Re: Fix var_eq_const: sum selectivity of all matching MCV entries instead of stopping at first match

HI,  hackers
  So far, I have only found compute_scalar_stats() and compute_distinct_stats() that
generate STATISTIC_KIND_MCV statistics. The way they allocate space for and populate
most_common_vals and most_common_freqs, as well as determine their number of elements,
is the same in both cases. In other words, these two arrays always have the same number of elements.

The relevant code is briefly shown below:
```C
/* Generate MCV slot entry */

                  mcv_values = palloc_array(Datum, num_mcv);
                  mcv_freqs = palloc_array(float4, num_mcv);
                  for (i = 0; i < num_mcv; i++)
                  {
                        mcv_values[i] = datumCopy(values[track[i].first].value,
                                                              stats->attrtype->typbyval,
                                                              stats->attrtype->typlen);
                        mcv_freqs[i] = (double) track[i].count / (double) samplerows;
                  }
                  MemoryContextSwitchTo(old_context);

                  stats->stakind[slot_idx] = STATISTIC_KIND_MCV;
                  stats->staop[slot_idx] = mystats->eqopr;
                  stats->stacoll[slot_idx] = stats->attrcollid;
                  stats->stanumbers[slot_idx] = mcv_freqs;
                  stats->numnumbers[slot_idx] = num_mcv;
                  stats->stavalues[slot_idx] = mcv_values;
                  stats->numvalues[slot_idx] = num_mcv;

regards,
--
ZizhuanLiu (X-MAN) 
44973863(at)qq(dot)com