#Postgresql alter table add column install#
#Postgresql alter table add column update#

Here are two ways to manually reproduce this behavior. These are caused by a number of different subsystems-like bulk data imports and merging offline updates-that as written require the guarantees a transaction provides. We’ve also seen transactions run into the hours as a non-rare occurrence. I’m currently working on a project that runs transactions into the minutes regularly. That is, if you have normal short transactions. Use ALTER TABLE ADD COLUMN statement to add a new column to existing table. Almost zero transaction blocking should occur. The point about these operations being “safe” is that while an ACCESS EXCLUSIVE lock is held, that lock is only needed for an incredibly short time. An ACCESS EXCLUSIVE lock is held unless explicitly noted. Note that the lock level required may differ for each subform.

There are several subforms described below.

What is also spelled out clearly in the documentation, but most discussion about these table modifications leaves out, is that most ALTER TABLE statements require an ACCESS EXCLUSIVE lock on the table being modified.ĪLTER TABLE changes the definition of an existing table. This information is all clearly spelled out in the PostgreSQL documentation for Alter Table and Modifying Tables.

#Postgresql alter table add column install#

To do that, create the index using CREATE INDEX CONCURRENTLY, and then install it as an official constraint using this syntax.

#Postgresql alter table add column update#

Default values only apply in subsequent INSERT or UPDATE commands they do not cause rows already in the table to change.Ĭoncurrent index creation can be used to avoid blocking table updates:Īdding a constraint using an existing index can be helpful in situations where a new constraint needs to be added without blocking table updates for a long time. These forms set or remove the default value for a column. So if you intend to fill the column with mostly non-default values, it’s best to add the column with no default, insert the correct values using UPDATE, and then add any desired default as described below.Īdding a default value on an existing column will only affect new rows: However, if no default is specified, PostgreSQL is able to avoid the physical update. Successful changes to running systems can happen, and the Postgres documentation is clear on what strategies avoid rewriting the table on disk.ĭefault values for new columns will force a table rewrite:Īdding a column with a default requires updating each row of the table (to store the new column value). ADD COLUMN without a default and allowing the column to be NULL. A number of articles say that some types of ALTER TABLE statements are perfectly safe, even for live, high-volume applications.