> A third alternative might be to adjust Arrow to move to field-index based execution instead of storing schemas in its relational operators.
This is an interesting idea - I do think there are operators that need column name information (e.g. a "checkpoint" node that writes to external storage). In this case, it sounds like any operator that requires knowing the column name needs to store additional information for the column names in addition to the field index, in its substrait representation. > The names at the relation root are a different story, since they specify the column names for the produced table, but there's already a mechanism for specifying those. I feel this is a bit limited because I don't think relation roots are the only thing that produces tables. I can think of two examples in Spark that produce tables and are not a relation root. (1) A checkpoint node that produces a table to write to external storage (2) A Python UDF node that materializes a slice of the table and produces a pd.DataFrame to pass down to a user function. I think we will see these cases in Arrow as well. I feel it might make sense to have a subclass of the operator that requires column names in addition to just root rel. *On schema-based vs index-based**:* >From personal experience working on Spark it would say that having schema available in each operator is useful for developers - I often print input/output rows and I feel not having the column names available might put a tax on the developer. (But maybe there are other ways to solve this). Are there other substrait consumers at the moment and how do they deal with this? For example, if we were to have Spark consumes substrait I'd imagine there would be similar problems because schema is part of Spark operators too and I don't think Substratit cannot convince Spark to go index based. I'd imagine during deserialization of the subtrait plan to Spark it would need to compute a schema object for each operator. That's why I think computing schema during serialization might be a better solution than going for index based in Arrow. On Wed, Apr 20, 2022 at 11:14 AM Phillip Cloud <cpcl...@gmail.com> wrote: > On Wed, Apr 20, 2022, 11:31 Jeroen van Straten < > jeroen.van.stra...@gmail.com> > wrote: > > > From a Substrait perspective, it would be up to Ibis to convert the > > column names to the correct indices. This can and should be transparent > > to the end user; front-ends already need to know what the schemas are > > in order to produce a valid Substrait plan, so Ibis should have all the > > information it needs to do this conversion. > > > > I would, however, agree that it's inconsistent for Substrait to require > > that column names are specified for read relations, but not for the > > other relations. AFAICT nothing on the consumer side can or should make > > use of those names. The only use case I can think of is to aid > > debugging; if that's the only reason they exist, it should indeed be > > possible to specify the names for each relation, but it should be in > > the form of as optional, non-functional hints in RelCommon. The names > > at the relation root are a different story, since they specify the > > column names for the produced table, but there's already a mechanism > > for specifying those. No idea if Arrow and Ibis use it (since it's > > optional), but Substrait supports it. > > > > ReadRels having column names is probably not strictly necessary for all the > ReadRels, but for NamedTable it's necessary. For example, if I wanted to > represent a ReadRel independent of storage, that an API could deserialize > for metadata, and present that metadata to the user then you need column > names. Ibis uses the column names from ReadRels to turn Substrait back into > expressions. > > The RelRoot is very much used in ibis. Every table expression compiles to a > RelRoot. > > > > If there *are* special cases I'm not seeing, where the execution engine > > really does need the names of intermediate columns, I suppose it would > > be up to the engine to figure out what they would be. The information > > could also be added via Substrait "advanced extensions," especially > > when a user overrides a name, but it wouldn't be a good idea for the > > consumer to rely on the existance of these extensions, for > > compatibility with other frameworks. > > > > If said special cases turn out to not be so special, a valid case could > > be made to pull that extension into Substrait itself. If they're only > > attached wherever the user manually overrides a name, I think the size > > explosion Phillip is talking about would be avoided. I *really* don't > > see why anyone would want to match *generated* names in any functional > > way; that's a recipe for undefined behavior. > > > > None of Substrait's built-in things make use of column names though, > > which implies that any Arrow execution node that *does* need it cannot > > be described with Substrait, and thus should never appear in an > > execution plan deserialized from Substrait. If it currently does, > > that's an issue with Arrow's Substrait consumer. If we'd want to > > correctly represent those kinds of nodes anyway, we'd already be > > dealing with an extension relation, to which we could attach whatever > > information we want. > > > > On Wed, 20 Apr 2022 at 16:05, Phillip Cloud <cpcl...