Contents
What does it mean to vacuum a database?
A vacuum is used for recovering space occupied by “dead tuples” in a table. A dead tuple is created when a record is either deleted or updated (a delete followed by an insert). As rows are inserted, deleted, and updated in a database, the column statistics also change.
What is vacuum freeze in Postgres?
Vacuum freeze marks a table’s contents with a very special transaction timestamp that tells postgres that it does not need to be vacuumed, ever. Next update this frozen id will disappear. For instance, the template0 database is frozen because it never changes (by default you cannot connect to it.)
How long does VACUUM full take?
For various reasons, it’s not uncommon for them to take 15-20 days, and we’ve seen some take upwards of 40 days. Vacuum has historically been a somewhat opaque process: it runs for a long time until it finishes, at which point it logs some basic information about what happened.
When should I VACUUM full?
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.
How does the vacuum function work in PostgreSQL?
The VACUUM operation performs below a series of operation within a ShareUpdateExclusive lock: Scan all pages of all tables (or specified table) of the database to get all dead tuples. Freeze old tuples if required. Remove the index tuple pointing to the respective DEAD tuples.
Why is it a vacuum not needed with MySQL compared to the?
The MySQL approximation of PostgreSQL’s vacuum is OPTIMIZE TABLE tablename (MySQL docs). It performs a similar function in MySQL as PostgreSQL in that, depending on the storage engine used, it reclaims unused space, reorganizes indexes and tables, and defragments data files.
What happens when you delete a tuple in PostgreSQL?
Whenever DELETE operations are performed, it marks the existing tuple as DEAD instead of physically removing those tuples. Similarly, whenever UPDATE operation is performed, it marks the corresponding existing tuple as DEAD and inserts a new tuple (i.e. UPDATE operation = DELETE + INSERT).
Why does index cleanup not work in PostgreSQL?
However, if index cleanup is not performed regularly, performance may suffer, because as the table is modified, indexes will accumulate dead tuples and the table itself will accumulate dead line pointers that cannot be removed until index cleanup is completed.