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alter table

Free space requirements for ALTER TABLE

One of the very cool features from 12.2 onwards is that the vast majority of segment operations the require rewriting the existing data can be done online without interruption to service. But whether an operation is being done online or not, since DDL operations need to be atomic, the general rule is that when an object is being reorganised this work cannot be done in place. The reorganised segment must be written elsewhere in order for the original segment to be left untouched in the event of the operation failing midway. (No-one wants their table to vanish just because someone pulled out the wrong power cable!)

General consensus from the “blog-sphere” is that you thus need enough space to hold the existing segment and space for the new segment to be created, but I thought I’d explore that a little further, and there’s a few surprises.

Column And Table Redefinition With Minimal Locking

TLDR: Note to future self… (1) Read this before you modify a table on a live PostgreSQL database. If you do it wrong then your app might totally hang. There is a right way to do it which avoids that. (2) Especially remember the lock_timeout step. Many blog posts around the ‘net are missing this and it’s very important.

Recently I was chatting with some PostgreSQL users (who, BTW, were doing rather large-scale cool stuff in PG) and they asked a question about making schema changes with minimal impact to the running application. They were specifically curious about changing a primary key from INT to BIGINT.  (Oh, you are making all your new PK fields BIGINT right?)

Column And Table Redefinition With Minimal Locking

TLDR: Note to future self… (1) Read this before you modify a table on a live PostgreSQL database. If you do it wrong then your app might totally hang. There is a right way to do it which avoids that. (2) Especially remember the lock_timeout step. Many blog posts around the ‘net are missing this and it’s very important.

Recently I was chatting with some PostgreSQL users (who, BTW, were doing rather large-scale cool stuff in PG) and they asked a question about making schema changes with minimal impact to the running application. They were specifically curious about changing a primary key from INT to BIGINT.  (Oh, you are making all your new PK fields BIGINT right?)

Column And Table Redefinition With Minimal Locking

TLDR: Note to future self… (1) Read this before you modify a table on a live PostgreSQL database. If you do it wrong then your app might totally hang. There is a right way to do it which avoids that. (2) Especially remember the lock_timeout step. Many blog posts around the ‘net are missing this and it’s very important.

Recently I was chatting with some PostgreSQL users (who, BTW, were doing rather large-scale cool stuff in PG) and they asked a question about making schema changes with minimal impact to the running application. They were specifically curious about changing a primary key from INT to BIGINT.  (Oh, you are making all your new PK fields BIGINT right?)