*** /tmp/DqhRGF_vacuum.c	2018-03-02 18:43:54.448046402 +0300
--- src/backend/commands/vacuum.c	2018-03-02 18:22:04.223070206 +0300
***************
*** 780,791 ****
  					   BlockNumber scanned_pages,
  					   double scanned_tuples)
  {
- 	BlockNumber old_rel_pages = relation->rd_rel->relpages;
  	double		old_rel_tuples = relation->rd_rel->reltuples;
! 	double		old_density;
! 	double		new_density;
  	double		multiplier;
- 	double		updated_density;
  
  	/* If we did scan the whole table, just use the count as-is */
  	if (scanned_pages >= total_pages)
--- 780,788 ----
  					   BlockNumber scanned_pages,
  					   double scanned_tuples)
  {
  	double		old_rel_tuples = relation->rd_rel->reltuples;
! 	double 		new_rel_tuples;
  	double		multiplier;
  
  	/* If we did scan the whole table, just use the count as-is */
  	if (scanned_pages >= total_pages)
***************
*** 801,839 ****
  		return old_rel_tuples;
  
  	/*
! 	 * If old value of relpages is zero, old density is indeterminate; we
! 	 * can't do much except scale up scanned_tuples to match total_pages.
  	 */
! 	if (old_rel_pages == 0)
! 		return floor((scanned_tuples / scanned_pages) * total_pages + 0.5);
  
  	/*
! 	 * Okay, we've covered the corner cases.  The normal calculation is to
! 	 * convert the old measurement to a density (tuples per page), then update
! 	 * the density using an exponential-moving-average approach, and finally
! 	 * compute reltuples as updated_density * total_pages.
! 	 *
! 	 * For ANALYZE, the moving average multiplier is just the fraction of the
! 	 * table's pages we scanned.  This is equivalent to assuming that the
! 	 * tuple density in the unscanned pages didn't change.  Of course, it
! 	 * probably did, if the new density measurement is different. But over
! 	 * repeated cycles, the value of reltuples will converge towards the
! 	 * correct value, if repeated measurements show the same new density.
! 	 *
! 	 * For VACUUM, the situation is a bit different: we have looked at a
! 	 * nonrandom sample of pages, but we know for certain that the pages we
! 	 * didn't look at are precisely the ones that haven't changed lately.
! 	 * Thus, there is a reasonable argument for doing exactly the same thing
! 	 * as for the ANALYZE case, that is use the old density measurement as the
! 	 * value for the unscanned pages.
! 	 *
! 	 * This logic could probably use further refinement.
  	 */
- 	old_density = old_rel_tuples / old_rel_pages;
- 	new_density = scanned_tuples / scanned_pages;
  	multiplier = (double) scanned_pages / (double) total_pages;
! 	updated_density = old_density + (new_density - old_density) * multiplier;
! 	return floor(updated_density * total_pages + 0.5);
  }
  
  
--- 798,825 ----
  		return old_rel_tuples;
  
  	/*
! 	 * Estimate the total number of tuples based on the density of scanned
! 	 * tuples.
  	 */
! 	new_rel_tuples = floor((scanned_tuples / scanned_pages) * total_pages + 0.5);
  
  	/*
! 	 * ANALYZE scans a representative subset of pages, so we trust its density
! 	 * estimate.
! 	 */
! 	if (is_analyze)
! 		return new_rel_tuples;
! 
! 	/*
! 	 * VACUUM scanned a nonrandom sample of pages, so we can't just scale up its
! 	 * result. For the portion of table it didn't scan, use the old number of tuples,
! 	 * and for the portion it did scan, use the number it reported. This is
! 	 * effectively an exponential moving average with adaptive factor.
  	 */
  	multiplier = (double) scanned_pages / (double) total_pages;
! 	return floor(old_rel_tuples * (1. - multiplier)
! 			+ new_rel_tuples * multiplier
! 			+ 0.5);
  }