cybertec-postgresql / Pg_squeeze

pg_squeeze is an extension that removes unused space from a table and optionally sorts tuples according to particular index (as if CLUSTER [2] command was executed concurrently with regular reads / writes). In fact we try to replace pg_repack [1] extension.

While providing very similar functionality, pg_squeeze takes a different approach as it

  1. Implements the functionality purely on server side.

  2. Utilizes recent improvements of PostgreSQL database server.

While 1) makes both configuration and use simpler (compared to [1] which uses both server and client side code), it also allows for rather smooth implementation of unattended processing using background workers [3].

As for 2), one important difference (besides the use of background workers) is that we use logical decoding [4] instead of triggers to capture concurrent changes.

Installation

1. Set PG_CONFIG environment variable to point to pg_config command of your PostgreSQL installation.

2. make

3. sudo make install

4. Apply the following settings to postgresql.conf:

   wal_level = logical

   max_replication_slots = 1 # ... or add 1 to the current value.

   shared_preload_libraries = 'pg_squeeze' # ... or add the library to the existing ones.

5. Start the PG cluster.

6. As a superuser, run

   CREATE EXTENSION pg_squeeze;

Note: If you want remove the extension from particular database, make sure the database has no "squeeze worker" (see "Enable / disable table processing" section) running. Otherwise the DROP EXTENSION command will hang until it's cancelled.

Note: when upgrading a database cluster with pg_squeeze installed (either using pg_dumpall/restore or pg_upgrade), make sure that the new cluster has pg_squeeze in shared_preload_libraries before you upgrade, otherwise the upgrade will fail.

Register table for regular processing

First, make sure that your table has either primary key or unique constraint. This is necessary to process changes other transactions might do while "pg_squeeze" is doing its work.

To make the "pg_squeeze" extension aware of the table, you need to insert a record into "squeeze.tables" table. Once added, statistics of the table are checked periodically. Whenever the table meets criteria to be "squeezed", a "task" is added to a queue. The tasks are processed sequentially, in the order they were created.

The simplest "registration" looks like

INSERT INTO squeeze.tables (tabschema, tabname, schedule)
VALUES ('public', 'foo', ('{30}', '{22}', NULL, NULL, '{3, 5}'));

Additional columns can be specified optionally, for example:

INSERT INTO squeeze.tables
(tabschema, tabname, schedule, free_space_extra, vacuum_max_age,
max_retry)
VALUES ('public', 'bar', ('{30}', '{22}', NULL, NULL, '{3, 5}'), 30,
'2 hours', 2);

Following is the complete description of table metadata.

• "tabschema" and "tabname" are schema and table name respectively.

• "schedule" column tells when the table should be checked, and possibly squeezed. The schedule is described by a value of the following composite data type, which resembles a crontab [6] entry:

  CREATE TYPE schedule AS ( minutes minute[], hours hour[], days_of_month dom[], months month[], days_of_week dow[] );

  Here, "minutes" (0 through 59) and "hours" (0 through 23) specify the time of the check within a day, while "days_of_month" (1 through 31), "months" (1 through 12) and "days_of_week" (0 through 7, where both 0 and 7 stand for Sunday) determine the day of the check.

  The check is performed if "minute", "hour" and "month" all match the current timestamp, while NULL value means any minute, hour and month respectively. As for "days_of_month" and "days_of_week", at least one of these needs to match the current timestamp, or both need to be NULL for the check to take place.

  For example, in the entries above tell that table "public"."bar" should be checked every Wednesday and Friday at 22:30.

• "free_space_extra" is the minimum percentage of "extra free space" needed to trigger processing of the table. The "extra" adjective refers to the fact that free space derived from "fillfactor" is not reason to squeeze the table.

  For example, if "fillfactor" is equal to 60, then at least 40 percent of each page should stay free during normal operation. If you want to ensure that 70 percent of free space makes pg_squeeze interested in the table, set "free_space_extra" to 30 (that is 70 percent required to be free minus the 40 percent free due to the "fillfactor").

  Default value of "free_space_extra" is 50.

• "min_size" is the minimum disk space in megabytes the table must occupy to be eligible for processing. The default value is 8.

• "vacuum_max_age" is the maximum time since the completion of the last VACUUM to consider the free space map (FSM) fresh. Once this interval has elapsed, the portion of dead tuples might be significant and so more effort than simply checking the FSM needs to be spent to evaluate the potential effect "pg_squeeze". The default value is 1 hour.

• "max_retry" is the maximum number of extra attempts to squeeze a table if the first processing of the corresponding task failed. Typical reason to retry the processing is that table definition got changed while the table was being squeezed. If the number of retries is achieved, processing of the table is considered complete. The next task is created as soon as the next scheduled time is reached.

  The default value of "max_retry" is 0 (i.e. do not retry).

• "clustering_index" is an existing index of the processed table. Once the processing is finished, tuples of the table will be physically sorted by the key of this index.

• "rel_tablespace" is an existing tablespace the table should be moved into. NULL means that the table should stay where it is.

• "ind_tablespaces" is a two-dimensional array in which each row specifies tablespace mapping of an index. The first and the second columns represent index name and tablespace name respectively. All indexes for which no mapping is specified will stay in the original tablespace.

  Regarding tablespaces, one special case is worth to mention: if tablespace is specified for table but not for indexes, the table gets moved to that tablespace but the indexes stay in the original one (i.e. the tablespace of the table is not the default for indexes as one might expect).

