[AQUMV] Answer Aggregation Query Directly.

This commits enable answer query which has aggregation directly. Use the results of view has aggregations to avoid compute those from origin table. This may lead to significant efficiency gains if the SQL has a large amount of data. AQUMV will always return results immediately.

If we have a valid view like:

create materialized view mv as
  select sum(c1) as mc1, count(c2) as mc2, avg(c3) as mc3, count(*) as
mc4
  from t where c1 > 90;

SQL:

select count(*), sum(c1), count(c2), avg(c3), abs(count(*) - 21) from t where c1 > 90;

Could be rewritten to:

select mc4, mc1, mc2, mc3, abs((mc4 - 21)) from mv;

Plan:

explain(verbose, costs off)
select count(*), sum(c1), count(c2), avg(c3), abs(count(*) - 21) from t where c1 > 90;
                                    QUERY PLAN
------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)
   Output: mc4, mc1, mc2, mc3, (abs((mc4 - 21)))
   ->  Seq Scan on mv
         Output: mc4, mc1, mc2, mc3, abs((mc4 - 21))
 Settings: enable_answer_query_using_materialized_views = 'on',
optimizer = 'off'
 Optimizer: Postgres query optimizer
(6 rows)

View query with Group By is not supported yet.

HAVING clause process:

If some HAVING quals only exist in origin query and they could be computed from view query's target list, then we could keep them like post_quals.But as the view has aggregations, the additional quals should be moved to WHERE instead of HAVING.

create table t(c1 int, c2 int, c3 int, c4 int);
create materialized view mv as
  select sum(c1) as mc1, count(c2) as mc2, avg(c3) as mc3, count(*) as
mc4
  from t where c1 > 90;

SQL:

select count(*), sum(c1) from t where c1 > 90 having abs(count(*) - 21)
> 0 and 2 > 1 and avg(c3) > 97;

Could be rewritten to (The HAVING clause has been rewritten to WHERE clause):

select mc4, mc1 from mv where mc3 > 97 and abs(mc4 - 21) > 0;

Plan:

explain(verbose, costs off)
select count(*), sum(c1) from t where c1 > 90 having abs(count(*) - 21)
> 0 and 2 > 1 and avg(c3) > 97;
                                    QUERY PLAN
------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)
   Output: mc4, mc1
   ->  Seq Scan on aqumv.mv
         Output: mc4, mc1
         Filter: ((mv.mc3 > '97'::numeric) AND (abs((mv.mc4 - 21)) > 0))
 Optimizer: Postgres query optimizer
(7 rows)

There are two additional HAVING quals: Expression: 2 > 1 (would be eliminated during planner). Expression: abs(count(*) - 21) > 0, it could be computed from view as:

    abs(mc4 - 21) > 0

And the new one is put to WHERE clause and acts as a Filter finally.

ORDER BY clause:

There is a trick for ORDER BY for both origin query and view query. As we has no Groupy By curretly, the aggregation results would be either one or zero rows that make the Order By clause pointless. We could avoid considering the sort columns if it's a junk for view matching.

LIMIT clause:

As we have no group by for view with aggs now, the final result would be either one or zero row. LIMIT, OFFSET clause of origin query could be applied to view if there are consts.

create incremental materialized view mv as
  select sum(c1) as mc1, count(c2) as mc2, avg(c3) as mc3, count(*) as
mc4
  from t where c1 > 90;

Query:

  select count(*), sum(c1) from t where c1 > 90 limit 2;

Could be rewritten to:

  select mc4, mc1 from mv limit 2;

Authored-by: Zhang Mingli avamingli@gmail.com

fix #ISSUE_Number

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