![]() ![]() Granting consent to receive the CYBERTEC Newsletter by electronic means is voluntary and can be withdrawn free of charge at any time.įurther information can be found in the privacy policy. Yes, I would like to receive information about new products, current offers and news about PostgreSQL via e-mail on a regular basis. Weitere Informationen finden Sie in der Datenschutzerklärung. Ich kann diese Zustimmung jederzeit widerrufen. Ja, ich möchte regelmäßig Informationen über neue Produkte, aktuelle Angebote und Neuigkeiten rund ums Thema PostgreSQL per E-Mail erhalten. In order to receive regular updates on important changes in PostgreSQL, subscribe to our newsletter, or follow us on Twitter, Facebook, or LinkedIn. If you want to learn more about PostgreSQL, see Michał’s post on practical examples of data normalization.Ĭheck out our CYBERTEC Migrator to migrate from Oracle to PostgreSQL: meet the CYBERTEC Migrator ALTER TABLE … DROP COLUMN … therefore (usually) doesn’t cause major locking issues, unless you’re having a hard time getting a short lock due to extremely high concurrency. ![]() However, we can’t see it and the space will be reclaimed later on. The size of the table doesn’t change since the physical column still exists. Test=# ALTER TABLE t_sample DROP COLUMN a3 ![]() Therefore DROP COLUMN simply marks the column as deleted: Just like ADD COLUMN, the DROP COLUMN command tries to avoid rewriting the table as much as possible. Once columns have been added, we also need to see what happens if we want to get rid of them. Note that size is not the primary problem here – locking usually is. Public | t_sample | table | hs | permanent | heap | 4223 MB | Therefore extensive table locking has to be avoided.ĭue to the fact that the additional column had to be written to disk, the size of the table has grown to over 4 GB in size. Having hundreds or possibly thousands of connections wait on this operation can seriously harm availability and cause all kinds of problems ranging from reaching connection limits to timeouts. While this is not much, it also means that the table is locked for the time being. Mind that the operation takes more than one minute. Therefore we really have to write the data to disk. However, the content of a table is supposed to be static – it must not change on the fly. Random() does not yield deterministic values. However, the situation starts to change if the value we want to add to the column is not constant anymore. Again, there is no need to persist this entry. In this case it can expect the value to be 10. However, if nothing can be found inside a row, PostgreSQL will know what the value is supposed to be. The engine knows that more columns are supposed to be around. Test=# ALTER TABLE t_sample ADD COLUMN a3 int DEFAULT 10 Īgain the operation is really quick and it does NOT affect the amount of storage we need because this additional column does not make it to disk in the first place: The system column knows that one more column should be on disk – as nothing is present the engine can safely assume that the value is indeed NULL.Ī similar trick can be applied if we add a constant default value: We do need a full table lock BUT it is a really short lock because PostgreSQL does not actually write this column to disk. The important point here is: This operation is really fast because all it does is to run some magic on the system catalog. Test=# ALTER TABLE t_sample ADD COLUMN a2 int The simplest one is to add a column without any default values: We have to address various scenarios here. But what if columns are added? How does it impact the database? Adding a column to a table in PostgreSQL Public | t_sample | table | hs | permanent | heap | 3458 MB | Schema | Name | Type | Owner | Persistence | Access method | Size | Description The initial table is around 3.4 GB in size as shown in the next listing: The following statement creates a simple table which is going to serve as a test dummy for the operations we plan to run:įROM generate_series(1, 100000000) AS a1 Getting started with ALTER TABLE… ADD COLUMN Let’s dive in and see how to run ALTER TABLE … ADD COLUMN in a way that doesn’t hurt operations due to locking or extensive I/O. Changing data structures is an important issue and happens often, therefore it’s important to understand what is really going on. Running ALTER TABLE … ADD COLUMN can have side effects which have the potential to cause serious issues in production. ADD COLUMN ALTER TABLE data structure lock postgresql random() ![]()
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