Thread: Push down Aggregates below joins
Currently, the planner always first decides the scan/join order, and
adds Group/Agg nodes on top of the joins. Sometimes it would be legal,
and beneficial, to perform the aggregation below a join. I've been
hacking on a patch to allow that.
For example:
create temp table a (id int4 primary key);
create temp table b (id int4);
insert into a select g from generate_series(1, 1000) g;
insert into b select g/10 from generate_series(1, 10000) g;
analyze a,b;
explain select b.id from a, b where a.id = b.id group by b.id;
Currently, you get a plan like this:
QUERY PLAN
-----------------------------------------------------------------------
HashAggregate (cost=323.64..333.65 rows=1001 width=4)
Group Key: b.id
-> Hash Join (cost=27.50..298.66 rows=9990 width=4)
Hash Cond: (b.id = a.id)
-> Seq Scan on b (cost=0.00..145.00 rows=10000 width=4)
-> Hash (cost=15.00..15.00 rows=1000 width=4)
-> Seq Scan on a (cost=0.00..15.00 rows=1000 width=4)
(7 rows)
With the patch, you get a plan like this:
QUERY PLAN
-------------------------------------------------------------------------
Hash Join (cost=192.52..221.27 rows=9990 width=4)
Hash Cond: (a.id = b.id)
-> Seq Scan on a (cost=0.00..15.00 rows=1000 width=4)
-> Hash (cost=180.01..180.01 rows=1001 width=4)
-> HashAggregate (cost=170.00..180.01 rows=1001 width=4)
Group Key: b.id
-> Seq Scan on b (cost=0.00..145.00 rows=10000 width=4)
(7 rows)
This is faster, because you need to join fewer rows. (That's not
reflected in the cost estimates above; cost estimation is not done
properly in this prototype yet.)
Implementation
--------------
Move the responsibility of planning aggregates from the "upper stages",
in grouping_planner(), into scan/join planning, in query_planner().
In query_planner(), after building the RelOptInfo for a scan or join
rel, also build a grouped RelOptInfo to shadow each RelOptInfo (if
aggregation can be done at that rel). The grouped RelOptInfo is stored
in a new 'grouped_rel' field in the parent RelOptInfo.
A grouped rel holds Paths where the grouping/aggregation is already
performed at that node, or below it. For a base rel, it represents
performing the aggregation on top of the scan, i.e. the Paths are like
Agg(Scan). For a grouped join rel, the paths look like an Agg(Join(A,
B)), or Join(Agg(A), B).
The first three of the attached patches just move existing code around.
The fourth patch contains the actual feature.
This is still a rough prototype, but any thoughts on the general approach?
- Heikki
Attachment
On 06/20/2018 10:12 PM, Heikki Linnakangas wrote: > Currently, the planner always first decides the scan/join order, and > adds Group/Agg nodes on top of the joins. Sometimes it would be legal, > and beneficial, to perform the aggregation below a join. I've been > hacking on a patch to allow that. > There was a patch [1] from Antonin Houska aiming to achieve something similar. IIRC it aimed to push the aggregate down in more cases, leveraging the partial aggregation stuff. I suppose your patch only aims to do the pushdown when the two-phase aggregation is not needed? regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
On 06/20/2018 10:12 PM, Heikki Linnakangas wrote: > Currently, the planner always first decides the scan/join order, and > adds Group/Agg nodes on top of the joins. Sometimes it would be legal, > and beneficial, to perform the aggregation below a join. I've been > hacking on a patch to allow that. > There was a patch [1] from Antonin Houska aiming to achieve something similar. IIRC it aimed to push the aggregate down in more cases, leveraging the partial aggregation stuff. I suppose your patch only aims to do the pushdown when the two-phase aggregation is not needed? [1] https://www.postgresql.org/message-id/flat/9666.1491295317@localhost regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote: > On 06/20/2018 10:12 PM, Heikki Linnakangas wrote: > > Currently, the planner always first decides the scan/join order, and > > adds Group/Agg nodes on top of the joins. Sometimes it would be legal, > > and beneficial, to perform the aggregation below a join. I've been > > hacking on a patch to allow that. > > > > There was a patch [1] from Antonin Houska aiming to achieve something > similar. IIRC it aimed to push the aggregate down in more cases, > leveraging the partial aggregation stuff. Yes, I interrupted the work when it become clear that it doesn't find its way into v11. I'm about to get back to it next week, and at least rebase it so it can be applied to the current master branch. -- Antonin Houska Cybertec Schönig & Schönig GmbH Gröhrmühlgasse 26, A-2700 Wiener Neustadt Web: https://www.cybertec-postgresql.com
