Re: Fast AT ADD COLUMN with DEFAULTs - Mailing list pgsql-hackers

On 10/9/16 11:02 PM, Corey Huinker wrote:
>
> There's actually another use case here that's potentially extremely
> valuable for warehousing and other "big data": compact
> representation of a default value.
>
>
> I too would benefit from tables having either a default value in the
> event of a NOT-NULL column being flagged null, or a flat-out constant.
>
> This would be a big win in partitioned tables where the partition can
> only hold one value of the partitioning column.

I hadn't thought of that use case... with rowcounts in the billions
becoming pretty common even the cost of a 4 byte enum starts to add up.

> I guess a constant would be a pg_type where the sole value is encoded,
> and the column itself is stored like an empty string.

Not empty string; the storage would look like NULL does today; the
difference being that we'd know that attribute wasn't NULL-able so if
it's marked as being "NULL" it actually means it has the default value.
Though obviously this would only work if the default was a Const, and
you wouldn't be able to change the default without ensuring no rows in
the table were using this trick. But I suspect there's still plenty of
scenarios where the advantage is worth it.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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