we have a database which we store some small files temporarily before they are pushed to S3. The problem I'm having at the moment is that once we clear the biniary in Postgresql (setting the binary column value = null) it does not seem to free up the memory. Are we missing anything?
You would need to perform a vacuum full in order to reclaim the free space, or just a vacuum to be able to re-use the space.
The doc says:
Plain VACUUM (without FULL) simply reclaims space and makes it
available for re-use. This form of the command can operate in parallel
with normal reading and writing of the table, as an exclusive lock is
not obtained. However, extra space is not returned to the operating
system (in most cases); it's just kept available for re-use within the
same table. VACUUM FULL rewrites the entire contents of the table into
a new disk file with no extra space, allowing unused space to be
returned to the operating system. This form is much slower and
requires an exclusive lock on each table while it is being processed.
Let's emphasis that this is true for both delete or update commands.
The FULL option is not recommended for routine use, but might be
useful in special cases. An example is when you have deleted or
updated most of the rows in a table and would like the table to
physically shrink to occupy less disk space and allow faster table
scans. VACUUM FULL will usually shrink the table more than a plain
VACUUM would.
Related
Is it possible to run PostgreSQL 11's VACUUM FULL for a short while and then get some benefit? Or does cancelling it midway cause all of its progress to be lost?
I've read about pg_repack (https://aws.amazon.com/blogs/database/remove-bloat-from-amazon-aurora-and-rds-for-postgresql-with-pg_repack/) but the way it works (creating new tables, copying data, etc.) sounds risky to me. Is that my paranoia or is it safe to use on a production database?
Backstory: I am working with a very large production database on AWS Aurora PostgreSQL 11. Many of the tables had tens of millions of records but have been pruned down significantly. The problem is that the table sizes on disk (and in the snapshots) have not decreased because DELETE and VACUUM (without FULL) do not shrink the files. These tables are in the hundreds of gigabytes range and I'm afraid running VACUUM FULL will take forever.
No. VACUUM FULL writes a new physical file for the table. Stopping it before it finishes voids the work done so far.
The manual:
VACUUM FULL rewrites the entire contents of the table into a new
disk file with no extra space, allowing unused space to be returned to
the operating system. This form is much slower and requires an ACCESS EXCLUSIVE lock on each table while it is being processed.
This is the main reason why community tools like pg_repack or pg_squeeze were created, which are more flexible, less blocking, and often faster, too. (I don't think pg_squeeze is available for Aurora, yet).
pg_repack might be a bit of overkill. You can instead just delete tuples from the end of the table and reinsert them towards the front of the table (reusing space already marked as free by an earlier VACUUM), at which point another ordinary VACUUM can truncate away the free space at the end of the table.
with d as (delete from mytable where ctid>='(50000,1)' returning *)
insert into mytable select * from d;
You can use pg_freespacemap to figure out where would be a good place to start the ctid criterion at.
This might not behave well if you have triggers or FK constraints, and it might bloat indexes such they would need to be rebuilt (but they probably do anyway). It will also lock a large number rows at a time, for the duration it takes for the re-insert to run and commit.
Improvements made since v11 will make the ctid scan more efficient than it will be in v11.
I have a table called EVENTS on my PostgreSQL DB schema.
It is empty, i.e. when I execute
SELECT * FROM EVENTS
I get an empty results set.
Nonetheless, the table occupies 5MB of disk space.
I'm executing
SELECT round(pg_total_relation_size('events') / 1024.0 / 1024.0, 2)
And I'm getting 5.13MB.
I tried to explicitly run VACUUM, but it didn't change anything.
Any ideas?
Truncate the table:
truncate events;
From the documentation:
TRUNCATE quickly removes all rows from a set of tables. It has the same effect as an unqualified DELETE on each table, but since it does not actually scan the tables it is faster. Furthermore, it reclaims disk space immediately, rather than requiring a subsequent VACUUM operation. This is most useful on large tables.
If you want to immediately reclaim disk space keeping existing rows of a non-empty table, you can use vacuum:
vacuum full events;
This locks exclusively the table and rewrite it (in fact, creates a new copy and drops the old one). It is an expensive operation and generally not recommended on larger tables.
In RDBMS some redundant usage of the disk space is a normal state. If you have a properly configured autovacuum daemon the unused space will be used when new rows are inserted.
If you have dead rows or bloat in your table, VACUUM will not actually reclaim its memory but make it reusable and this is used when you insert data to the table next time.
To reclaim the memory used, try
VACUUM FULL events;
I just want to check that my understanding of these two things is correct. If it's relevant, I am using Postgres 9.4.
I believe that one should vacuum a database when looking to reclaim space from the filesystem, e.g. periodically after deleting tables or large numbers of rows.