On 21/06/18 09:11, Antonin Houska wrote: > Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote: > >> On 06/20/2018 10:12 PM, Heikki Linnakangas wrote: >>> Currently, the planner always first decides the scan/join order, and >>> adds Group/Agg nodes on top of the joins. Sometimes it would be legal, >>> and beneficial, to perform the aggregation below a join. I've been >>> hacking on a patch to allow that. >> >> There was a patch [1] from Antonin Houska aiming to achieve something >> similar. IIRC it aimed to push the aggregate down in more cases, >> leveraging the partial aggregation stuff. > > Yes, I interrupted the work when it become clear that it doesn't find its way > into v11. I'm about to get back to it next week, and at least rebase it so it > can be applied to the current master branch. Ah, cool! I missed that thread earlier. Yes, seems like we've been hacking on the same feature. Let's compare! I've been using this paper as a guide: "Including Group-By in Query Optimization", by Surajit Chaudhuri and Kyuseok Shim: https://pdfs.semanticscholar.org/3079/5447cec18753254edbbd7839f0afa58b2a39.pdf Using the terms from that paper, my patch does only "Invariant Grouping transormation", while yours can do "Simple Coalescing Grouping", which is more general. In layman terms, my patch can push the Aggregate below a join, while your patch can also split an Aggregate so that you do a partial aggregate below the join, and a final stage above it. My thinking was to start with the simpler Invariant Grouping transformation first, and do the more advanced splitting into partial aggregates later, as a separate patch. Doing partial aggregation actually made your patch simpler in some ways, though. I had to make some changes to grouping_planner(), because in my patch, it is no longer responsible for aggregation. In your patch, it's still responsible for doing the final aggregation. There's some difference in the way we're dealing with the grouped RelOptInfos. You're storing them completely separately, in PlannerInfo's new 'simple_grouped_rel_array' array and 'join_grouped_rel_list/hash'. I'm attaching each grouped RelOptInfo to its parent. I got away with much less code churn in allpaths.c, indxpath.c, joinpath.c. You're adding new 'do_aggregate' flags to many functions. I'm not sure if you needed that because you do partial aggregation and I don't, but it would be nice to avoid it. You're introducing a new GroupedVar expression to represent Aggrefs between the partial and final aggregates, while I'm just using Aggref directly, above the aggregate node. I'm not thrilled about introducing an new Var-like expression. We already have PlaceHolderVars, and I'm always confused on how those work. But maybe that's necessary to supprot partial aggregation? In the other thread, Robert Haas wrote: > Concretely, in your test query "SELECT p.i, avg(c1.v) FROM > agg_pushdown_parent AS p JOIN agg_pushdown_child1 AS c1 ON c1.parent = > p.i GROUP BY p.i" you assume that it's OK to do a Partial > HashAggregate over c1.parent rather than p.i. This will be false if, > say, c1.parent is of type citext and p.i is of type text; this will > get grouped together that shouldn't. It will also be false if the > grouping expression is something like GROUP BY length(p.i::text), > because one value could be '0'::numeric and the other '0.00'::numeric. > I can't think of a reason why it would be false if the grouping > expressions are both simple Vars of the same underlying data type, but > I'm a little nervous that I might be wrong even about that case. > Maybe you've handled all of this somehow, but it's not obvious to me > that it has been considered. Ah, I made the same mistake. I did consider the "GROUP BY length(o.i::text)", but not the cross-datatype case. I think we should punt on that for now, and only do the substitution for simple Vars of the same datatype. That seems safe to me. Overall, your patch is in a more polished state than my prototype. For easier review, though, I think we should try to get something smaller committed first, and build on that. Let's punt on the Var substitution. And I'd suggest adopting my approach of attaching the grouped RelOptInfos to the parent RelOptInfo, that seems simpler. And if we punt on the partial aggregation, and only allow pushing down the whole Aggregate, how much smaller would your patch get? (I'll be on vacation for the next two weeks, but I'll be following this thread. After that, I plan to focus on this feature, as time from reviewing patches in the commitfest permits.) - Heikki
Heikki Linnakangas <hlinnaka@iki.fi> wrote: > Ah, cool! I missed that thread earlier. Yes, seems like we've been hacking on > the same feature. Let's compare! > > I've been using this paper as a guide: > > "Including Group-By in Query Optimization", by Surajit Chaudhuri and Kyuseok > Shim: > https://pdfs.semanticscholar.org/3079/5447cec18753254edbbd7839f0afa58b2a39.pdf > Using the terms from that paper, my patch does only "Invariant Grouping > transormation", while yours can do "Simple Coalescing Grouping", which is more > general. In layman terms, my patch can push the Aggregate below a join, while > your patch can also split an Aggregate so that you do a partial aggregate > below the join, and a final stage above it. Thanks for the link. I've just checked the two approaches briefly so far. > My thinking was to start with the simpler Invariant Grouping transformation > first, and do the more advanced splitting into partial aggregates later, as > a separate patch. I think for this you need to make sure that no join duplicates already processed groups. I tried to implement PoC of "unique keys" in v5 of my patch [1], see 09_avoid_agg_finalization.diff. The point is that the final aggregation can be replaced by calling pg_aggregate(aggfinalfn) function if the final join generates only unique values of the GROUP BY expression. Eventually I considered this an additional optimization of my approach and postponed work on this to later, but I think something like this would be necessary