[PROPOSAL] Platform-native resource usage stats for Windows

Hello,

I'd like to propose an enhancement to how PostgreSQL reports resource 
usage statistics on Windows, building on a 2017 discussion initiated by 
Justin Pryzby [1].

I've been spending some time reviewing windows specific code in the 
postgresql codebase.  I've notice several "opportunities" for 
enhancements and cleanup-- especially if we are only supporting Windows 
10+.  One of the areas I noticed was getrusage().

The current Windows getrusage() shim in src/port/getrusage.c only 
reports CPU times. Everything else is zero. Windows has better 
performance APIs than Unix getrusage() in several areas - we're just not 
using them.

Back in 2017, Justin Pryzby proposed exposing ru_maxrss [1], and Robert 
Haas suggested that if we're going to customize by platform, we should 
show only meaningful values per platform [2]. Nobody followed through 
with a patch.

Problem

Windows administrators get this:
! 0/0 [0/0] filesystem blocks in/out
! 0/39 [0/1281] page faults/reclaims, 0 [0] swaps

The zeros are uninformative. Windows has the data - I/O byte counts,
detailed memory stats, handle counts - but we're trying to jam it into
struct rusage where it doesn't fit.

I propose we let each platform report what it actually measures, using 
native APIs:

Windows: GetProcessMemoryInfo, GetProcessIoCounters, QueryProcessCycleTime
Linux/BSD: Keep using getrusage, maybe expose more fields later

This means platform-specific output formats, but that's better than 
reporting zeros. This follows the precedent Robert Haas suggested in 
2017 [2]: "if we're going to go to the trouble of customizing this code 
by platform, we ought to get rid of values that are meaningless on that 
platform."

We already have extensive platform-specific code in 
src/backend/port/win32/. This follows the same pattern.

I've included a rough proof-of-concept. It:

(1) Adds PgWin32ResourceUsage extension to PGRUsage
(2) Implements Windows-native collection via additions to pg_rusage_init()
(3) Shows actual I/O bytes, memory details, handle/thread counts
(4) Leaves Unix paths completely untouched

On Windows you'd see:
! Memory: 45232 KB working set (48192 KB peak), 38416 KB private
! I/O: 2457600 bytes read (147 ops), 819200 bytes written (43 ops)
! Resources: 247 handles, 8 threads

Before I invest time in a full patch:

(1) Is there support for this general approach of platform-specific 
resource reporting?
(2) Should this extend to non-Windows Unix platforms as well (e.g., 
exposing more Linux-specific fields)?
(3) Any concerns about diverging output formats across platforms?
(4) Would you prefer all platforms to show only common fields, or 
platform-native output?

I believe this aligns with PostgreSQL's existing philosophy - we already 
have extensive platform-specific code in src/backend/port/win32/, and 
this would be a natural extension of that approach.

I'm happy to implement this if there's community interest. Feedback welcome!


[1] 
https://www.postgresql.org/message-id/20170610222026.GE18003@telsasoft.com
[2] https://www.mail-archive.com/pgsql-hackers@postgresql.org/msg316929.html

BG