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From Stamatis Zampetakis <zabe...@gmail.com>
Subject Re: [DISCUSSION] Extension of Metadata Query
Date Tue, 04 Jun 2019 22:44:23 GMT
Thanks for bringing this up Danny.

I guess the discussion came up due to CALCITE-2885 [1]. Looking back into
it, I am not sure that the intention there is to make the RelMetadataQuery
pluggable. We could possibly solve the performance problem without
extending the RelMetadataQuery. We have to look again into this case.

For more details regarding the existence of the two methods in
RelMetadataProvider have a look in CALCITE-604 [2]. More general for the
design of RelMetadataProvider you may find useful the description in [3].

Best,
Stamatis

[1] https://issues.apache.org/jira/browse/CALCITE-2885
[2] https://issues.apache.org/jira/browse/CALCITE-604
[3]
https://web.archive.org/web/20140624040836/www.hydromatic.net/wiki/RelationalExpressionMetadata/

On Tue, Jun 4, 2019 at 7:48 PM Julian Hyde <jhyde@apache.org> wrote:

> > 1. Why we have 2 methods in RelMetadataProvider?
>
> The metadata system is complicated. We need to allow multiple handlers
> for any given call. So, making a metadata call involves multiple
> dispatch [1] based on the metadata method being called, the type of
> RelNode, and the handlers that are registered. Also it needs to cache
> results, and detect cycles. And the dispatch needs to be efficient, so
> we generate janino code to do the dispatch, and re-generate when new
> handlers or sub-classes of RelNode are added.
>
> I forget details, the two methods are basically required to allow us
> to generate code to do the dispatch.
>
> > 2. We should make the RelMetadataQuery in RelOptCluster pluggable.
>
> I disagree. RelMetadataQuery is not an extension point. Its sole
> purpose is to to keep state (the cache and cycle-checking).
> RelMetadataProvider is the extension point. (By analogy, if you are
> un-parsing an AST, you let each AST sub-class handle unparsing itself,
> but the unparsed text goes to a simple StringBuilder. RelMetadataQuery
> is in the role of the StringBuilder. In a complex system, it is nice
> to keep some of the components simple, or at least keep them to
> prescribed roles.)
>
> Julian
>
> [1] https://en.wikipedia.org/wiki/Multiple_dispatch
>
> On Sun, Jun 2, 2019 at 11:19 PM Yuzhao Chen <yuzhao.cyz@gmail.com> wrote:
> >
> > Currently we provide answer to metadata query through
> RelMetadataProvider [1], there are some sub-classes of it:
> >
> > RelMetadataProvider
> > |
> > |- VolcanoRelMetadataProvider
> > |- ChainedRelMetadataProvider/DefaultRelMetadataProvider
> > |- HepRelMetadataProvider
> > |- CachingRelMetadataProvider
> > |- ReflectiveRelMetadataProvider
> > |- JaninoRelMetadataProvider
> >
> > The RelMetadataProvider has two methods: #apply and #handlers, the
> #apply method seems a programming interface and there is a demo code how we
> can use it:
> >
> > RelMetadataProvider provider;
> > LogicalFilter filter;
> > RexNode predicate;
> > Function<RelNode, Metadata> function =
> > provider.apply(LogicalFilter.class, Selectivity.class};
> > Selectivity selectivity = function.apply(filter);
> > Double d = selectivity.selectivity(predicate);
> >
> > But let's see our RelOptCluster's member variables[2], there are
> MetadataFactory and RelMetadataQuery which all can be used to query the
> metadata, for MetadataFactory, there is a default impl named
> MetadataFactoryImpl which will invoke RelMetadataProvider#apply internally,
> for RelMetadataQuery, it will invoke RelMetadataProvider#handlers (finally
> composed and codeden by JaninoRelMetadataProvider).
> >
> > In our planning phrase, we can invoke RelOptRuleCall#getMetadataQuery to
> get the MQ and query the metadata.
> >
> > For extension of metadata handlers, we can set our customized
> RelMetadataProvider in RelOptCluster[3]. But for RelMetadataQuery, we have
> no way to extend it now, because the RelOptCluster always has a singleton
> instance [4] which is only the default implementation.
> >
> >
> > My question is as follows:
> >
> > 1. Why we have 2 methods in RelMetadataProvider, and why we need the
> MetadataFactory and RelMetadataProvider#apply ? It seems that it's function
> is already been overriden by RelMetadataQuery(The difference is that
> MetadataFactory use Reflection and RelMetadataQuery use gened bytes code).
> > 2. We should make the RelMetadataQuery in RelOptCluster pluggable.
> >
> >
> > [1]
> https://github.com/apache/calcite/blob/b0e83c469ff57257c1ea621ff943ca76f626a9b7/core/src/main/java/org/apache/calcite/rel/metadata/RelMetadataProvider.java#L38
> > [2]
> https://github.com/apache/calcite/blob/b0e83c469ff57257c1ea621ff943ca76f626a9b7/core/src/main/java/org/apache/calcite/plan/RelOptCluster.java#L49
> > [3]
> https://github.com/apache/calcite/blob/b0e83c469ff57257c1ea621ff943ca76f626a9b7/core/src/main/java/org/apache/calcite/plan/RelOptCluster.java#L135
> > [4]
> https://github.com/apache/calcite/blob/b0e83c469ff57257c1ea621ff943ca76f626a9b7/core/src/main/java/org/apache/calcite/plan/RelOptCluster.java#L151
> >
> >
> >
> > Best,
> > Danny Chan
>

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