for your approach. As soon as you find out that a grouped relation is joined to another relation in a way that duplicates the grouping expression, you cannot proceed in creating grouped path. > There's some difference in the way we're dealing with the grouped > RelOptInfos. You're storing them completely separately, in PlannerInfo's new > 'simple_grouped_rel_array' array and 'join_grouped_rel_list/hash'. I'm > attaching each grouped RelOptInfo to its parent. In the first version of my patch I added several fields to RelOptInfo (reltarget for the grouped paths, separate pathlist, etc.) but some people didn't like it. In the later versions I introduced a separate RelOptInfo for grouped relations, but stored it in a separate list. Your approach might make the patch a bit less invasive, i.e. we don't have to add those RelOptInfo lists / arrays to standard_join_search() and subroutines. > I got away with much less code churn in allpaths.c, indxpath.c, > joinpath.c. You're adding new 'do_aggregate' flags to many functions. I'm not > sure if you needed that because you do partial aggregation and I don't, but it > would be nice to avoid it. IIRC, the do_aggregate argument determines how the grouped join should be created. If it's false, the planner joins a grouped relation (if it exists) to non-grouped one. If it's true, it joins two non-grouped relations and applies (partial) aggregation to the result. > You're introducing a new GroupedVar expression to represent Aggrefs between > the partial and final aggregates, while I'm just using Aggref directly, above > the aggregate node. I'm not thrilled about introducing an new Var-like > expression. We already have PlaceHolderVars, and I'm always confused on how > those work. But maybe that's necessary to supprot partial aggregation? The similarity of GroupedVar and PlaceHolderVar is that they are evaluated at some place in the join tree and the result is only passed to the joins above and eventually to the query target, w/o being evaluated again. In contrast, generic expressions are evaluated in the query target (only the input Vars get propagated from lower nodes), but that's not what we want for 2-stage aggregation. In my patch GroupedVar represents either the result of partial aggregation (the value of the transient state) or a grouping expression which is more complex than a plain column reference (Var expression). > In the other thread, Robert Haas wrote: > > Concretely, in your test query "SELECT p.i, avg(c1.v) FROM > > agg_pushdown_parent AS p JOIN agg_pushdown_child1 AS c1 ON c1.parent = > > p.i GROUP BY p.i" you assume that it's OK to do a Partial > > HashAggregate over c1.parent rather than p.i. This will be false if, > > say, c1.parent is of type citext and p.i is of type text; this will > > get grouped together that shouldn't. It will also be false if the > > grouping expression is something like GROUP BY length(p.i::text), > > because one value could be '0'::numeric and the other '0.00'::numeric. > > I can't think of a reason why it would be false if the grouping > > expressions are both simple Vars of the same underlying data type, but > > I'm a little nervous that I might be wrong even about that case. > > Maybe you've handled all of this somehow, but it's not obvious to me > > that it has been considered. > > Ah, I made the same mistake. I did consider the "GROUP BY length(o.i::text)", > but not the cross-datatype case. I think we should punt on that for now, and > only do the substitution for simple Vars of the same datatype. That seems safe > to me. Yes, I reached the same conclusion. I'll add this restriction to the next version of the patch. > Overall, your patch is in a more polished state than my prototype. Probably I spent much more time on it. > For easier review, though, I think we should try to get something smaller > committed first, and build on that. Let's punt on the Var substitution. As mentioned above, I think we can only live without the Var substitution (in other words without 2-stage aggregation) if we can check the uniqueness of grouping keys of any path. So the question is how much effort this check requires. > And I'd suggest adopting my approach of attaching the grouped RelOptInfos to > the parent RelOptInfo, that seems simpler. o.k., I'll try this in the next version. > And if we punt on the partial aggregation, and only allow pushing down the > whole Aggregate, how much smaller would your patch get? I can't tell now, need to spend some time looking at the code. > (I'll be on vacation for the next two weeks, but I'll be following this > thread. After that, I plan to focus on this feature, as time from reviewing > patches in the commitfest permits.) Likewise, I'll be off from July 5th to 22nd. I'll post what I have before I leave and will see what you could make out of it :-) It's cool that you intend to work on this feature too! [1] https://www.postgresql.org/message-id/18007.1513957437%40localhost -- Antonin Houska Cybertec Schönig & Schönig GmbH Gröhrmühlgasse 26, A-2700 Wiener Neustadt Web: https://www.cybertec-postgresql.com
On 20.06.18 22:12, Heikki Linnakangas wrote:
> Currently, the planner always first decides the scan/join order, and
> adds Group/Agg nodes on top of the joins. Sometimes it would be legal,
> and beneficial, to perform the aggregation below a join. I've been
> hacking on a patch to allow that.
