Hi Christian,
Yes, I do agree it is better to generate SEMI/ANTI during Sql-to-Rel conversion
if possible.
With regard to the missing Rel-to-Sql conversion for SEMI/ANTI joins, you can
log a JIRA and open a pull request for it, people are happy to take a look.
- Haisheng
------------------------------------------------------------------
发件人：Christian Beikov<christian.bei...@gmail.com>
日 期：2020年02月20日 03:11:02
收件人：Haisheng Yuan<h.y...@alibaba-inc.com>; Apache Calcite dev
list<dev@calcite.apache.org>
主 题：Re: Translation of SQL EXISTS
Hey Haisheng,
it is nice to have a rule that detects such patterns but do you agree that it
would be better to generate SEMI- and ANTI-joins already during Sql-to-Rel
conversion?
I implemented the Rel-to-Sql conversion for SEMI- and ANTI-join and it seems to
work for my use cases, but it feels a bit ugly as I have to do some
transformations which might be ad-hoc. I had to replace identifiers that refer
to the outcome of the SEMI- and ANTI-joins with the select items of the LHS
because the SEMI- and ANTI-join does not get an alias.
See for yourself:
https://github.com/beikov/calcite/commit/6ca67e2e8df44c2081e71407ee6893a961f36271
I'll add a few tests and a comment, but could anyone please comment on whether
this approach looks ok?
Regards,
Christian
Am 19.02.2020 um 19:03 schrieb Haisheng Yuan:
Hi Christian,
For the query in your example, Calcite first generates inner join plan with
aggregate child, then through SemJoinRule transform the inner join to semi or
antisemi join. The reason to have inner join is that it allows join
commutativity, which is good for generating a potential better plan with
nestedloop join or hash join.
Admittedly, this process in Calcite is counter intuitive. It should be in
reverse order, first generate a semi or anti-semi join, then generate an
inner/outer join.
- Haisheng
------------------------------------------------------------------
发件人：Christian Beikov<christian.bei...@gmail.com>
日 期：2020年02月19日 21:12:13
收件人：<dev@calcite.apache.org>
主 题：Translation of SQL EXISTS
Hello,
I'm a bit confused about how the SQL EXISTS predicate is translated. I'd
assume that an EXISTS is translated in relational algebra to a SEMI- and
NOT EXISTS to an ANTI-join, but it's not.
PlannerImpl p = new PlannerImpl(config);
SqlNode sqlNode = p.parse("delete from _order o where exists (select 1
from order_position p where o.id = p.order_id)");
p.validate(sqlNode);
RelRoot rel = p.rel(sqlNode);
RelToSqlConverter sqlConverter = new RelToSqlConverter(dialect);
SqlImplementor.Result result = sqlConverter.visitChild(0, rel.rel);
sqlWriter.format(result.asStatement());
Worse, when printing this, I only get DELETE FROM "public"."_order" i.e.
the EXISTS part is not rendered. This is the plan I get.
LogicalTableModify(table=[[adhoc, _order]], operation=[DELETE],
flattened=[true])
LogicalProject(inputs=[0])
LogicalProject(inputs=[0], exprs=[[CAST($1):BIGINT, CAST($2):BOOLEAN]])
LogicalJoin(condition=[=($0, $1)], joinType=[inner])
JdbcTableScan(table=[[adhoc, _order]])
LogicalAggregate(group=[{0}], agg#0=[MIN($1)])
LogicalProject(exprs=[[$1, true]])
JdbcTableScan(table=[[adhoc, order_position]])
I'd expect something along the lines of
LogicalTableModify(table=[[adhoc, _order]], operation=[DELETE],
flattened=[true])
LogicalProject(inputs=[0])
LogicalJoin(condition=[=($0, $1)], joinType=[semi])
JdbcTableScan(table=[[adhoc, _order]])
JdbcTableScan(table=[[adhoc, order_position]])
and for NOT EXISTS
LogicalTableModify(table=[[adhoc, _order]], operation=[DELETE],
flattened=[true])
LogicalProject(inputs=[0])
LogicalJoin(condition=[=($0, $1)], joinType=[anti])
JdbcTableScan(table=[[adhoc, _order]])
JdbcTableScan(table=[[adhoc, order_position]])
Am I missing something and the current aggregate function translation
makes sense?
I constructed relational algebra structures for some other statements
with SEMI- and ANTI-joins and already noticed that these join types
weren't handled in
org.apache.calcite.rel.rel2sql.RelToSqlConverter#visit(org.apache.calcite.rel.core.Join),
which I fixed locally. Is the lack of a translation intentional?
Is such a translation of SEMI- and ANTI-join to EXISTS and NOT EXISTS an
over-simplification or would you say it's correct? As far as I
understood from https://en.wikipedia.org/wiki/Relational_algebra this is
correct.
I'd be happy to contribute that back. I didn't look into the Sql-to-Rel
translation for EXISTS and NOT EXISTS to SEMI- and ANTI-join yet, but I
assume that's not that hard and I could add that.
Regards,
Christian