@gmail.com> wrote: > > > > > A third alternative might be to adjust Arrow to move to field-index > based > > > execution instead of storing schemas in its relational operators. > > > > > > I suspect that deserializing the schema on the arrow side is the right > > > solution in at least the near term. Repeating field names where they > > aren't > > > strictly necessary has a tendency to enlarge plans, and the effect gets > > > worse as plans go through transformations. > > > > > > On Wed, Apr 20, 2022 at 6:08 AM Yaron Gvili <rt...@hotmail.com> wrote: > > > > > > > Hi Weston, > > > > > > > > To reiterate, the scenario is not a user writing a Substrait plan by > > > hand, > > > > but a user writing an Ibis expression and the remaining steps - > > Substrait > > > > compilation, serialization to protobuf, deserialization to an Arrow > > plan, > > > > and execution of the plan - are done systematically. This issue is > > > related > > > > to Ibis, Ibis-Substrait, and Arrow together, so multiple persons may > > need > > > > to bear on this issue. > > > > Index based field references should still work here. For example, if > > > > I have tables: > > > > > > > > Key | LeftPayload -- Key | RightPayload > > > > > > > > The join expression would be field(0) == field(2) > > > > Right, but the point in this use case is that it would not be > > convenient > > > > for a user writing an Ibis expression to specify each key of each > > table, > > > > especially when plenty of (say, 100) tables are being joined. It > would > > be > > > > much more convenient for the user to specify once the name of the key > > > that > > > > exists in all tables. This is a use case where the key should (not > > must) > > > be > > > > specified by name for convenience. > > > > In both of those cases the field names are not part of the plan > itself. > > > > In the second use case, the user writing the ibis expression is > > > specifying > > > > a string-name as a parameter to a relation that would later get > passed > > to > > > > an Arrow execution node in an options instance and used to > dynamically > > > set > > > > up field-names. This string-name does in fact appear explicitly in > the > > > > Substrait plan, not for convenience but of necessity. What does not > > > appear > > > > in the Substrait plan are output field-names of intermediate > relations; > > > > this in itself is not a problem, because these field-names can (in > > > > principle) be recomputed to allow them to be matched to the > dynamically > > > > set-up field-names. But the current implementation of Arrow (in its > > > > Substrait module) does not do this because the schemata are not > > available > > > > during deserialization, when the option instances (such as > > > > ProjectNodeOptions) that require the schemata are created. Instead, > > Arrow > > > > (in its Substrait module) ends up with non-natural field-names like > > > > "FieldPath(1)" that fail to be matched. > > > > > > > > That's why this is not a Substrait specific problem, but one that is > > > > related to Ibis, Ibis-Substrait, and Arrow together. We think it can > be > > > > resolved either by changing Substrait to assist Arrow by specifying > the > > > > field-names of intermediate relations in the plan, so that they are > > > readily > > > > available during deserialization, or by changing Arrow (in its > > Substrait > > > > module) to reproduce the field-names during deserialization, like by > > > > computing the schemata before options and node instances are created. > > We > > > > might come up with other solutions in this discussion, of course. > > Either > > > > way, for this second use case, the field-names of intermediate > > relations > > > > must be natural; they cannot be left as something like > "FieldPath(1)". > > > > > > > > > > > > Yaron. > > > > ________________________________ > > > > From: Weston Pace <weston.p...@gmail.com> > > > > Sent: Tuesday, April 19, 2022 7:12 PM > > > > To: dev@arrow.apache.org <dev@arrow.apache.org> > > > > Cc: Li Jin <ice.xell...@gmail.com> > > > > Subject: Re: [C++] output field names in Arrow Substrait > > > > > > > > > However, the problem is there are natural cases in > > > > > which an execution node should or must take in a string-name > > > > > > > > If we can come up with such a case then I agree it would be a problem > > > > for Substrait's current definition. I don't think we can come up > with > > > > such a case. Every column that can be referenced by name has a > unique > > > > index we could use instead. > > > > > > > > > One use case is an execution node that is joining N > > > > > input tables on a column-name that exists in all of them. > > > > > > > > Index based field references should still work here. For example, if > > > > I have tables: > > > > > > > > Key | LeftPayload -- Key | RightPayload > > > > > > > > The join expression would be field(0) == field(2) > > > > > > > > > The execution node would make itself more convenient > > > > > for the user by allowing specifying a string-name than by > > > > > specifying N FieldRef instances (each with the same > > > > > name but on a different input table) like the above > > > > > requirement would force > > > > > > > > Substrait plans aren't normally created by humans. I'm not sure > > > > convenience is a factor here. > > > > > > > > > Another use case for a string-name is when it is used within > > > > > the execution node to dynamically set up field-names, e.g., a > > > > > node that operates on the input's columns whose name starts > > > > > with the given string-name or a node that operates on an input > > > > > column whose name is given as data in another input column. > > > > > > > > In both of those cases the field names are not part of the plan > itself. > > > > > > > > On Tue, Apr 19, 2022 at 9:16 AM Yaron Gvili <rt...@hotmail.com> > wrote: > > > > > > > > > > Hi Weston, > > > > > > > > > > Thanks for the quick response. > > > > > I think you might have forgotten the links for [1][2][3] > > > > > Sorry about the confusion; I use these not as references to links > but > > > as > > > > markers of points I make in the beginning that I elaborate on later, > in > > > the > > > > places where I reuse the markers. > > > > > Are you going from Substrait to an Arrow execution plan? > > > > > Yes. > > > > > For Substrait -> Arrow most of our execution nodes should take in a > > > > FieldRef which can be a name but can also be index-based. So I > wouldn't > > > > expect Substrait's exclusive use of index-based references to be an > > > issue. > > > > > Yes, I agree this is a design feature of Substrait and that if the > > > > execution node takes in a FieldRef then there won't be a problem with > > > > Substrait use of an index-based reference for it. If any > > to-be-developed > > > > execution node must indeed use a FieldRef when taking in a parameter > > that > > > > refers to columns, then this requirement should be documented for > > > > developers of execution nodes. > > > > > > > > > > However, the problem is there are natural cases in which an > execution > > > > node should or must take in a string-name; these suffer from the > above > > > > design feature due to the non-natural field names that it leads to. > One > > > use > > > > case is an execution node that is joining N input tables on a > > column-name > > > > that exists in all of them. The execution node would make itself more > > > > convenient for the user by allowing specifying a string-name than by > > > > specifying N FieldRef instances (each with the same name but on a > > > different > > > > input table) like the above requirement would force. Another use case > > > for a > > > > string-name is when it is used within the execution node to > dynamically > > > set > > > > up field-names, e.g., a node that operates on the input's columns > whose > > > > name starts with the given string-name or a node that operates on an > > > input > > > > column whose name is given as data in another input column. > > > > > > > > > > This is the main reason we think there should be appropriate > > (natural) > > > > field names defined for each relation/node. > > > > > Also keep in mind that Substrait isn't exactly a query language for > > > > user's to be typing by hand. > > > > > Yes, I took that into consideration. Namely, the above discussion > > > refers > > > > to the scenario of a user writing an Ibis expression and the > remaining > > > > steps - Substrait compilation, serialization to protobuf,, > > > deserialization > > > > to an Arrow plan, and execution of the plan - are done > systematically. > > > > > > > > > > > > > > > Yaron. > > > > > ________________________________ > > > > > From: Weston Pace <weston.p...@gmail.com> > > > > > Sent: Tuesday, April 19, 2022 1:01 PM > > > > > To: dev@arrow.apache.org <dev@arrow.apache.org> > > > > > Cc: Li Jin <ice.xell...@gmail.com> > > > > > Subject: Re: [C++] output field names in Arrow Substrait > > > > > > > > > > Hi Yaron, I think you might have forgotten the links for [1][2][3] > so > > > > > I'm not entirely sure of the context. Are you going from Substrait > > to > > > > > an Arrow execution plan? Or are you going from an Arrow execution > > > > > plan to Substrait? > > > > > > > > > > For Substrait -> Arrow most of our execution nodes should take in a > > > > > FieldRef which can be a name but can also be index-based. So I > > > > > wouldn't expect Substrait's exclusive use of index-based references > > to > > > > > be an issue. Also keep in mind that Substrait isn't exactly a > query > > > > > language for user's to be typing by hand. So, for example, if a > user > > > > > wants a join and is using Ibis they could type: > > > > > > > > > > table.semi_join(s, table.x == table.y) > > > > > > > > > > Ibis is then responsible for converting "x" and "y" to the > > appropriate > > > > > indices (and it should have all the information needed to do so). > > The > > > > > Substrait plan will refer to these nodes by index and the > > > > > corresponding Arrow execution plan will use integer-based FieldRef. > > > > > > > > > > For Arrow -> Substrait then I agree that this could become a > problem. > > > > > Right now the Arrow -> Substrait path is mainly used for internal > > > > > testing with the hope that Substrait -> Arrow -> Substrait should > > > > > generally be possible (with zero loss of information). This does > not > > > > > mean that every Arrow plan will be convertible into Substrait. > That > > > > > is certainly a potential goal, and PRs to add that capability would > > be > > > > > welcome, but I don't know if anyone working on the Arrow/Substrait > > > > > integration has that goal in mind. If that is your goal I might be > > > > > curious to learn more about your use cases. > > > > > > > > > > On Tue, Apr 19, 2022 at 6:11 AM Yaron Gvili <rt...@hotmail.com> > > wrote: > > > > > > > > > > > > Hi, > > > > > > > > > > > > > > > > > > We ran into an issue due to the fact that, for intermediate > > > relations, > > > > Substrait does not automatically compute output field names nor > allows > > > one > > > > to explicitly name output fields [1]. This leads to trouble when one > > > needs > > > > to refer to these output fields by name [2]. We run into this trouble > > > when > > > > deserializing in Arrow [3] (based on commit 8e13c2dd). > > > > > > > > > > > > > > > > > > > > > > > > One use case where [1] occurs is: > > > > > > > > > > > > import ibis > > > > > > > > > > > > import pyarrow as pa > > > > > > > > > > > > loaded_table = ibis.local_table(...) > > > > > > > > > > > > cast_table = ibis.mutate(loaded_table, > > > > time=loaded_table.time.cast(pa.int64()) > > > > > > > > > > > > Even if loaded_table has nicely named fields, like "time" and > > > "value1" > > > > and "value2", the resulting cast_table (when deserialized in Arrow) > has > > > > inconvenient field names like "FieldPath(1)" and "FieldPath(2)" for > all > > > but > > > > the "time" field. I believe that Substrait has all the information to > > > > automatically compute the field name "value*" for cast_table, but it > > > > doesn't do this. Moreover, the caller has to know the schema of > > > > loaded_table in order to set the output field names for cast_table, > and > > > > this is not convenient. When cast_table is an intermediate relation > > > (i.e., > > > > not the root relation of the plan), Substrait also doesn't allow the > > > caller > > > > to explicitly name the output fields, and there is no place for these > > > field > > > > names in the Substrait protobuf. > > > > > > > > > > > > > > > > > > > > > > > > One use case where [2] occurs is in a join-type operation over > > > > cast_table. The caller would normally like to use a field name (and > not > > > an > > > > index) to refer to the join-key. Even if the caller knows the field > > name > > > > for the join-key in loaded_table, many field names in cast_table > (when > > > > deserialized in Arrow) are different (and each includes an index in > it) > > > > than those of loaded_table. > > > > > > > > > > > > > > > > > > > > > > > > The case of [3] occurs because Arrow deserializes ExecNodeOption > > > > instances before it has Schema instances at hand. At this stage, > > without > > > > schemata, Arrow cannot compute field names that should be placed in a > > > > ExecNodeOption that needs them, in particular ProjectNodeOptions. > > > > Currently, Arrow creates an expression vector for the > > ProjectNodeOptions > > > > instance but leaves the names vector empty, and later defaults each > > name > > > to > > > > the ToString of each expression. > > > > > > > > > > > > > > > > > > > > > > > > We'd like your input on this issue. Currently, we see two viable > > ways > > > > to resolve this issue: (1) add output field name for intermediate > > > relations > > > > in the Substrait plan so that Arrow can directly access them, or (2) > > > > reproduce the field names in Arrow during deserialization by creating > > > > Schema instances before ExecNodeOptions instances. In any case, we > > think > > > > there should be appropriate field names defined for each > relation/node. > > > > > > > > > > > > > > > > > > > > > > > > Note that due to this issue, any Arrow execution node that > prompts > > > the > > > > user to refer to fields by name might not play nicely with Substrait. > > > > > > > > > > > > > > > > > > Cheers, > > > > > > Yaron. > > > > > > > > > >
