Re: Add index scan progress to pg_stat_progress_vacuum

After speaking with Nathan offline, A few changes have been made to the patch.

As mentioned earlier in the thread, tracking how many indexes are processed in PARALLEL vacuum mode is not very straightforward since only the workers or leader process have ability to inspect the Vacuum shared parallel state.

The latest version of the patch introduces a shared memory to track indexes vacuumed/cleaned by each worker ( or leader ) in a PARALLEL vacuum. In order to present this data in the pg_stat_progress_vacuum view, the value of the new column "indexes_processed" is retrieved from shared memory by pg_stat_get_progress_info. For non-parallel vacuums, the value of "indexes_processed" is retrieved from the backend progress array directly.

The patch also includes the changes to implement the new view pg_stat_progress_vacuum_index which exposes the index being vacuumed/cleaned up.

postgres=# \d+ pg_stat_progress_vacuum ;
View "pg_catalog.pg_stat_progress_vacuum"
Column | Type | Collation | Nullable | Default | Storage | Description
--------------------+---------+-----------+----------+---------+----------+-------------
pid | integer | | | | plain |
datid | oid | | | | plain |
datname | name | | | | plain |
relid | oid | | | | plain |
phase | text | | | | extended |
heap_blks_total | bigint | | | | plain |
heap_blks_scanned | bigint | | | | plain |
heap_blks_vacuumed | bigint | | | | plain |
index_vacuum_count | bigint | | | | plain |
max_dead_tuples | bigint | | | | plain |
num_dead_tuples | bigint | | | | plain |
indexes_total | bigint | | | | plain | <<<-- new column
indexes_processed | bigint | | | | plain | <<<-- new column

<<<--- new view --->>>

postgres=# \d pg_stat_progress_vacuum_index
View "pg_catalog.pg_stat_progress_vacuum_index"
Column | Type | Collation | Nullable | Default
----------------+---------+-----------+----------+---------
pid | integer | | |
datid | oid | | |
datname | name | | |
indexrelid | bigint | | |
leader_pid | bigint | | |
phase | text | | |
tuples_removed | bigint | | |

On 1/26/22, 8:07 PM, "Imseih (AWS), Sami" <simseih(at)amazon(dot)com> wrote:

Attached is the latest patch and associated documentation.

This version addresses the index_ordinal_position column confusion. Rather than displaying the index position, the pg_stat_progress_vacuum view now has 2 new column(s):
index_total - this column will show the total number of indexes to be vacuumed
index_complete_count - this column will show the total number of indexes processed so far. In order to deal with the parallel vacuums, the parallel_workers ( planned workers ) value had to be exposed and each backends performing an index vacuum/cleanup in parallel had to advertise the number of indexes it vacuumed/cleaned. The # of indexes vacuumed for the parallel cleanup can then be derived the pg_stat_progress_vacuum view.

postgres=# \d pg_stat_progress_vacuum
View "pg_catalog.pg_stat_progress_vacuum"
Column | Type | Collation | Nullable | Default
----------------------+---------+-----------+----------+---------
pid | integer | | |
datid | oid | | |
datname | name | | |
relid | oid | | |
phase | text | | |
heap_blks_total | bigint | | |
heap_blks_scanned | bigint | | |
heap_blks_vacuumed | bigint | | |
index_vacuum_count | bigint | | |
max_dead_tuples | bigint | | |
num_dead_tuples | bigint | | |
index_total | bigint | | |. <<<---------------------
index_complete_count | numeric | | |. <<<---------------------

The pg_stat_progress_vacuum_index view includes:

Indexrelid - the currently vacuumed index
Leader_pid - the pid of the leader process. NULL if the process is the leader or vacuum is not parallel
tuples_removed - the amount of indexes tuples removed. The user can use this column to see that the index vacuum has movement.

postgres=# \d pg_stat_progress_vacuum_index
View "pg_catalog.pg_stat_progress_vacuum_index"
Column | Type | Collation | Nullable | Default
----------------+---------+-----------+----------+---------
pid | integer | | |
datid | oid | | |
datname | name | | |
indexrelid | bigint | | |
phase | text | | |
leader_pid | bigint | | |
tuples_removed | bigint | | |

On 1/12/22, 9:52 PM, "Imseih (AWS), Sami" <simseih(at)amazon(dot)com> wrote:

On 1/12/22, 1:28 PM, "Bossart, Nathan" <bossartn(at)amazon(dot)com> wrote:

On 1/11/22, 11:46 PM, "Masahiko Sawada" <sawada(dot)mshk(at)gmail(dot)com> wrote:
> Regarding the new pg_stat_progress_vacuum_index view, why do we need
> to have a separate view? Users will have to check two views. If this
> view is expected to be used together with and joined to
> pg_stat_progress_vacuum, why don't we provide one view that has full
> information from the beginning? Especially, I think it's not useful
> that the total number of indexes to vacuum (num_indexes_to_vacuum
> column) and the current number of indexes that have been vacuumed
> (index_ordinal_position column) are shown in separate views.

> I suppose we could add all of the new columns to
> pg_stat_progress_vacuum and just set columns to NULL as appropriate.
> But is that really better than having a separate view?

To add, since a vacuum can utilize parallel worker processes + the main vacuum process to perform index vacuuming, it made sense to separate the backends doing index vacuum/cleanup in a separate view.
Besides what Nathan suggested, the only other clean option I can think of is to perhaps create a json column in pg_stat_progress_vacuum which will include all the new fields. My concern with this approach is that it will make usability, to flatten the json, difficult for users.

> Also, I’m not sure how useful index_tuples_removed is; what can we
> infer from this value (without a total number)?

> I think the idea was that you can compare it against max_dead_tuples
> and num_dead_tuples to get an estimate of the current cycle progress.
> Otherwise, it just shows that progress is being made.

The main purpose is to really show that the "index vacuum" phase is actually making progress. Note that for certain types of indexes, i.e. GIN/GIST the number of tuples_removed will end up exceeding the number of num_dead_tuples.

Nathan

[0] https://postgr.es/m/7874FB21-FAA5-49BD-8386-2866552656C7%40amazon.com