Re: Monitoring multixact members growth

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My goal here is to predict where multixact members are growing the fastest so I can perform manual VACUUM FREEZE only on those tables

(typically with multi-billion row count) when system is relatively idle as opposed to just sit and wait for wraparound protection to take over

when autovacuum_multixact_freeze_max_age threshold is reached (slowing the whole system down). 

I think that you're probably approaching this wrong. Vacuum is something that you generally want to run more aggressively, not less. But to be fair, it's a very common misunderstanding that waiting to do vacuum processing until later can be a good idea... even though in fact it works in the opposite way - on systems with significant load (where it matters) - sometimes a long-running report or query that needs old row versions for its own processing might cause a lot of table and index bloat and negatively impact real-time transactional performance. (For really long-running stuff, it's sometimes better to use a snapshot of the DB or maybe a standby system that's disconnected from the primary for reporting and periodically replays logs to catch up. But obviously you start simple and don't add this complexity to the architecture until it's truly needed.)

Funny thing is that I've had to do exactly what you're asking about, as part of troubleshooting problems - but the goal wasn't to run vacuum later but to run a vacuum freeze IMMEDIATELY.  🙂  As one example, pile-ups on LWLock multixact_offset.

Here's one pageinspect query that did the trick for me. In the first line (WITH...) you change public.my_test to the table you want to inspect. This only looks at a single table and it was for troubleshooting the aforementioned wait event, so it's actually breaking down mxid's by SLRU page numbers. If you're seeing a large number of SLRU pages (lots of rows coming back) then that means you might want to proactively run a manual vacuum freeze. (And then see if you can update the app code to reduce mxid usage!)

I'm not answering your question, but thought it was a nice excuse to share a related query and pontificate a bit... hopefully useful to someone!

-Jeremy

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pg-14.4 rw root@db1=# create extension pageinspect;
CREATE EXTENSION
Time: 7.561 ms

pg-14.4 rw root@db1=# with tab_name as (select 'public.my_test' t)
select min(now()) current_timestamp, count(*) number_mxid_on_page, min(xmax) min_mxid, max(xmax) max_mxid,
  trunc((xmax)/(8192/4)) page_no
from (
  select ((attrs).t_infomask::bit(16) & x'1000'::bit(16))::int::boolean is_multixact,
     (attrs).t_infomask::bit(16) infomask,
     (attrs).t_xmax::text::integer xmax
   from (
     select page,heap_page_item_attrs(get_raw_page((select t from tab_name),page),
                                      (select t from tab_name)::regclass) attrs
     from generate_series(0,(select relpages from pg_class where oid=(select t from tab_name)::regclass)-1) page
   ) subq where (attrs).t_infomask is not null
 ) subq3 where is_multixact
group by trunc((xmax)/(8192/4));
 current_timestamp | number_mxid_on_page | min_mxid | max_mxid | page_no
-------------------+---------------------+----------+----------+---------
(0 rows)

Time: 2223.640 ms (00:02.224)

pg-14.4 rw root@db1=# begin;
BEGIN
Time: 0.466 ms

pg-14.4 rw root@db1=# select * from my_test where i<5 for update;
 i |         data
---+----------------------
 1 | XXXXXXXXXXXXXXXXXXXX
 2 | XXXXXXXXXXXXXXXXXXXX
 3 | XXXXXXXXXXXXXXXXXXXX
 4 | XXXXXXXXXXXXXXXXXXXX
(4 rows)

Time: 50.074 ms

pg-14.4 rw root@db1=# savepoint a;
SAVEPOINT
Time: 0.605 ms

pg-14.4 rw root@db1=# update my_test set i=i-10 where i<5;
UPDATE 4
Time: 49.481 ms

pg-14.4 rw root@db1=# with tab_name as (select 'public.my_test' t)
select min(now()) current_timestamp, count(*) number_mxid_on_page, min(xmax) min_mxid, max(xmax) max_mxid,
  trunc((xmax)/(8192/4)) page_no
from (
  select ((attrs).t_infomask::bit(16) & x'1000'::bit(16))::int::boolean is_multixact,
     (attrs).t_infomask::bit(16) infomask,
     (attrs).t_xmax::text::integer xmax
   from (
     select page,heap_page_item_attrs(get_raw_page((select t from tab_name),page),
                                      (select t from tab_name)::regclass) attrs
     from generate_series(0,(select relpages from pg_class where oid=(select t from tab_name)::regclass)-1) page
   ) subq where (attrs).t_infomask is not null
 ) subq3 where is_multixact
group by trunc((xmax)/(8192/4));
       current_timestamp       | number_mxid_on_page | min_mxid | max_mxid | page_no
-------------------------------+---------------------+----------+----------+---------
 2022-08-19 23:05:43.349723+00 |                   4 |        1 |        1 |       0
(1 row)

Time: 2117.555 ms (00:02.118)

pg-14.4 rw root@db1=#

-- 
http://about.me/jeremy_schneider