtx);
 }
@@ -132,6 +136,7 @@ spgbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 	 */
 	initSpGistState(&buildstate.spgstate, index);
 	buildstate.spgstate.isBuild = true;
+	buildstate.indtuples = 0;
 
 	buildstate.tmpCtx = AllocSetContextCreate(CurrentMemoryContext,
 											  "SP-GiST build temporary context",
@@ -146,7 +151,8 @@ spgbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 	SpGistUpdateMetaPage(index);
 
 	result = (IndexBuildResult *) palloc0(sizeof(IndexBuildResult));
-	result->heap_tuples = result->index_tuples = reltuples;
+	result->heap_tuples = reltuples;
+	result->index_tuples = buildstate.indtuples;
 
 	return result;
 }