VACUUM FULL

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This document is obsolete for PostgreSQL 9.0 and above. Most of the content applies only to PostgreSQL 8.4 and below.


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VACUUM vs VACUUM FULL (PostgreSQL 8.4 and older)


This document is obsolete for PostgreSQL 9.0 and above. Most of the content applies only to PostgreSQL 8.4 and below.


The VACUUM command and associated autovacuum process are PostgreSQL's way of controlling MVCC bloat. The VACUUM command has two main forms of interest - ordinary VACUUM, and VACUUM FULL. These two commands are actually quite different and should not be confused.

VACUUM scans a table, marking tuples that are no longer needed as free space so that they can be overwritten by newly inserted or updated data. See Introduction to VACUUM, ANALYZE, EXPLAIN, and COUNT and the PostgreSQL documentation on MVCC for a detailed explanation of this. Note that you should rarely need to use the VACUUM command directly on a modern PostgreSQL database, as autovacuum should take care of it for you if properly set up.

VACUUM FULL, unlike VACUUM, touches data that has not been deleted. On pre-9.0 versions of PostgreSQL, it moves data into spaces earlier in the file that have been freed. Once it has created a free space at the end of the file, it truncates the file so that the OS knows that space is free and may be reused for other things. Moving in-use data around this way can have adverse side-effects, including taking heavy weight locks, increased i/o, and adding index bloat. On older systems, there are better ways to free space if you need to, and better ways to optimize tables (see below) so you should essentially never use VACUUM FULL on a pre-9.x system. Even on 9.x and above, the system is designed with the goal that you should never be running VACUUM FULL regularly, and doing so can have costs like huge WAL archive output and high loads on any streaming replication servers.

For clarity, 9.0 changes VACUUM FULL. As covered in the documentation, the VACUUM FULL implementation has been changed to one that's similar to using CLUSTER in older versions. This gives a slightly different set of trade-offs from the older VACUUM FULL described here. While the potential to make the database slower via index bloating had been removed by this change, it's still something you may want to avoid doing, due to the locking and general performance overhead of a VACUUM FULL.

When to use VACUUM FULL and when not to

Many people, either based on misguided advice on the 'net or on the assumption that it must be "better", periodically run VACUUM FULL on their tables. This is generally not recommended and in some cases can make your database slower, not faster.

VACUUM FULL is only needed when you have a table that is mostly dead rows - ie, the vast majority of its contents have been deleted. It should not be used for table optimization or periodic maintenance, as it's generally counterproductive. In most cases the freed space will be promptly re-allocated, possibly increasing file-system-level fragmentation and requiring file system space allocations that're slower than just re-using existing free space within a table.

When you run VACUUM FULL on a table, that table is locked for the duration of the operation, so nothing else can work with the table. VACUUM FULL is much slower than a normal VACUUM, so the table may be unavailable for a while.

More importantly, on pre-9.0 systems, while VACUUM FULL compacts the table, it does not compact the indexes - and in fact may increase their size, thus slowing them down, causing more disk I/O when the indexes are used, and increasing the amount of memory they require. A REINDEX may be required after VACUUM FULL on PostgreSQL versions older than 9.0. See the main documentation's notes on VACUUM vs VACUUM FULL.

What to use instead

If you shouldn't use regularly use VACUUM FULL (or use it at all on versions older than 9.0) ... what should you be using?

Autovacuum

If autovacuum is running frequently enough and aggressively enough, your tables should never grow ("bloat") due to unreclaimed dead rows, so you should never need to return "dead" space to the OS.

Autovacuum continues to improve dramatically with every PostgreSQL version, and is a very good reason to make sure you are running the latest version. For example, with 8.4 the free space map is now managed automatically, removing a no-longer-necessary tuning parameter and eliminating a major source of table bloat.

If autovacuum isn't doing enough to keep your tables and indexes bloat-free, tune it, don't supplement it with manual vacuuming and reindexing. You may need to increase your free space map settings (pre-8.4), tune autovacuum to run more frequently, and/or tell autovacuum to vacuum certain frequently-updated tables more aggressively than others.

VACUUM

Unless you need to return space to the OS so that other tables or other parts of the system can use that space, or you are trying to repair a table that has bloated out of control due to insufficient autovacuum, you should use VACUUM instead of VACUUM FULL.

