Re: Optimize MCV stats for sortable types and utilize sorted-order properties

Hi,

In v12, the CF tests showed failures in the following two TAP tests:
  271/429 postgresql:scripts / scripts/102_vacuumdb_stages  ERROR  6.22s exit status 2
  272/429 postgresql:scripts / scripts/100_vacuumdb               ERROR 10.90s exit status 2

It was also able to reproduce the issue manually with the following commands:
SELECT pg_catalog.set_config('search_path', '', false);
SET default_statistics_target=10;
RESET vacuum_cost_delay;
ANALYZE pg_catalog.pg_proc;

The issue was caused by the MCV tracking logic. The fix is to remove the rightmost entry
with the smallest count and shift the following elements when the number of tracked
values reaches num_mcv, making room for the new value at the end:
/*
 * If track_cnt reaches num_mcv, remove the rightmost
 * entry with the smallest count and shift the
 * following elements to the left, making room for the
 * new value at the end.
 */

After applying this fix, the three related test cases pass successfully in my environment.

Thanks again.

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