Because this patch moves a lot of code around, there are nontrivial
conflicts now. I was able to apply it on top of
fb6accd27b99f5f91a7e9e5bd32b98a53fc6d6b8 based on the date.
With that, I'm getting test failures in partition_aggregate, like this
Sort
Sort Key: t2.y, (sum(t1.y)), (count(*))
- -> Append
- -> HashAggregate
- Group Key: t2.y
...
+ -> Result
+ -> Append
+ -> HashAggregate
+ Group Key: t1.x
...
And there is apparently no expected/aggregate_pushdown.out file in the
patch set.
--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
This is what I managed to hack so far. Now the patch can also handle the AGGSPLIT_SIMPLE aggregates. Antonin Houska <ah@cybertec.at> wrote: > Heikki Linnakangas <hlinnaka@iki.fi> wrote: > > > Ah, cool! I missed that thread earlier. Yes, seems like we've been hacking on > > the same feature. Let's compare! > > > > I've been using this paper as a guide: > > > > "Including Group-By in Query Optimization", by Surajit Chaudhuri and Kyuseok > > Shim: > > https://pdfs.semanticscholar.org/3079/5447cec18753254edbbd7839f0afa58b2a39.pdf > > > Using the terms from that paper, my patch does only "Invariant Grouping > > transormation", while yours can do "Simple Coalescing Grouping", which is more > > general. In layman terms, my patch can push the Aggregate below a join, while > > your patch can also split an Aggregate so that you do a partial aggregate > > below the join, and a final stage above it. > > Thanks for the link. I've just checked the two approaches briefly so far. > > > My thinking was to start with the simpler Invariant Grouping transformation > > first, and do the more advanced splitting into partial aggregates later, as > > a separate patch. > > I think for this you need to make sure that no join duplicates already > processed groups. I tried to implement PoC of "unique keys" in v5 of my patch > [1], see 09_avoid_agg_finalization.diff. The point is that the final > aggregation can be replaced by calling pg_aggregate(aggfinalfn) function if > the final join generates only unique values of the GROUP BY expression. > > Eventually I considered this an additional optimization of my approach and > postponed work on this to later, but I think something like this would be > necessary for your approach. As soon as you find out that a grouped relation > is joined to another relation in a way that duplicates the grouping > expression, you cannot proceed in creating grouped path. The current patch version does not check the uniqueness of grouping keys, so it actually doe not produce the plans you wanted to see. For expeimental purposes, you can comment out the (agg_kind == REL_AGG_KIND_SIMPLE) branch near the bottom of make_join_rel_common_grouped() and see that the agg_pushdown regression test will generate plans with AGGSPLIT_SIMPLE pushed down. For example Finalize HashAggregate Group Key: p.i -> Hash Join Hash Cond: (p.i = c1.parent) -> Seq Scan on agg_pushdown_parent p -> Hash -> Partial HashAggregate Group Key: c1.parent -> Seq Scan on agg_pushdown_child1 c1 will become Hash Join Hash Cond: (p.i = c1.parent) -> Seq Scan on agg_pushdown_parent p -> Hash -> HashAggregate Group Key: c1.parent -> Seq Scan on agg_pushdown_child1 c1 The plan will look correct but it can generate duplicate grouping keys. > > There's some difference in the way we're dealing with the grouped > > RelOptInfos. You're storing them completely separately, in PlannerInfo's new > > 'simple_grouped_rel_array' array and 'join_grouped_rel_list/hash'. I'm > > attaching each grouped RelOptInfo to its parent. > > In the first version of my patch I added several fields to RelOptInfo > (reltarget for the grouped paths, separate pathlist, etc.) but some people > didn't like it. In the later versions I introduced a separate RelOptInfo for > grouped relations, but stored it in a separate list. Your approach might make > the patch a bit less invasive, i.e. we don't have to add those RelOptInfo > lists / arrays to standard_join_search() and subroutines. Done. I think this concept might eventually lead to simplification of create_grouping_paths(), especially the in the special cases related to partitioning. I just think so because it's more generic, but haven't tried yet. > > You're introducing a new GroupedVar expression to represent Aggrefs between > > the partial and final aggregates, while I'm just using Aggref directly, above > > the aggregate node. I'm not thrilled about introducing an new Var-like > > expression. We already have PlaceHolderVars, and I'm always confused on how > > those work. But maybe that's necessary to supprot partial aggregation? > > The similarity of GroupedVar and PlaceHolderVar is that they are evaluated at > some place in the join tree and the result is only passed to the joins above > and eventually to the query target, w/o being evaluated again. In contrast, > generic expressions are evaluated in the query