> > Thanks Micah, for the latter, I meant the type of denormalization of > repeating a 3-part name as opposed to using an ID.
Is the concern here just metadata size or something else? For size I think if this is really anticipated to be a problem that it is likely for the state map in general, and we could investigate some more sophisticated encodings for the State map even without the overlap. I think maybe this could be handled if it proves to be a problem but hopefully engines are placing a reasonable cap on view depth + number of tables per view which puts an upper bound on overall size. Thanks, Micah On Fri, Aug 16, 2024 at 4:56 PM Walaa Eldin Moustafa <wa.moust...@gmail.com> wrote: > Thanks Micah, for the latter, I meant the type of denormalization of > repeating a 3-part name as opposed to using an ID. > > On Fri, Aug 16, 2024 at 4:52 PM Micah Kornfield <emkornfi...@gmail.com> > wrote: > >> However, this still does not address the semantic issue which is more >>> fundamental in my opinion. The Iceberg table spec is not aware of catalog >>> table identifiers and this use will be the first break of this abstraction. >> >> >> IIUC, based on Jan's comments, we are not going to modify the table >> specification. I thought the state map was effectively opaque metadata >> from the table specification perspective? If this is the case I feel like >> that is OK and not a blocker, I think by their nature as already discussed >> MVs need catalog information to function properly and the choice to put >> catalog information into the table metadata is pragmatic and preserves >> other desirable properties. It might be a more important point if we want >> to update the table specification (IMO, I still think it would probably be >> OK). >> >> On a side note, it does not address the denormalization issue either if >>> we ever want to introduce the lineage in the view as a nice-to-have. >> >> >> I think if lineage is introduced to the View metadata, it should only >> hold direct dependencies for the reasons already discussed. IMO, I think >> the potential overlap is OK as they serve two different purposes. >> >> Cheers, >> Micah >> >> >> >> >> >> On Fri, Aug 16, 2024 at 4:27 PM Walaa Eldin Moustafa < >> wa.moust...@gmail.com> wrote: >> >>> That is right. I agree that in the case of using catalog identifiers in >>> state information, using them in lineage information would be a >>> nice-to-have and not a requirement. >>> >>> However, this still does not address the semantic issue which is more >>> fundamental in my opinion. The Iceberg table spec is not aware of catalog >>> table identifiers and this use will be the first break of this abstraction. >>> >>> On a side note, it does not address the denormalization issue either if >>> we ever want to introduce the lineage in the view as a nice-to-have. >>> >>> Thanks, >>> Walaa. >>> >>> >>> On Fri, Aug 16, 2024 at 10:09 AM Jan Kaul <jank...@mailbox.org.invalid> >>> wrote: >>> >>>> Hi Walaa, >>>> >>>> I would argue that for the refresh operation the query engine has to >>>> parse the query and then somehow execute it. For a full refresh it will >>>> directly execute the query and for a incremental refresh it will execute a >>>> modified version. Therefore it has to fully expand the query tree. >>>> >>>> Best wishes, >>>> >>>> Jan >>>> >>>> Am 16.08.2024 18:13 schrieb Walaa Eldin Moustafa <wa.moust...@gmail.com >>>> >: >>>> >>>> Thanks Jan for the summary. >>>> >>>> For this point: >>>> >>>> > For a refresh operation the query engine has to parse the SQL and >>>> fully expand the lineage with it's children anyway. So the lineage is not >>>> strictly required. >>>> >>>> If the lineage is provided at creation time by the respective engine, >>>> the refresh operation does not need to parse the SQL, correct? >>>> >>>> Thanks, >>>> Walaa. >>>> >>>> On Fri, Aug 16, 2024 at 12:24 AM Jan Kaul <jank...@mailbox.org.invalid> >>>> wrote: >>>> >>>> As the table I created is not properly shown in the mailing list I'll >>>> reformat the summary of the different drawbacks again: >>>> >>>> Drawbacks of (no lineage, refresh-state key = identifier): >>>> >>>> - introduces catalog identifiers into table metadata (#4) >>>> - query engine has to expand lineage at refresh time (happens anyway) >>>> >>>> Drawbacks of (normalized lineage, refresh-state key = uuid): >>>> >>>> - recursive calls to catalog to expand lineage at read time (#2) >>>> - fragile by requiring child views to have lineage field >>>> >>>> Drawbacks of (denormalized lineage, refresh-state key = uuid): >>>> >>>> - update of materialized view version required if child view is updated >>>> (#5) >>>> On 16.08.24 09:17, Jan Kaul wrote: >>>> >>>> Hi, >>>> >>>> Thanks Micah for clearly stating the requirements. I think this gives >>>> better clarity for the discussion. >>>> >>>> It seems like we don't have a solution that satisfies all requirements >>>> at once. So we would need to choose which has the fewest drawbacks. >>>> >>>> I would like to summarize the different drawbacks that came up in the >>>> discussion. >>>> no lineage >>>> + refresh-state key = identifier >>>> normalized lineage >>>> + refresh-state key = uuid >>>> denormalized lineage >>>> + refresh-state key = uuid >>>> - introduces catalog identifiers into table metadata (#4) >>>> - query engine has to expand lineage at refresh time (happens anyway) >>>> - recursive calls to catalog to expand lineage at read time (#2) >>>> - fragile by requiring child views to have lineage field >>>> - update of materialized view version required if child view is updated >>>> (#5) >>>> >>>> With identifiers as the refresh-state keys, the lineage is not strictly >>>> required and becomes an orthogonal proposal. That's why I left it out if >>>> the comparison. >>>> >>>> In my opinion introducing catalog identifiers into the table metadata >>>> (requirement #4) is the least significant drawback as it is not a technical >>>> reason but more about semantics. Especially as the identifiers are not >>>> introduced into the table spec but are rather stored in the snapshot >>>> summary. That's why I'm in favor of using the catalog identifiers as the >>>> refresh-state keys. >>>> >>>> Regarding your last point Walaa: >>>> >>>> The option of using catalog identifiers in the state map still requires >>>> keeping lineage information in the view because REFRESH MV needs the latest >>>> fully expanded children (which could have changed from the set of children >>>> currently in the state map), without reparsing the view tree. >>>> >>>> For a refresh operation the query engine has to parse the SQL and fully >>>> expand the lineage with it's children anyway. So the lineage is not >>>> strictly required. >>>> >>>> If I understand correctly, most of you are also in favor of using >>>> catalog identifiers + ref as the refresh-state keys and postponing the >>>> lineage proposal. >>>> >>>> I hope that we can move the discussion forward. >>>> >>>> Jan >>>> On 16.08.24 08:07, Walaa Eldin Moustafa wrote: >>>> >>>> The option of using catalog identifiers in the state map still requires >>>> keeping lineage information in the view because REFRESH MV needs the latest >>>> fully expanded children (which could have changed from the set of children >>>> currently in the state map), without reparsing the view tree. Therefore, >>>> catalog identifiers in the state map, does not eliminate the need for >>>> tracking children in the form of catalog identifiers in the lineage side >>>> (but in this case lineage will be a set instead of just a map). >>>> >>>> Hence, my concerns with using catalog identifiers (as opposed to UUIDs) >>>> are: >>>> * The fundamental issue where the table spec depends on/refers to the >>>> view spec (because such catalog identifiers are not defined in the table >>>> spec and the only place they have a meaning is in the view spec lineage >>>> information). >>>> * (less fundamental) The denormalization introduced by this >>>> arrangement, where each identifier is 3-parts and all of them repeat in >>>> both lineage info and state map. >>>> >>>> I am not very concerned with recursive expansion (through multiple >>>> calls), as it is always the case with views. >>>> >>>> On a positive note, looks like we agree to move past sequence numbers >>>> :) >>>> >>>> Thanks, >>>> Walaa. >>>> >>>> >>>> >>>> >>>> On Thu, Aug 15, 2024 at 4:07 PM Micah Kornfield <emkornfi...@gmail.com> >>>> wrote: >>>> >>>> I think given the constraint that catalog lookup has to be by >>>> identifier and not UUID, I'd prefer using identifier in the refresh state. >>>> If we use identifiers, we can directly parallelize the catalog calls to >>>> fetch the latest state. If we use UUID, the engine has to go back to the >>>> MV and possibly additional views to reconstruct the lineage map. It's just >>>> a lot slower and more work for the engine when there is a MV that >>>> references a lot of views (and those views reference additional views). >>>> >>>> >>>> I'm +1 on using catalog identifiers as the key. As you point out this >>>> is inline with #2 (try to minimize serial catalog lookups) in addition to >>>> supporting requirement #3. >>>> >>>> On Thu, Aug 15, 2024 at 3:27 PM Benny Chow <btc...@gmail.com> wrote: >>>> >>>> I think given the constraint that catalog lookup has to be by >>>> identifier and not UUID, I'd prefer using identifier in the refresh state. >>>> If we use identifiers, we can directly parallelize the catalog calls to >>>> fetch the latest state. If we use UUID, the engine has to go back to the >>>> MV and possibly additional views to reconstruct the lineage map. It's just >>>> a lot slower and more work for the engine when there is a MV that >>>> references a lot of views (and those views reference additional views). >>>> >>>> Thanks >>>> Benny >>>> >>>> >>>> On Thu, Aug 15, 2024 at 2:14 PM Walaa Eldin Moustafa < >>>> wa.moust...