Re: Spilling hashed SetOps and aggregates to disk

On 06/07/2018 02:11 AM, David Rowley wrote:
> On 7 June 2018 at 08:11, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
>> On 06/06/2018 04:11 PM, Andres Freund wrote:
>>> Consider e.g. a scheme where we'd switch from hashed aggregation to
>>> sorted aggregation due to memory limits, but already have a number of
>>> transition values in the hash table. Whenever the size of the transition
>>> values in the hashtable exceeds memory size, we write one of them to the
>>> tuplesort (with serialized transition value). From then on further input
>>> rows for that group would only be written to the tuplesort, as the group
>>> isn't present in the hashtable anymore.
>>>
>>
>> Ah, so you're suggesting that during the second pass we'd deserialize
>> the transition value and then add the tuples to it, instead of building
>> a new transition value. Got it.
> 
> Having to deserialize every time we add a new tuple sounds terrible
> from a performance point of view.
> 

I don't think Andres was suggesting that. I think the scheme was to sort
the tuples and read all the tuples for a group at once (so we would
deserialize just once).

> Can't we just:
> 
> 1. HashAgg until the hash table reaches work_mem.
> 2. Spill the entire table to disk.
> 3. Destroy the table and create a new one.
> 4. If more tuples: goto 1
> 5. Merge sort and combine each dumped set of tuples.
> 

... and this is pretty much what Jeff Davis suggested, I think. The
trouble is, each of those cases behaves nicely/terribly in different
corner cases.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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