Before this PR, we were trusting on the columns of group by about
guaranteeing the uniqueness of the results. However, this assumption
is correct only if the columns in the group by is subset of columns
in the distinct clause. It can be wrong if we have part of group by
columns and some aggregation columns in the distinct clause. With
this PR, we add distinct plan on top of aggregate plan when necessary.
With #1804 (and related PRs), Citus gained the ability to
plan subqueries that are not safe to pushdown.
There are two high-level requirements for pushing down subqueries:
* Individual subqueries that require a merge step (i.e., GROUP BY
on non-distribution key, or LIMIT in the subquery etc). We've
handled such subqueries via #1876.
* Combination of subqueries that are not joined on distribution keys.
This commit aims to recursively plan some of such subqueries to make
the whole query safe to pushdown.
The main logic behind non colocated subquery joins is that we pick
an anchor range table entry and check for distribution key equality
of any other subqueries in the given query. If for a given subquery,
we cannot find distribution key equality with the anchor rte, we
recursively plan that subquery.
We also used a hacky solution for picking relations as the anchor range
table entries. The hack is that we wrap them into a subquery. This is only
necessary since some of the attribute equivalance checks are based on
queries rather than range table entries.
We used to only support pushdownable set operations inside a
subquery, however, we could easily expand the restriction
checks to cover top level set operations as well.
We use PostgreSQL hooks to accumulate the join restrictions
and PostgreSQL gives us all the join paths it tries while
deciding on the join order. Thus, for queries that have many
joins, this function is likely to remove lots of duplicate join
restrictions. This becomes relevant for Citus on query pushdown
check peformance.
We were allowing count distict queries even if they were
not directly on columns if the query is grouped on
distribution column.
When performing these checks we were skipping subqueries
because they also perform this check in a more concise manner.
We relied on oid SUBQUERY_RELATION_ID (10000) to decide if
a given RTE relation id denotes a subquery, however, we also
use SUBQUERY_PUSHDOWN_RELATION_ID (10001) for some subqueries.
We skip both type of subqueries with this change.
Marco Slot
Metin Döşlü
Metin Doslu
Marco Slot
Burak Velioglu
velioglu
Önder Kalacı
Markus Sintonen
Jason Petersen
Burak Yücesoy
Murat Tuncer
Brian Cloutier