![]() ![]() 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. In fact, VARCHAR and TEXT fields are stored in the same way in Postgres.ADD COLUMN ALTER TABLE data structure lock postgresql random() The size specification in VARCHAR is only used to check the size of the values which are inserted, it does not affect the disk layout. Stored in background tables so that they do not interfere with rapid Physical requirement on disk might be less. Long strings are compressed by the system automatically, so the Longer strings have 4 bytes of overhead instead of 1. Plus the actual string, which includes the space padding in the case The storage requirement for a short string (up to 126 bytes) is 1 byte Postgres, unlike some other databases, is smart enough to only use just enough space to fit the string (even using compression for longer strings), so even if your column is declared as VARCHAR(255) - if you store 40-character strings in the column, the space usage will be 40 bytes + 1 byte of overhead. THERE'S NO NEED TO RESIZE THE COLUMN IN YOUR CASE! Constraints you can change around without this table lock/rewrite problem, and they can do more integrity checking than just the weak length check. If you don't care about multi-database compatibility, consider storing your data as TEXT and add a constraint to limits its length. VARCHAR is a terrible type that exists in PostgreSQL only to comply with its associated terrible part of the SQL standard. You'll need to figure out how to truncate those manually-so you're back some locks just on oversize ones-because if someone tries to update anything on that row it's going to reject it as too big now, at the point it goes to store the new version of the row. You would be wise to do a scan over the whole table looking for rows where the length of the field is >40 characters after making the change. ![]() If you hack a lower limit in there, that will not reduce the size of existing values at all. The length check is done when values are stored into the rows. All sorts of weird cases to be aware of here. Make sure you read the Character Types section of the docs before changing this. The only way to do this officially is with ALTER TABLE, and as you've noted that change will lock and rewrite the entire table while it's running. You have to hack the database catalog data. There's a description of how to do this at Resize a column in a PostgreSQL table without changing data. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |