Re: More stable query plans via more predictable column statistics

Hi Alex,

Thanks for excellent research.

I've ran your queries against Trustly's production database and I can
confirm your findings, the results are similar:

WITH ...
SELECT count(1),
       min(hist_ratio)::real,
       avg(hist_ratio)::real,
       max(hist_ratio)::real,
       stddev(hist_ratio)::real
  FROM stats2
 WHERE histogram_bounds IS NOT NULL;

-[ RECORD 1 ]----
count  | 2814
min    | 0.193548
avg    | 0.927357
max    | 1
stddev | 0.164134


WHERE distinct_hist < num_hist
-[ RECORD 1 ]----
count  | 624
min    | 0.193548
avg    | 0.672407
max    | 0.990099
stddev | 0.194901


WITH ..
SELECT schemaname ||'.'|| tablename ||'.'|| attname || (CASE inherited
WHEN TRUE THEN ' (inherited)' ELSE '' END) AS columnname,
       n_distinct, null_frac,
       num_mcv, most_common_vals, most_common_freqs,
       mcv_frac, (mcv_frac / (1 - null_frac))::real AS nonnull_mcv_frac,
       distinct_hist, num_hist, hist_ratio,
       histogram_bounds
  FROM stats2
 ORDER BY hist_ratio
 LIMIT 1;

 -[ RECORD 1 ]-----+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
columnname        | public.x.y
n_distinct        | 103
null_frac         | 0
num_mcv           | 10
most_common_vals  | {0,1,2,3,4,5,6,7,8,9}
most_common_freqs |
{0.4765,0.141733,0.1073,0.0830667,0.0559667,0.0373333,0.0251,0.0188,0.0141,0.0113667}
mcv_frac          | 0.971267
nonnull_mcv_frac  | 0.971267
distinct_hist     | 18
num_hist          | 93
hist_ratio        | 0.193548387096774
histogram_bounds  |
{10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,12,12,12,12,12,12,12,12,12,12,12,12,13,13,13,13,13,13,13,13,13,13,14,14,14,14,14,15,15,15,15,16,16,16,16,21,23,5074,5437,5830,6049,6496,7046,7784,14629,21285}