I believe that one should analyse a database after creating new indexes, or (periodically) after adding or deleting large numbers of rows from a table, so that the query planner can make good calls.
Does that sound right?
vacuum analyze;
collects statistics and should be run as often as much data is dynamic (especially bulk inserts). It does not lock objects exclusive. It loads the system, but is worth of. It does not reduce the size of table, but marks scattered freed up place (Eg. deleted rows) for reuse.
vacuum full;
reorganises the table by creating a copy of it and switching to it. This vacuum requires additional space to run, but reclaims all not used space of the object. Therefore it requires exclusive lock on the object (other sessions shall wait it to complete). Should be run as often as data is changed (deletes, updates) and when you can afford others to wait.
Both are very important on dynamic database
Correct.
I would add that you can change the value of the default_statistics_target parameter (default to 100) in the postgresql.conf file to a higher number, after which, you should restart your server and run analyze to obtain more accurate statistics.
I use PostgreSQL on an embedded system with limited drive space. Now the DB-drive is full. When I delete data, it doesn't seem to free up any space. I tried to VACUUM FULL, but that requires space. So does deleting the last remaining index.
Any ideas on how to free up space without randomly deleting stuff? I can afford to lose some of the data from back when, but I can't seem to actually do it, since there isn't enough space to VACUUM FULL.
PostgreSQL uses MVCC model which means that deleted records mark their space as free (after the transaction which deleted them had been committed) but it is still reserved by the table.
Prior to PostgreSQL 9.0, VACUUM FULL used to move the data inside the table without need for additional space.
In PostgreSQL 9.0, behavior of VACUUM FULL had changed and now it requires additional space for the full copy of the table.
You may try to drop the indexes from the tables and vacuum them one by one, starting from the least one.
The easiest answer at this point would be to dump the database to a different drive/computer (for instance, using pg_dump, or pg_dumpall if you have more than one db, and keeping in mind things like Large Objects that need special backup/restore processes) then drop and recreate the database.
If there's a tiny bit of space left, you might try vacuum full smallesttable, which might be able to finish and free up some space to vacuum the next smallest table, and so on.
If you end up filling the drive completely, the database server will probably refuse to start and you won't be able to do either of those. In that case, you could move the entire data directory to another computer with the same CPU architecture and more disk space, then start postgresql there to perform the vacuum.
In certain situations VACUUM (not full) can reclaim some disk space. (I think it will return pages that are totally dead to the OS.) That might free up enough space to begin with the VACUUM FULL. But it's not a good idea to let one table grow to more than the amount of disk free space.
Since Postgres can only add columns at the end of tables, I end up re-ordering by adding new columns at the end of the table, setting them equal to existing columns, and then dropping the original columns.
So, what does PostgreSQL do with the memory that's freed by dropped columns? Does it automatically re-use the memory, so a single record consumes the same amount of space as it did before? But that would require a re-write of the whole table, so to avoid that, does it just keep a bunch of blank space around in each record?
The question is old, but since both answers are wrong or misleading, I'll add another one.
When updating a row, Postgres writes a new row version and the old one is eventually removed by VACUUM after no running transaction can see it any more.
Plain VACUUM does not return disk space from the physical file that contains the table to the system, unless it finds completely dead or empty blocks at the physical end of the table. You need to run VACUUM FULL or CLUSTER to aggressively compact the table and return excess space to the system. This is not typically desirable in normal operation. Postgres can re-use dead tuples to keep new row versions on the same data page, which benefits performance.
In your case, since you update every row, the size of the table is doubled (from its minimum size). It's advisable to run VACUUM FULL or CLUSTER to return the bloat to the system.
Both take an exclusive lock on the table. If that interferes with concurrent access, consider pg_repack, which can do the same without exclusive locks.
To clarify: Running CLUSTER reclaims the space completely. No VACUUM FULL is needed after CLUSTER (and vice versa).
More details:
PostgreSQL 9.2: vacuum returning disk space to operating system
From the docs:
The DROP COLUMN form does not physically remove the column, but simply makes it invisible to SQL operations. Subsequent insert and update operations in the table will store a null value for the column. Thus, dropping a column is quick but it will not immediately reduce the on-disk size of your table, as the space occupied by the dropped column is not reclaimed. The space will be reclaimed over time as existing rows are updated.
You'll need to do a CLUSTER followed by a VACUUM FULL to reclaim the space.
Why do you "reorder" ? There is no order in SQL, it doesn't make sence. If you need a fixed order, tell your queries what order you need or use a view, that's what views are made for.
Diskspace will be used again after vacuum, auto_vacuum will do the job. Unless you disabled this process.
Your current approach will kill overall performance (table locks), indexes have to be recreated, statistics go down the toilet, etc. etc. And in the end, you end up with the same situation you allready had. So why the effort?