Re: Improve pg_stat_statements scalability

Hello!

I noticed that the patch has some issues with minmax_only, and these
differences seem to be unintended to me:

1. minmax only resets updates mean/sum_var_time (but then when it
calculates them again, it also uses the data before the minmax reset)

+ /*
+ * Reset only min/max timing values and update minmax_stats_since.
+ * Iterate matching entries in the dshash, reset the corresponding
+ * pgstat counters' min/max fields.
+ */
...
+ shared->counters.mean_time[kind] = 0;
+ shared->counters.sum_var_time[kind] = 0;

2. min_time doesn't recover after a minmax reset:

+ if (pending->min_time[kind] < shared->min_time[kind])
+ shared->min_time[kind] = pending->min_time[kind];

As it is only updated if it's smaller, but it is 0 after it, and
because the call count remains > 0 it doesn't get initialized with the
current value.

+ if ((int) pg_atomic_read_u64(&pgss_shared->nentries) <= pgss_max *
(100 - USAGE_DEALLOC_PERCENT) / 100)
+ break;

isn't there a possible overflow here? The similar deleted check had
explicit uint64 casts for pgss_max multiplication.

- /*
- * Don't proceed if file does not exceed 512 bytes per possible entry.
- */
- if ((uint64) extent < (uint64) 512 * pgss_max)
- return false;