• "skip_analyze" indicates that table processing should not be followed by ANALYZE command. The default value is "false", meaning ANALYZE is performed by default.

CAUTION! "squeeze.table" is the only table user should modify. If you want to change anything else, make sure you perfectly understand what you are doing.

Ad-hoc processing for any table

It's also possible to "squeeze" tables manually without registering, skipping any time and bloat checks.

Function signature: squeeze.squeeze_table(tabchema name, tabname name, clustering_index name, rel_tablespace name, ind_tablespaces name[])

Sample execution:

SELECT squeeze.squeeze_table('public', 'pgbench_accounts', null, null, null);

Enable / disable table processing

To enable automated processing, run this statement as superuser:

SELECT squeeze.start_worker();

The function starts a background worker ("scheduler worker") that periodically checks which of the registered tables should be checked according to its schedule, and creates a new task for them. Another worker ("squeeze worker") is also launched that processes those tasks - note that the processing includes check whether the table is bloated enough.

If the workers are already running for the current database, the function does not report any error but the new workers will exit immediately.

If the background workers are running, you can use the following statement to stop them:

SELECT squeeze.stop_worker();

CAUTION! Only the functions mentioned in this section are considered user interface. If you want to call any other one, make sure you perfectly understand what you're doing.

If you want the background workers to start automatically during startup of the whole PostgreSQL cluster, add entries like this to postgresql.conf file

squeeze.worker_autostart = 'my_database your_database'
squeeze.worker_role = postgres

Next time you start the cluster, one worker will be launched for "my_database" and one for "your_database". If you take this approach, note that any worker will either reject to start or will stop without doing any work if

 1. The "pg_squeeze" extension does not exist in the database..

 or

 2. squeeze.worker_role parameter specifies role which does not have the
 superuser privileges.

Note: The functions/configuration variables explained above use singular form of the word "worker" although there are actually two workers. This is because only one worker existed in the previous versions of pg_squeeze, which ensured both scheduling and execution of the tasks. This implementation change is probably not worth to force all users to adjust their configuration files during upgrade.

Control the impact on other backends

Although the table being squeezed is available for both read and write operations by other transactions most of the time, exclusive lock is needed to finalize the processing. If pg_squeeze occasionally seems to block access to tables too much, consider setting "squeeze.max_xlock_time" GUC parameter. For example

SET squeeze.max_xlock_time TO 100;

tells that the exclusive lock shouldn't be held for more than 0.1 second (100 milliseconds). If more time is needed for the final stage, pg_squeeze releases the exclusive lock, processes changes committed by other transactions in between and tries the final stage again. Error is reported if the lock duration is exceeded a few more times. If that happens, you should either increase the setting or schedule processing of the problematic table to a different daytime, when the write activity is lower.

Monitoring

"squeeze.log" table contains one entry per successfully squeezed table.

"squeeze.errors" table contains errors that happened during squeezing. An usual problem reported here is that someone changed definition (e.g. added or removed column) of the table whose processing was just in progress.

Unregister table

If particular table should no longer be subject to periodical squeeze, simply delete the corresponding row from "squeeze.tables" table.

It's also a good practice to unregister table that you're going to drop, although the background worker does unregister non-existing tables periodically.

Upgrade from pg_squeeze 1.2.x

CAUTION! As there's no straightforward way to migrate the scheduling information (see the notes on the "schedule" column of the "squeeze"."tables" table) automatically, and as the "schedule" column must not contain NULL values, the upgrade deletes the contents of the "squeeze"."tables" table. Please export the table contents to a file before you perform the upgrade and configure the checks of those tables again as soon as the upgrade is done.

1. Set PG_CONFIG environment variable to point to pg_config command of your PostgreSQL installation.

2. make

3. Stop the PG instance

4. sudo make install

5. Restart the PG instance

6. Connect to each database containing pg_squeeze 1.2.x and run this command:

   ALTER EXTENSION pg_squeeze UPDATE;

Notes on Concurrency

1. The extension does not prevent other transactions from altering table at certain stages of the processing. If a "disruptive command" (i.e. ALTER TABLE, VACUUM FULL, CLUSTER or TRUNCATE) manages to commit before the squeeze could finish, the squeeze_table() function aborts and all changes done to the table are rolled back. The "max_retry" column of "squeeze.tables" table determines how many times the squeeze worker will retry. Besides that, change of schedule might help you to avoid disruptions.

2. Like [1], pg_squeeze also changes visibility of rows and thus allows for MVCC-unsafe behavior described in the first paragraph of [5].

References

[1] https://reorg.github.io/pg_repack/

[2] https://www.postgresql.org/docs/13/static/sql-cluster.html

[3] https://www.postgresql.org/docs/13/static/bgworker.html

[4] https://www.postgresql.org/docs/13/static/logicaldecoding.html

[5] https://www.postgresql.org/docs/13/static/mvcc-caveats.html

[6] https://www.freebsd.org/cgi/man.cgi?query=crontab&sektion=5&apropos=0&manpath=FreeBSD+12.1-RELEASE+and+Ports

Authors

The extension was developed by Cybertec Schönig & Schönig GmbH (https://www.cybertec-postgresql.com). Please use this page to report issues: https://github.com/cybertec-postgresql/pg_squeeze/issues