Re: eqjoinsel_semi still sucks ...

On 2/5/2012 20:34, Tom Lane wrote:

On reflection I think that the idea of clamping ndistinct beforehand is
just wrong, and what we ought to do instead is apply a multiplier to the
selectivity estimate afterwards.  In the case of a base rel we could
just multiply by the selectivity of its baserestrictinfo list.  For join
rels it's a bit harder to guess how much a given input relation might
have been decimated, but if the join's estimated size is smaller than
the output size of the base rel the correlation var came from, we could
multiply by that ratio (on top of whatever correction came from the base
rel's restriction clauses).

I got stuck in some cases where (due to a tree of filters) the planner underestimates the JOIN just because the ndistinct conveys a huge number to the selectivity estimation formula. However, the estimation of both input relations is made correctly and is limited.
I've tried to understand the logic through commits 0d3b231eebf, 97930cf578e and 7f3eba30c9d. But it is still not clear.
So, why the idea of clamping ndistinct is terrible in general? Could you explain your reasons a bit more?

Hi, I also have an interest in understanding the same issue, and I dug into the above commits and the topic . I hope this letter will help to sort out this issue. I found some information on this topic in the discussion [1], more specifically in the email [2].

First of all, according to the post [2] and 0d3b231ee commit, clampling ndistinct is necessary because it allows us to eliminate non-MCV values, or potentially even the least-common MCVs for the inner relation that might not be analyze operation (part of the code is given below when not all different values are taken into account):static void
compute_distinct_stats(VacAttrStatsP stats,
                       AnalyzeAttrFetchFunc fetchfunc,
                       int samplerows,
                       double totalrows)
{
...

/*
     * Decide how many values are worth storing as most-common values. If
     * we are able to generate a complete MCV list (all the values in the
     * sample will fit, and we think these are all the ones in the table),
     * then do so.  Otherwise, store only those values that are
     * significantly more common than the values not in the list.
     *
     * Note: the first of these cases is meant to address columns with
     * small, fixed sets of possible values, such as boolean or enum
     * columns.  If we can *completely* represent the column population by
     * an MCV list that will fit into the stats target, then we should do
     * so and thus provide the planner with complete information.  But if
     * the MCV list is not complete, it's generally worth being more
     * selective, and not just filling it all the way up to the stats
     * target.
     */
    if (track_cnt < track_max && toowide_cnt == 0 &&
      stats->stadistinct > 0 &&
      track_cnt <= num_mcv)
    {
      /* Track list includes all values seen, and all will fit */
      num_mcv = track_cnt;
    }
    else
    {
      int       *mcv_counts;

      /* Incomplete list; decide how many values are worth keeping */
      if (num_mcv > track_cnt)
        num_mcv = track_cnt;

      if (num_mcv > 0)
      {
        mcv_counts = (int *) palloc(num_mcv * sizeof(int));
        for (i = 0; i < num_mcv; i++)
          mcv_counts[i] = track[i].count;

        num_mcv = analyze_mcv_list(mcv_counts, num_mcv,
                       stats->stadistinct,
                       stats->stanullfrac,
                       samplerows, totalrows);
      }
    }

...
}

The reason why the idea of clamping is used may be related to the fact that during the processing of eqjoinsel_inner, the ndistinct estimate for the join key column decreases (in proportion to the selectivity of the baserel restrictions), which leads to a proportional increase in the number of selectivities for the join condition; we have to eliminate values other than MCV or potentially the least-common MCV for the inner relation, since the most common values are the ones that are most likely to survive the decimation resulting from a lower restriction clause.

This is discussed in [2], I copied a fragment where it is clearly seen below:

If you think about what is happening in eqjoinsel_inner with the patch, we are reducing the ndistinct estimate for the join key column proportionally to the selectivity of whatever baserel restrictions apply.  This then results in proportionally increasing the selectivity number for the join condition --- in other words, we're more or less cancelling out the effects of one or the other relation's base restrictions.  So that's pretty broken in general.  The reason it is important for semi/antijoin inner relations is that this is actually the only way that restrictions applied to the inner rel get to impact the join size estimate at all, since set_joinrel_size_estimates is not going to factor the inner rel size into what it multiplies the join selectivity against.    In short, I was mistakenly extrapolating from the observation that it helped to hack the ndistinct estimate for a semijoin's inner rel, to the conclusion that we should do that for all join input rels.  
...
Yeah, those are estimating that all the outer rows have join partners, because there are more distinct values in the sub-select than there are in the outer relation.

In addition, the answer to the question why clamping is necessary is also in the comment to commit 97930cf5, and this not only reduced the number of rows coming out of a table and  the number of possibly-distinct values of a join variable, but join restriction clauses that might have been applied at a lower level of join.

That patch accounted for baserel restriction clauses that reduced the number of rows coming out of a table (and hence the number of possibly-distinct values of a join variable), but not for join restriction clauses that might have been applied at a lower level of join.  To account for the latter, look up the sizes of the min_lefthand and min_righthand inputs of the current join, and clamp with those in the same way as for the base relations.

1. https://www.postgresql.org/message-id/flat/5156.1314829311%40sss.pgh.pa.us#86609b6ac6af405dec67316bfbe9868f

2. https://www.postgresql.org/message-id/raw/5156.1314829311%40sss.pgh.pa.us

----

Regards,

Alena Rybakina