target (only the input Vars get > propagated from lower nodes), but that's not what we want for 2-stage > aggregation. > In my patch GroupedVar represents either the result of partial aggregation > (the value of the transient state) or a grouping expression which is more > complex than a plain column reference (Var expression). I'm still not convinced that GroupedVar should be removed. First, RelOptInfo can currently have either Var or PlaceHolderVar in its reltarget, so I prefer to add no more than one kind of expression (GroupedVar can represent either Aggref or a generic (non-Var) grouping expression). Second, GroupedVar indicates during cost estimation that the value has been evaluated at lower node of the join tree and thus the higher nodes should not account for the evaluation again, see set_pathtarget_cost_width(). > > In the other thread, Robert Haas wrote: > > > Concretely, in your test query "SELECT p.i, avg(c1.v) FROM > > > agg_pushdown_parent AS p JOIN agg_pushdown_child1 AS c1 ON c1.parent = > > > p.i GROUP BY p.i" you assume that it's OK to do a Partial > > > HashAggregate over c1.parent rather than p.i. This will be false if, > > > say, c1.parent is of type citext and p.i is of type text; this will > > > get grouped together that shouldn't. It will also be false if the > > > grouping expression is something like GROUP BY length(p.i::text), > > > because one value could be '0'::numeric and the other '0.00'::numeric. > > > I can't think of a reason why it would be false if the grouping > > > expressions are both simple Vars of the same underlying data type, but > > > I'm a little nervous that I might be wrong even about that case. > > > Maybe you've handled all of this somehow, but it's not obvious to me > > > that it has been considered. > > > > Ah, I made the same mistake. I did consider the "GROUP BY length(o.i::text)", > > but not the cross-datatype case. I think we should punt on that for now, and > > only do the substitution for simple Vars of the same datatype. That seems safe > > to me. > > Yes, I reached the same conclusion. I'll add this restriction to the next > version of the patch. Done. Rleated problem is "binary equality" of grouping keys. We need to avoid aggregation push-down if it can discard information that JOIN/ON or WHERE clause needs. The patch does not solve the problem yet. This is where the discussion ended up: [1] > > Overall, your patch is in a more polished state than my prototype. > > Probably I spent much more time on it. > > > For easier review, though, I think we should try to get something smaller > > committed first, and build on that. Let's punt on the Var substitution. > > And if we punt on the partial aggregation, and only allow pushing down the > > whole Aggregate, how much smaller would your patch get? > > I can't tell now, need to spend some time looking at the code. I didn't have enough time to separate "your functionality" and can do it when I'm back from vacation. If you're courious and want to try yourself, this is what you need to do to eliminate "my functionality": * Remove the REL_AGG_KIND_PARTIAL constant (or the whole RelAggKind enumeration) * Remove the needs_final_agg field of the RelOptGrouped structure (or the whole structure and make RelOptInfo grouped point directly to the RelOptInfo that no_final_agg currently points to). * Remove code paths "if (GroupedVar.agg_partial != NULL) ..." * Revert my changes of create_ordinary_grouping_paths() * Revert the related part of my changes of set_upper_references() (see comments) And then try to adjust the code so it can compile. Of course the patch needs quite some polishing, but first we need to achieve consensus on the concepts. > > > (I'll be on vacation for the next two weeks, but I'll be following this > > thread. After that, I plan to focus on this feature, as time from reviewing > > patches in the commitfest permits.) > > Likewise, I'll be off from July 5th to 22nd. I'll post what I have before I > leave and will see what you could make out of it :-) > > It's cool that you intend to work on this feature too! > > [1] https://www.postgresql.org/message-id/18007.1513957437%40localhost A few more notes: * I didn't have time to check if all the regression tests succeed. Besides the tests that the patch adds I only ran the partition_join test with enable_agg_pushdown enabled, to see that the patch does push aggregation down to partitions. * Older version of my patch contains the postgres_fdw part. I can adjust it when I'm back. [1] https://www.postgresql.org/message-id/11966.1530875502%40localhost -- Antonin Houska Cybertec Schönig & Schönig GmbH Gröhrmühlgasse 26, A-2700 Wiener Neustadt Web: https://www.cybertec-postgresql.com
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Antonin Houska <ah@cybertec.at> wrote:
> I didn't have enough time to separate "your functionality" and can do it when
> I'm back from vacation.
