Re: [PATCH] Incremental sort (was: PoC: Partial sort)

On Mon, Sep 9, 2019 at 5:55 PM Tomas Vondra
<tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> The "patched" column means all developer GUCs disabled, so it's expected
> to produce the same plan as master (and it is). And then there's one
> column for each developer GUC. If the column is just TRUE it means the
> GUC does not affect any of the synthetic queries. There are 4 of them:
>
> - devel_add_paths_to_grouping_rel_parallel
> - devel_create_partial_grouping_paths
> - devel_gather_grouping_paths
> - devel_standard_join_search
>
> The places controlled by those GUCs are either useless, or the query
> affected by them is not included in the list of queries.

I'd previously found (in my reverse engineering efforts) the query:

select *
from tenk1 t1
join tenk1 t2 on t1.hundred = t2.hundred
join tenk1 t3 on t1.hundred = t3.hundred
order by t1.hundred, t1.twenty
limit 50;

can change plans to use incremental sort when
generate_useful_gather_paths() is added to standard_join_search().
Specifically, we get a merge join between t1 and t3 as the top level
(besides limit) node where the driving side of the join is a gather
merge with incremental sort. This does rely on these gucs set in the
test harness:

set local max_parallel_workers_per_gather=4;
set local min_parallel_table_scan_size=0;
set local parallel_tuple_cost=0;
set local parallel_setup_cost=0;

So I think we can reduce the number of unused gucs to 3.

James