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.
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