So I've separated the code that does not use the 2-stage replication (and
therefore the feature is not involved in parallel queries).
Note on coding: so far most of the functions to which I added the "grouped"
argument can get the same information from rel->agg_info (it's set iff the
relation is grouped). So we can remove it for this "simple aggregation", but
the next (more generic) part of the patch will have to add some argument
anyway, to indicate whether AGGSPLIT_SIMPLE, AGGSPLIT_INITIAL_SERIAL (or no
aggregation) should be performed.
Besides splitting the code I worked on the determination whether join
duplicates grouping keys or not. Currently it still fails to combine
"uniquekeys" of joined relation in some (important) cases when the information
needed is actually available. I think ECs should be used here, like it is for
pathkeys.
So currently you should comment out this code
if (!match_uniquekeys_to_group_pathkeys(root, result, target))
*keys_ok = false;
in pathnode.c:make_uniquekeys_for_join() if you want the patch to at leat
produce interesting EXPLAIN output.
One example:
CREATE TABLE a(i int primary key);
CREATE TABLE b(j int primary key, k int);
SET enable_agg_pushdown TO true;
EXPLAIN
SELECT j, sum(k)
FROM a, b
WHERE i = j
GROUP BY j
QUERY PLAN
-----------------------------------------------------------------------
Hash Join (cost=94.75..162.43 rows=2260 width=12)
Hash Cond: (a.i = b.j)
-> Seq Scan on a (cost=0.00..35.50 rows=2550 width=4)
-> Hash (cost=66.50..66.50 rows=2260 width=12)
-> HashAggregate (cost=43.90..66.50 rows=2260 width=12)
Group Key: b.j
-> Seq Scan on b (cost=0.00..32.60 rows=2260 width=8)
However there are cases like this
EXPLAIN
SELECT i, sum(k)
FROM a, b
WHERE i = j
GROUP BY i
which currently does not work. The reason is that the column b.j which is not
in the GROUP BY clause needs to be in the grouped output of the "b" (grouped)
table output, otherwise the join condition cannot be evaluated.
While separating the code that only uses 1-stage aggregation I removed the
code that adds such extra grouping keys to per-relation AggPath because in
general this is only safe if the final aggregation is performed, and the final
aggregation uses no added columns. However I forgot that grouping keys can be
added in cases like shown above, i.e. the grouping expression b.j is derived
from GROUP BY using equivalence class.
I'll fix this (and various other problems) asap. I believe it's worth to at
least show the current code. I'm curious if it's something we can build on or
if another rework will be needed.
(I'll be off next week.)
--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26, A-2700 Wiener Neustadt
Web: https://www.cybertec-postgresql.com
Attachment
On Fri, Aug 03, 2018 at 04:50:11PM +0200, Antonin Houska wrote: > Antonin Houska <ah@cybertec.at> wrote: > > > I didn't have enough time to separate "your functionality" and can do it when > > I'm back from vacation. > > So I've separated the code that does not use the 2-stage replication (and > therefore the feature is not involved in parallel queries). This patch has been waiting for input from its author for a couple of months now, so I am switching it to "Returned with Feedback". If a new version can be provided, please feel free to send a new one. -- Michael