If you need to manually VACUUM your tables at any time other than when running major admin or update tasks that rewrite large parts of your tables, you probably don't have autovacuum set up well enough.

CLUSTER

If you're trying to "optimise" your tables by packing them down and removing table bloat that's accumulated due to (say) insufficiently aggressive autovacuuming, or you're trying to return dead space in a table to the operating system, it's fine to use VACUUM FULL in PostgreSQL 9.0 and above.

Consider setting a FILLFACTOR of less than the default 100, so the rewritten table has some free space pre-alloacated within it for updates and new inserts; otherwise you'll just get file system allocations as soon as you do anything to the table.

In older versions, it's preferable to use CLUSTER. It runs a lot faster than pre-9.0 VACUUM FULL and will compact and optimise the indexes as well as the table its self. However, you will need enough free space to hold all the in-use data from the table while CLUSTER runs. As with post-9.0 VACUUM FULL, a non-default FILLFACTOR may be wise.

TRUNCATE TABLE

If you've been using VACUUM FULL to free space from a table that's periodically completely emptied using DELETE FROM tablename; (without a WHERE clause), you can use TRUNCATE TABLE to replace those two steps with one much, much faster one.

Instead of:

DELETE FROM tablename;
VACUUM FULL tablename;

write:

TRUNCATE TABLE tablename;

Please make sure to read the caveats in the notes on the TRUNCATE TABLE documentation. If TRUNCATE TABLE isn't suitable for your needs, you can use DELETE followed by CLUSTER instead.

If using DELETE on a table that is the target of foreign key references, consider adding an index to the referencing columns. That will allow checks for foreign key enforcement to avoid a sequential scan on the referencing table, making DELETE from the referenced table vastly faster.

ALTER TABLE .. SET DATA TYPE (relevant for 8.4 and below only)

This section is obsolete for PostgreSQL 9.0 and above. Skip it unless you use a very old version.

The problem with CLUSTER is that it reorders the table following an index. If the table is not already approximately in that index' order, this will take a long time because it will have to do a scattered read of the table pages over and over as it looks for each tuple. A faster alternative is to request a full table rewrite without requiring a particular order. PostgreSQL versions prior to 9.0 do not offer any direct way to invoke this operation; however, you can use the following workaround. Choose any table column, and use ALTER TABLE to change its type to the same type. This is obviously going to cause no logical change to the table, but the server will have to rewrite the table, getting rid of dead tuples while at it.

For example, assuming the an_integer_column is of type INTEGER:

ALTER TABLE your_table ALTER an_integer_column SET DATA TYPE integer;

This trick will not work in PostgreSQL versions 9.1 or later, as it detects that the change in data type is degenerate and so no rewrite is necessary.

SELECT ... INTO (relevant for 8.4 and below only)

This section is obsolete for PostgreSQL 9.0 and above. Skip it unless you use a very old version.

Sometimes it can be faster to use a SELECT ... INTO command to copy data from a bloated table into a new table, then re-create the indexes and finally rename the tables to replace the old one with the new one. It's rarely worth doing this instead of using CLUSTER, though, as CLUSTER does almost the same thing automatically and can rebuild indexes in parallel. The main reason you may want to use SELECT ... INTO instead of CLUSTER is if you don't want to sort the table.


Recovering from index bloat caused by VACUUM FULL (relevant for 8.4 and below only)

This section is obsolete for PostgreSQL 9.0 and above. Skip it unless you use a very old version.

If you have indexes badly bloated by regular use of VACUUM FULL, your best bet is usually going to be to use CLUSTER to rewrite the table and rebuild the indexes.

If you can't afford to have the table locked for that long, you can rebuild each index individually while queries continue to run on the table. PostgreSQL unfortunately doesn't have a REINDEX CONCURRENTLY command, but it can be simulated with appropriate use of CREATE INDEX ... CONCURRENTLY, ALTER INDEX ... RENAME and DROP INDEX to create new indexes, swap the old and new ones by renaming, and drop the old indexes. Note that since you can't drop some indexes, such as primary keys, this may not be a possible cleanup technique for all of them.

Originally by --Ringerc 03:48, 26 November 2009 (UTC)

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