I have been working on AWS PostgreSQL RDS for a few years, but still not
very experienced when it comes to performance issues.  Plus RDS is slightly
different from the pure PostgreSQL.

I am trying to comprehend exactly how vacuum works.

Here is what I am trying to do.

I am purging old records from a table (500 million rows, but I am doing it
in sets of  50,000,000 with a smaller loop of 100,000).  That works just
fine.

Because of the amount of data/rows deleted, I disabled the autovacuum for
this table (I want to have control over vacuum, autovacuum does not
complete anyway due to the timeout, sizing, etc settings that I do not want
to change system wide).  I will put the autovacuum back once I am done of
course.

The issue is when I start vacuuming.  This table has 4 indexes and a PK
that I worry about.  The PK takes about 30 minutes to vacuum and two of the
indexes take about an hour each.  The problem comes in for the other 2
indexes - they take 12+ hours each to vacuum:

psql:/home/backup/leo/fws_vacuum.sql:6: INFO:  vacuuming
"public.pc_workflowlog"

psql:/home/backup/leo/fws_vacuum.sql:6: INFO:  launched 4 parallel vacuum
workers for index vacuuming (planned: 4)

psql:/home/backup/leo/fws_vacuum.sql:6: INFO:  scanned index
"pc_workflowlog_pk" to remove 50000000 row versions

DETAIL:  CPU: user: 191.03 s, system: 12.43 s, elapsed: 1711.22 s

psql:/home/backup/leo/fws_vacuum.sql:6: INFO:  scanned index
"workflowlo_u_publicid_g6uqp9lkn6e8" to remove 50000000 row versions

DETAIL:  CPU: user: 325.75 s, system: 19.75 s, elapsed: 2674.24 s

psql:/home/backup/leo/fws_vacuum.sql:6: INFO:  scanned index
"workflowlo_n_workflow_2tc9k2hdtry9v" to remove 50000000 row versions

DETAIL:  CPU: user: 312.17 s, system: 16.94 s, elapsed: 3097.88 s

*psql:/home/backup/leo/fws_vacuum.sql:6: INFO:  scanned index
"workflowlo_n_frozenseti_2kjkbjgf3c6ro" to remove 50000000 row versions*

*DETAIL:  CPU: user: 41187.70 s, system: 216.14 s, elapsed: 42749.36 s*

*psql:/home/backup/leo/fws_vacuum.sql:6: INFO:  scanned index
"workflowlo_n_userid_14kqw6qdsnndw" to remove 50000000 row versions *

*DETAIL:  CPU: user: 41280.66 s, system: 216.67 s, elapsed: 42832.16 s*

psql:/home/backup/leo/fws_vacuum.sql:6: INFO:  "pc_workflowlog": removed
50000000 row versions in 1129870 pages

I've increased max_parallel_maintenance_workers to 8 for the session and it
used parallel 4 (one for each index I assume) to handle it and the two
indexes were done in ~ an hour.  What I am trying to figure out is how to
force the other two large indexes to be vacuumed in parallel - a few
workers going against an index.  It seems it is possible to do, the index
size is large enough to kick in, but I have not been able to figure it out
yet.  Most of the parameters are at default values.

What am I missing?

I have a few other questions.  Does vacuum time depend on the number of
dead rows only and the size of the table, or does the entire storage
allocation (including dead tuples) also affect it?

Would it be more beneficial to drop the two large indexes, purge, vacuum,
and recreate the indexes after make more sense (I know it needs to be
tested)?  The reason I am doing it in stages is to make sure I have enough
time to vacuum, but maybe it would not take much longer to vacuum after the
complete purge?

Lastly, is it better to delete all the rows (500 mil) instead of doing it
in smaller batches, and vacuum only once?

The current size of the table is about 1T and the indexes add another 1.5T
to it.

Truncate is not an option as I am only deleting rows older than 6 months.
Client was not doing purging for years, but will do it after the clean up.

P.S. This is my very first post here, please advise if it is the wrong
channel.  Thank you in advance.

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