Additional fixes of memory alignment in pg_mcv_list code

tvondra · tvondra · commit ea4e1c0e8f58 · 2019-03-30T18:34:59.000+01:00
Commit d85e0f3 tried to fix memory alignment issues in serialization and deserialization of pg_mcv_list values, but it was a few bricks shy. The arrays of uint16 indexes in serialized items was not aligned, and the both the values and isnull flags were using the same pointer. Per investigation by Tom Lane on gaur.
diff --git a/src/backend/statistics/mcv.c b/src/backend/statistics/mcv.c
@@ -51,7 +51,7 @@
  *	 ndim * (sizeof(uint16) + sizeof(bool)) + 2 * sizeof(double)
  */
 #define ITEM_SIZE(ndims)	\
-	((ndims) * (sizeof(uint16) + sizeof(bool)) + 2 * sizeof(double))
+	MAXALIGN((ndims) * (sizeof(uint16) + sizeof(bool)) + 2 * sizeof(double))
 
 /*
  * Macros for convenient access to parts of a serialized MCV item.
@@ -929,8 +929,8 @@ statext_mcv_deserialize(bytea *data)
 	mcvlen = MAXALIGN(offsetof(MCVList, items) + (sizeof(MCVItem) * nitems));
 
 	/* arrays of values and isnull flags for all MCV items */
-	mcvlen += MAXALIGN(sizeof(Datum) * ndims * nitems);
-	mcvlen += MAXALIGN(sizeof(bool) * ndims * nitems);
+	mcvlen += nitems * MAXALIGN(sizeof(Datum) * ndims);
+	mcvlen += nitems * MAXALIGN(sizeof(bool) * ndims);
 
 	/* we don't quite need to align this, but it makes some assers easier */
 	mcvlen += MAXALIGN(datalen);
@@ -942,9 +942,9 @@ statext_mcv_deserialize(bytea *data)
 	valuesptr = (char *) mcvlist
 		+ MAXALIGN(offsetof(MCVList, items) + (sizeof(MCVItem) * nitems));
 
-	isnullptr = valuesptr + (MAXALIGN(sizeof(Datum) * ndims * nitems));
+	isnullptr = valuesptr + (nitems * MAXALIGN(sizeof(Datum) * ndims));
 
-	dataptr = isnullptr + (MAXALIGN(sizeof(bool) * ndims * nitems));
+	dataptr = isnullptr + (nitems * MAXALIGN(sizeof(bool) * ndims));
 
 	/*
 	 * Build mapping (index => value) for translating the serialized data into
@@ -1043,10 +1043,11 @@ statext_mcv_deserialize(bytea *data)
 		MCVItem    *item = &mcvlist->items[i];
 
 		item->values = (Datum *) valuesptr;
-		valuesptr += (sizeof(Datum) * ndims);
+		valuesptr += MAXALIGN(sizeof(Datum) * ndims);
+
+		item->isnull = (bool *) isnullptr;
+		isnullptr += MAXALIGN(sizeof(bool) * ndims);
 
-		item->isnull = (bool *) valuesptr;
-		valuesptr += (sizeof(bool) * ndims);
 
 		/* just point to the right place */
 		indexes = ITEM_INDEXES(ptr);
