What is the effect on record size of reordering columns in PostgreSQL?

Question

Since Postgres can only add columns at the end of tables, I end up re-ordering by adding new columns at the end of the table, setting them equal to existing columns, and then dropping the original columns.

So, what does PostgreSQL do with the memory that's freed by dropped columns? Does it automatically re-use the memory, so a single record consumes the same amount of space as it did before? But that would require a re-write of the whole table, so to avoid that, does it just keep a bunch of blank space around in each record?

Answer 1

From the docs:

The DROP COLUMN form does not physically remove the column, but simply makes it invisible to SQL operations. Subsequent insert and update operations in the table will store a null value for the column. Thus, dropping a column is quick but it will not immediately reduce the on-disk size of your table, as the space occupied by the dropped column is not reclaimed. The space will be reclaimed over time as existing rows are updated.

You'll need to do a CLUSTER followed by a VACUUM FULL to reclaim the space.

Answer 2

Why do you "reorder" ? There is no order in SQL, it doesn't make sence. If you need a fixed order, tell your queries what order you need or use a view, that's what views are made for.

Diskspace will be used again after vacuum, auto_vacuum will do the job. Unless you disabled this process.

Your current approach will kill overall performance (table locks), indexes have to be recreated, statistics go down the toilet, etc. etc. And in the end, you end up with the same situation you allready had. So why the effort?

Answer 3

_{The question is old, but since both answers are wrong or misleading, I'll add another one.}

When updating a row, Postgres writes a new row version and the old one is eventually removed by VACUUM after no running transaction can see it any more.

Plain VACUUM does not return disk space from the physical file that contains the table to the system, unless it finds completely dead or empty blocks at the physical end of the table. You need to run VACUUM FULL or CLUSTER to aggressively compact the table and return excess space to the system. This is not typically desirable in normal operation. Postgres can re-use dead tuples to keep new row versions on the same data page, which benefits performance.

In your case, since you update every row, the size of the table is doubled (from its minimum size). It's advisable to run VACUUM FULL or CLUSTER to return the bloat to the system.
Both take an exclusive lock on the table. If that interferes with concurrent access, consider pg_repack, which can do the same without exclusive locks.

To clarify: Running CLUSTER reclaims the space completely. No VACUUM FULL is needed after CLUSTER (and vice versa).

More details:

• PostgreSQL 9.2: vacuum returning disk space to operating system