Re: Considering fractional paths in Append node

On 5/3/2025 03:27, Alexander Korotkov wrote:
> On Mon, Mar 3, 2025 at 1:04 PM Andrei Lepikhov <lepihov(at)gmail(dot)com> wrote:
>>> 2. As usage of root->tuple_fraction RelOptInfo it has been criticized,
>>> do you think we could limit this to some simple cases? For instance,
>>> check that RelOptInfo is the final result relation for given root.
>> I believe that using tuple_fraction is not an issue. Instead, it serves
>> as a flag that allows the upper-level optimisation to consider
>> additional options. The upper-level optimiser has more variants to
>> examine and will select the optimal path based on the knowledge
>> available at that level. Therefore, we're not introducing a mistake
>> here; we're simply adding extra work in the narrow case. However, having
>> only the bottom-up planning process, I don't see how we could avoid this
>> additional workload.
>
> Yes, but if we can assume root->tuple_fraction applies to result of
> Append, it's strange we apply the same tuple fraction to all the child
> rels. Latter rels should less likely be used at all and perhaps
> should have less tuple_fraction.
Of course, it may happen. But I'm not sure it is a common rule.
Using LIMIT, we usually select data according to specific clauses.
Imagine, we need TOP-100 ranked goods. Appending partitions of goods, we
will use the index on the 'rating' column. But who knows how top-rated
goods are spread across partitions? Maybe a single partition contains
all of them? So, we need to select 100 top-rated goods from each partition.
Hence, applying the same limit to each partition seems reasonable, right?

--
regards, Andrei Lepikhov