Re: [Postgres-xc-developers] FQS for queries with subquery

[Amit K. <ami...@en...>]

Hi Ashutosh,

I only have following few points:

-------------
 trailing whitespace characters in the patch.

--------------

Re: "Subqueries and permission checks",  Just to make sure I understand the
scope of the patch ...
The statement "In very near future, we should be able to use the
infrastructure for FQS for shipping sub-queries without planning at the
coordinator, the whole query is not shippable" , can you please elaborate
on this point ? Is it that individual subqueries are not fast-shipped in
some cases if the whole query is not shippable ? For e.g. :

create table t2 (c1 int, c2 int, c3 int) distribute by hash(c2);
create table t1 (c1 int, c2 int, c3 int) distribute by replication;

explain select * from (select avg(c1), c2 from t2 group by c2) a1 join
(select avg(c1), c2 from t1 group by c2) b1 using
(c2);                                 QUERY
PLAN
----------------------------------------------------------------------------
 Data Node Scan on "__REMOTE_FQS_QUERY__"  (cost=0.00..0.00 rows=0 width=0)
   Node/s: datanode_1, datanode_2

Now if I disable FQS, the join does not get reduced. That means, if the
above query is used as a subquery in a FROM clause of an outer query, the
subquery won't get pushed ? I guess this is expected, because as you said
"Subqueries are not shipped if the whole query is not-shippable". But not
sure.


------------

There is a comment below in xc_FQS_join.sql . I guess we need to correct
the comment now.

-- JOIN between two distributed relations is not shippable as of now,
because we
-- don't store the distribution information completely while deducing nodes
in
-- the planner. If we come to that some time in the future, we would see
change
-- in the plan below


------------

Can you please correct some typos in the comment :

 * pgxc_replace_dist_vars_subquery
 * The function looks up the *member so ExecNodes::en_dist*_var in the
 * query->targetList. If found, they are re-stamped with the given varno and
 * resno of the TargetEntry found and added to the new distribution var list
 * being created. This function is useful to re-stamp the distribution
columns
 * of a subquery.

------------------

Other than this, I have no more issues, and you can go ahead with the
commit.


Thanks
-Amit



On 26 February 2013 18:07, Ashutosh Bapat
<ash...@en...>wrote:

> Hi All,
> PFA the patch for using FQS for queries with subqueries. Subqueries appear
> in the query as sublinks or as relations in From clause. The sublinks are
> in-general shippable if all the relations involved are replicated. I
> haven't got a rule to handle subqueries where distributed tables are
> involved. For example, looking at the datanodes where individual sublinks
> in query be below, should be shipped, I can not come up with a rule to
> merge them so as to find a set of datanodes where the query can be shipped.
>
> select (select tab1.val from tab1 where tab1.val = tab2.val2), tab2.val2
> from tab2 where tab2.val = (select sum(val2) from tab1 group by val)
>
> In fact, you will notice that the above query is unshippable by FQS.
>
> This patch deals with the subqueries in the From clause of the query. The
> idea is every query result has a set of datanodes where the result can be
> computed and the distribution columns (sometime computed ones) on which the
> distribution of the result depends. With this information the existing
> rules for shippability of join are applicable. This patch includes fixes
> discussed in threads with subjects "Subqueries and permission checks" and
> "Annotating ExecNodes with distribution column information".
>
> Regression shows no new failures.
> --
> Best Wishes,
> Ashutosh Bapat
> EntepriseDB Corporation
> The Enterprise Postgres Company
>
>
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