Date: Sun, 7 Jul 2019 14:34:49 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: James Coleman <jtc331@gmail.com>
Cc: Peter Geoghegan <pg@bowt.ie>, Simon Riggs <simon@2ndquadrant.com>,
	Rafia Sabih <rafia.pghackers@gmail.com>,
	Shaun Thomas <shaun.thomas@2ndquadrant.com>,
	Dmitry Dolgov <9erthalion6@gmail.com>,
	Alexander Korotkov <a.korotkov@postgrespro.ru>,
	Tom Lane <tgl@sss.pgh.pa.us>,
	PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Subject: Re: [PATCH] Incremental sort (was: PoC: Partial sort)
Message-ID: <20190707123449.w3b2vetwv4y42evj@development>
References: 
 <CAAaqYe9pzNZnF328bFZfzR8zJK9Mvft3i6TN0NjxjCkWeVm1iQ@mail.gmail.com>
 <20190625160235.6ctryjzvw5je7cet@development>
 <CAAaqYe_JMQh0FeSSynr_j66ZNNiX3BjKoLiMgMF4zna1m0tcVg@mail.gmail.com>
 <CAH2-Wz=ptCW_dnRi3z4n8eUCzc82Y2qsd9ReWLr0-nxfhh=Hsg@mail.gmail.com>
 <CAAaqYe_pPWgcgjM5OVkwwgDPn9e63ZjFbv6ychiuHF_22B-FjQ@mail.gmail.com>
 <CAH2-Wz=L8dZNttnvSXvH6NTMWw2T5VsBBC4oU4tLTMV9CoLRvg@mail.gmail.com>
 <20190625203207.lvmmat7d2ilnr64t@development>
 <CAAaqYe-i7D+D+oih1AZ5QyyAMSWKjzNT7+eBz-RMr9i18zG96g@mail.gmail.com>
 <20190625232205.ssecrwplyb7faxbb@development>
 <CAAaqYe8PVSO3dzysZZ-RB8HRMAZ_JMOPwW--veY9ZEuTCcbe8A@mail.gmail.com>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="da2fpsfr6m73d3xi"
Content-Disposition: inline
In-Reply-To: 
 <CAAaqYe8PVSO3dzysZZ-RB8HRMAZ_JMOPwW--veY9ZEuTCcbe8A@mail.gmail.com>
User-Agent: NeoMutt/20180716-1444-295967
Precedence: bulk


--da2fpsfr6m73d3xi
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Disposition: inline

On Thu, Jul 04, 2019 at 09:29:49AM -0400, James Coleman wrote:
>On Tue, Jun 25, 2019 at 7:22 PM Tomas Vondra
><tomas.vondra@2ndquadrant.com> wrote:
>>
>> On Tue, Jun 25, 2019 at 04:53:40PM -0400, James Coleman wrote:
>> >
>> >Unrelated: if you or someone else you know that's more familiar with
>> >the parallel code, I'd be interested in their looking at the patch at
>> >some point, because I have a suspicion it might not be operating in
>...
>> So I've looked into that, and the reason seems fairly simple - when
>> generating the Gather Merge paths, we only look at paths that are in
>> partial_pathlist. See generate_gather_paths().
>>
>> And we only have sequential + index paths in partial_pathlist, not
>> incremental sort paths.
>>
>> IMHO we can do two things:
>>
>> 1) modify generate_gather_paths to also consider incremental sort for
>> each sorted path, similarly to what create_ordered_paths does
>>
>> 2) modify build_index_paths to also generate an incremental sort path
>> for each index path
>>
>> IMHO (1) is the right choice here, because it automatically does the
>> trick for all other types of ordered paths, not just index scans. So,
>> something like the attached patch, which gives me plans like this:
>...
>> But I'm not going to claim those are total fixes, it's the minimum I
>> needed to do to make this particular type of plan work.
>
>Thanks for looking into this!
>
>I intended to apply this to my most recent version of the patch (just
>sent a few minutes ago), but when I apply it I noticed that the
>partition_aggregate regression tests have several of these failures:
>
>ERROR:  could not find pathkey item to sort
>
>I haven't had time to look into the cause yet, so I decided to wait
>until the next patch revision.
>

I wanted to investigate this today, but I can't reprodure it. How are
you building and running the regression tests?

Attached is a patch adding the incremental sort below gather merge, and
also tweaking the costing. But that's mostly for and better planning
decisions, I don't get any pathkey errors even with the first patch.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services 

--da2fpsfr6m73d3xi
Content-Type: text/plain; charset=us-ascii
Content-Disposition: attachment; filename="parallel-incremental-sort-v2.patch"

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 3efc807164..d7bf33f64d 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -2719,6 +2719,8 @@ generate_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_rows)
 	{
 		Path	   *subpath = (Path *) lfirst(lc);
 		GatherMergePath *path;
+		bool		is_sorted;
+		int			presorted_keys;
 
 		if (subpath->pathkeys == NIL)
 			continue;
@@ -2727,6 +2729,26 @@ generate_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_rows)
 		path = create_gather_merge_path(root, rel, subpath, rel->reltarget,
 										subpath->pathkeys, NULL, rowsp);
 		add_path(rel, &path->path);
+
+		/* consider incremental sort */
+		is_sorted = pathkeys_common_contained_in(root->sort_pathkeys,
+												 subpath->pathkeys, &presorted_keys);
+
+		if (!is_sorted && (presorted_keys > 0))
+		{
+			/* Also consider incremental sort. */
+			subpath = (Path *) create_incremental_sort_path(root,
+															rel,
+															subpath,
+															root->sort_pathkeys,
+															presorted_keys,
+															-1);
+
+			path = create_gather_merge_path(root, rel, subpath, rel->reltarget,
+											subpath->pathkeys, NULL, rowsp);
+
+			add_path(rel, &path->path);
+		}
 	}
 }
 
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 7f820e7351..c6aa17ba67 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -1875,16 +1875,8 @@ cost_incremental_sort(Path *path,
 				   limit_tuples);
 
 	/* If we have a LIMIT, adjust the number of groups we'll have to return. */
-	if (limit_tuples > 0 && limit_tuples < input_tuples)
-	{
-		output_tuples = limit_tuples;
-		output_groups = floor(output_tuples / group_tuples) + 1;
-	}
-	else
-	{
-		output_tuples = input_tuples;
-		output_groups = input_groups;
-	}
+	output_tuples = input_tuples;
+	output_groups = input_groups;
 
 	/*
 	 * Startup cost of incremental sort is the startup cost of its first group

--da2fpsfr6m73d3xi--