Waiting for PostgreSQL 13 – Allow vacuum command to process indexes in parallel.

On 20th of January 2020, Amit Kapila committed patch:

Allow vacuum command to process indexes in parallel.
 
This feature allows the vacuum to leverage multiple CPUs in order to
process indexes.  This enables us to perform index vacuuming and index
cleanup with background workers.  This adds a PARALLEL option to VACUUM
command where the user can specify the number of workers that can be used
to perform the command which is limited by the number of indexes on a
table.  Specifying zero as a number of workers will disable parallelism.
This option can't be used with the FULL option.
 
Each index is processed by at most one vacuum process.  Therefore parallel
vacuum can be used when the table has at least two indexes.
 
The parallel degree is either specified by the user or determined based on
the number of indexes that the table has, and further limited by
max_parallel_maintenance_workers.  The index can participate in parallel
vacuum iff it's size is greater than min_parallel_index_scan_size.
 
Author: Masahiko Sawada and Amit Kapila
 
Mahendra Singh and Sergei Kornilov
 
Discussion:
https://postgr.es/m/CAD21AoDTPMgzSkV4E3SFo1CH_x50bf5PqZFQf4jmqjk-C03BWg@mail.gmail.com
https://postgr.es/m/CAA4eK1J-VoR9gzS5E75pcD-OH0mEyCdp8RihcwKrcuw7J-Q0+w@mail.gmail.com

Continue reading Waiting for PostgreSQL 13 – Allow vacuum command to process indexes in parallel.

Waiting for PostgreSQL 12 – Allow VACUUM to be run with index cleanup disabled.

On 4th of April 2019, Robert Haas committed patch:

Allow VACUUM to be run with index cleanup disabled.
 
 
This commit adds a new reloption, vacuum_index_cleanup, which
controls whether index cleanup is performed for a particular
relation by default.  It also adds a new option to the VACUUM
command, INDEX_CLEANUP, which can be used to override the
reloption.  If neither the reloption nor the VACUUM option is
used, the default is true, as before.
 
Masahiko Sawada, reviewed and tested by Nathan Bossart, Alvaro
Herrera, Kyotaro Horiguchi, Darafei Praliaskouski, and me.
The wording of the documentation is mostly due to me.
 
Discussion: http://postgr.es/m/CAD21AoAt5R3DNUZSjOoXDUY=naYPUOuffVsRzuTYMz29yLzQCA@mail.gmail.com

Continue reading Waiting for PostgreSQL 12 – Allow VACUUM to be run with index cleanup disabled.

Waiting for 9.6 – Add simple VACUUM progress reporting.

On 15th of March, Robert Haas committed patch:

Add simple VACUUM progress reporting. 
 
There's a lot more that could be done here yet - in particular, this
reports only very coarse-grained information about the index vacuuming
phase - but even as it stands, the new pg_stat_progress_vacuum can
tell you quite a bit about what a long-running vacuum is actually
doing.
 
Amit Langote and Robert Haas, based on earlier work by Vinayak Pokale
and Rahila Syed.

Continue reading Waiting for 9.6 – Add simple VACUUM progress reporting.

Waiting for 9.6 – Add a generic command progress reporting facility.

On 9th of March, Robert Haas committed patch:

Add a generic command progress reporting facility.
 
Using this facility, any utility command can report the target relation
upon which it is operating, if there is one, and up to 10 64-bit
counters; the intent of this is that users should be able to figure out
what a utility command is doing without having to resort to ugly hacks
like attaching strace to a backend.
 
As a demonstration, this adds very crude reporting to lazy vacuum; we
just report the target relation and nothing else.  A forthcoming patch
will make VACUUM report a bunch of additional data that will make this
much more interesting.  But this gets the basic framework in place.
 
Vinayak Pokale, Rahila Syed, Amit Langote, Robert Haas, reviewed by
Kyotaro Horiguchi, Jim Nasby, Thom Brown, Masahiko Sawada, Fujii Masao,
and Masanori Oyama.

Continue reading Waiting for 9.6 – Add a generic command progress reporting facility.

Waiting for 9.5 – vacuumdb: enable parallel mode

On 23rd of January, Alvaro Herrera committed patch:

vacuumdb: enable parallel mode
 
This mode allows vacuumdb to open several server connections to vacuum
or analyze several tables simultaneously.
 
Author: Dilip Kumar.  Some reworking by Álvaro Herrera
Reviewed by: Jeff Janes, Amit Kapila, Magnus Hagander, Andres Freund

Continue reading Waiting for 9.5 – vacuumdb: enable parallel mode

What does “Fix VACUUM’s tests to see whether it can update relfrozenxid” really mean?

In release notes to latest release you can find:

Fix VACUUM's tests to see whether it can update relfrozenxid (Andres Freund)
 
In some cases VACUUM (either manual or autovacuum) could incorrectly advance
a table's relfrozenxid value, allowing tuples to escape freezing, causing
those rows to become invisible once 2^31 transactions have elapsed. The
probability of data loss is fairly low since multiple incorrect advancements
would need to happen before actual loss occurs, but it's not zero. In 9.2.0
and later, the probability of loss is higher, and it's also possible to get
"could not access status of transaction" errors as a consequence of this
bug. Users upgrading from releases 9.0.4 or 8.4.8 or earlier are not
affected, but all later versions contain the bug.
 
The issue can be ameliorated by, after upgrading, vacuuming all tables in
all databases while having vacuum_freeze_table_age set to zero. This will
fix any latent corruption but will not be able to fix all pre-existing data
errors. However, an installation can be presumed safe after performing this
vacuuming if it has executed fewer than 2^31 update transactions in its
lifetime (check this with SELECT txid_current() < 2^31).

What does it really mean?

Continue reading What does “Fix VACUUM's tests to see whether it can update relfrozenxid" really mean?

Bloat removal by tuples moving

Looong time ago, I wrote a piece about removing bloat by moving rows away from the end of table, and vacuuming it.

This is/was very slow, and was optimized (to some extent) by Nathan Thom, but his blogpost vanished. Besides, later on we got great tool: pg_reorg (or, as it's currently named: pg_repack).

But recently I was in position where I couldn't pg_reorg. So I had to look for other way. And I found it 🙂

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Explaining the unexplainable – part 5

In previous posts in this series, I talked about how to read EXPLAIN output, and what each line (operation/node) means.

Now, in the final post, I will try to explain how it happens that Pg chooses “Operation X" over “Operation Y".

Continue reading Explaining the unexplainable – part 5