[PATCH] Add features to pg_stat_statements

This is a proposal to add some features to pg_stat_statements.
Attached is the patch of this.

pg_stat_statements uses a hash table to hold statistics,
and the maximum number of its entries can be configured through 
pg_stat_statements.max.
When the number of entries exceeds the pg_stat_statements.max,
pg_stat_statements deallocate existing entries.

Currently, it is impossible to know how many times/when this 
deallocation happened.
But, with this information, more detailed performance analysis would be 
possible.
So, this patch provides access to this information.

This patch provides a view (pg_stat_statements_ctl) and
a function (pg_stat_statements_ctl).
The pg_stat_statements_ctl view is defined
in terms of a function also named pg_stat_statements_ctl.

The entry of pg_stat_statements_ctl view is removed
when pg_stat_statements_reset(0,0,0) is called.
Maybe, it is convenient to provide a function named 
pg_stat_statements_ctl_reset
that removes only the entry of pg_stat_statements_ctl.

The following is an example of this feature.
Here, a deallocation is shown with the INFO message.
pg_stat_statements_ctl tracks the number of deallocations
and timestamp of last deallocation.


testdb=# select * from pg_stat_statements_ctl;
  dealloc |         last_dealloc
---------+-------------------------------
        2 | 2020-09-18 16:55:31.128256+09
(1 row)

testdb=# select count(*) from test1;
2020-09-18 16:59:20.745 JST [3920] INFO:  deallocate
2020-09-18 16:59:20.745 JST [3920] STATEMENT:  select count(*) from 
test1;
INFO:  deallocate
  count
-------
     90
(1 row)

testdb=# select * from pg_stat_statements_ctl;
  dealloc |         last_dealloc
---------+-------------------------------
        3 | 2020-09-18 16:59:20.745652+09
(1 row)


Regards,
Katsuragi Yuta