Avoid integer overflow in the loop that extracts histogram entries from

ANALYZE's total sample.  The original coding is at risk of overflow for
statistics targets exceeding about 2675; this was not a problem before
8.4 but it is now.  Per bug #4793 from Dennis Noordsij.

src/backend/commands/analyze.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/analyze.c,v 1.135 2009/03/31 22:12:46 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/analyze.c,v 1.136 2009/05/05 18:02:11 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2260,6 +2260,10 @@ compute_scalar_stats(VacAttrStatsP stats,
 			MemoryContext old_context;
 			Datum	   *hist_values;
 			int			nvals;
+			int			pos,
+						posfrac,
+						delta,
+						deltafrac;
 
 			/* Sort the MCV items into position order to speed next loop */
 			qsort((void *) track, num_mcv,
@@ -2313,15 +2317,35 @@ compute_scalar_stats(VacAttrStatsP stats,
 			/* Must copy the target values into anl_context */
 			old_context = MemoryContextSwitchTo(stats->anl_context);
 			hist_values = (Datum *) palloc(num_hist * sizeof(Datum));
+
+			/*
+			 * The object of this loop is to copy the first and last values[]
+			 * entries along with evenly-spaced values in between.  So the
+			 * i'th value is values[(i * (nvals - 1)) / (num_hist - 1)].  But
+			 * computing that subscript directly risks integer overflow when
+			 * the stats target is more than a couple thousand.  Instead we
+			 * add (nvals - 1) / (num_hist - 1) to pos at each step, tracking
+			 * the integral and fractional parts of the sum separately.
+			 */
+			delta = (nvals - 1) / (num_hist - 1);
+			deltafrac = (nvals - 1) % (num_hist - 1);
+			pos = posfrac = 0;
+
 			for (i = 0; i < num_hist; i++)
 			{
-				int			pos;
-
-				pos = (i * (nvals - 1)) / (num_hist - 1);
 				hist_values[i] = datumCopy(values[pos].value,
 										   stats->attr->attbyval,
 										   stats->attr->attlen);
+				pos += delta;
+				posfrac += deltafrac;
+				if (posfrac >= (num_hist - 1))
+				{
+					/* fractional part exceeds 1, carry to integer part */
+					pos++;
+					posfrac -= (num_hist - 1);
+				}
 			}
+
 			MemoryContextSwitchTo(old_context);
 
 			stats->stakind[slot_idx] = STATISTIC_KIND_HISTOGRAM;