Re: estimating # of distinct values

On Tue, Jan 18, 2011 at 12:23 PM, Jim Nasby <jim@nasby.net> wrote:
> On Jan 17, 2011, at 8:11 PM, Robert Haas wrote:
>> On Mon, Jan 17, 2011 at 7:56 PM, Jim Nasby <jim@nasby.net> wrote:
>>> - Forks are very possibly a more efficient way to deal with TOAST than having separate tables. There's a fair
amountof overhead we pay for the current setup. 
>>
>> That seems like an interesting idea, but I actually don't see why it
>> would be any more efficient, and it seems like you'd end up
>> reinventing things like vacuum and free space map management.
>
> The FSM would take some effort, but I don't think vacuum would be that hard to deal with; you'd just have to free up
thespace in any referenced toast forks at the same time that you vacuumed the heap. 

How's that different from what vacuum does on a TOAST table now?

>>> - Dynamic forks would make it possible to do a column-store database, or at least something approximating one.
>>
>> I've been wondering whether we could do something like this by
>> treating a table t with columns pk, a1, a2, a3, b1, b2, b3 as two
>> tables t1 and t2, one with columns pk, a1, a2, a3 and the other with
>> columns pk, b1, b2, b3.  SELECT * FROM t would be translated into
>> SELECT * FROM t1, t2 WHERE t1.pk = t2.pk.
>
> Possibly, but you'd be paying tuple overhead twice, which is what I was looking to avoid with forks.

What exactly do you mean by "tuple overhead"?

--
Robert Haas
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