index 47581ab705f2c92df871a08deb445181b843ea2d..6285a5788894478c21b6012eeb190994fe636013 100644 (file)
*
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/commands/analyze.c,v 1.135 2009年03月31日 22:12:46 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/commands/analyze.c,v 1.136 2009年05月05日 18:02:11 tgl Exp $
*
*-------------------------------------------------------------------------
*/
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,
/* 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;