@gmail.com> wrote: >>>> >>>> Thanks Jan, Micah, and Karuppayya for chiming in. >>>> >>>> I do not think 3 and 4 are at odds with each other (for example >>>> maintaining both lineage map and state map through UUID can achieve both). >>>> Also, I do not think we can drop the lineage map since in many catalogs, >>>> the only lookup method is by the catalog identifier, and not the UUID. >>>> >>>> I think if we go with UUIDs in the state, we should have a lineage map >>>> (from identifiers to UUIDs) to go with it. >>>> >>>> Thanks, >>>> Walaa. >>>> >>>> >>>> On Thu, Aug 15, 2024 at 1:45 PM karuppayya <karuppayya1...@gmail.com> >>>> wrote: >>>> >>>> +1 to storing the refresh state as a map of UUIDs to snapshot IDs, and >>>> deferring the inclusion of lineage to a future iteration.(like Micha >>>> mentioned) >>>> This would greatly simplify the current design. >>>> >>>> Also in terms of identifiers to use(UUID or catalog identifier) for the >>>> refresh state >>>> We will not be able to fetch the table/View using the UUID alone, for >>>> example from Hive based catalog. >>>> We do not have the direct mapping between UUID and table/view. >>>> Which leaves us only with the catalog identifiers? >>>> >>>> Thanks & Regards >>>> Karuppayya >>>> >>>> >>>> On Thu, Aug 15, 2024 at 9:16 AM Micah Kornfield <emkornfi...@gmail.com> >>>> wrote: >>>> >>>> I think it might be worth restating perceived requirements and making >>>> sure there is alignment on them. >>>> >>>> If I am reading correctly, I think the following are perceived >>>> requirements: >>>> 1. An engine must be able to unambiguously detect that an underlying >>>> queried entity has changed or not via metadata to decide if materialized >>>> table data can be used. >>>> 2. The number of sequential catalog reads an engine needs to make to >>>> make use of a materialized table state at read time is minimized. >>>> 3. Engines that don't understand a SQL dialect can still use MV >>>> information if it is not stale. >>>> 4. Table refs (catalog identifiers) should not appear in the >>>> materialized table metadata (i.e. state). >>>> 5. The view part of the MV definition should not need a new revision >>>> for any changes to objects it queries as long as their schemas stay >>>> compatible (only state information on the materialized table need to >>>> change). >>>> >>>> In my mind, requirement 1, is the only true requirement. I think this >>>> necessitates having UUID + snapshot ID as part of the state information >>>> (not necessarily part of the Lineage). I think it also necessitates having >>>> a denormalized view of all entities that are inputs into the MV in the >>>> state information (a view object might not change but its underlying tables >>>> or views could change and that must be detected). >>>> >>>> Requirements 2 and 5 are somewhat at odds with each other. If >>>> information is denormalized (fully expanded) in Lineage, it means if table >>>> information is somehow dropped from an intermediate view, one would need to >>>> update the view (or make excess calls to the catalog). In my mind, this >>>> argues for normalization of the lineage stored on the view (with the cost >>>> of potentially 1 additional serial catalog lookup once the state >>>> information is retrieved). >>>> >>>> I think #3 is at odds with #4. I think #3 is more worthwhile, then >>>> keeping #4 (and as Jan noted #4 adds complexity). >>>> >>>> I think the last remaining question is if lineage serves any purpose. >>>> I think it is useful for the following reasons: >>>> a) When there are no intermediate views queried, it allows for fully >>>> parallelized lookup calls to the catalog without having to parse the SQL >>>> statement first >>>> b) Allows tools that don't need to lookup state information or parse >>>> SQL but still navigate MV/view trees. >>>> >>>> Both of these seem relatively minor, so lineage could perhaps be left >>>> out in the first iteration. >>>> >>>> As it applies to Jan's questions: >>>> >>>> 1. Should we move the identifiers out of the refresh-state into a new >>>> lineage record that is stored as part of the view metadata? >>>> >>>> No, I don't think so, I think #5 is a reasonable requirement and I >>>> think this violates it. >>>> >>>> >>>> 2. If yes, should the lineage in the view be fully expanded? >>>> >>>> No, I think only the state should be fully expanded (for reasons >>>> mentioned above, it potentially requires more updates to the view then >>>> necessary). >>>> >>>> >>>> 3. What should be used as an identifier in the lineage to reference >>>> entries in the refresh-state? >>>> >>>> >>>> Catalog identifiers make sense to me. If we agree requirement #3 is >>>> not a requirement then it seems like this could also be UUIDs. >>>> >>>> Thanks, >>>> Micah >>>> >>>> On Thu, Aug 15, 2024 at 7:57 AM Benny Chow <btc...@gmail.com> wrote: >>>> >>>> If we go with either UUID or Table Identifier + VersionID/SnapshotId in >>>> the refresh state, then this list is fully expanded already. So, to >>>> validate the freshness of a materialization, the engine doesn't even need >>>> to look at the view lineage. IMO, the view lineage is nice to have but not >>>> a necessary requirement for MVs. The view lineage makes sharing of views >>>> between engines without common SQL dialects possible. >>>> >>>> Benny >>>> >>>> On Thu, Aug 15, 2024 at 12:22 AM Jan Kaul <jank...@mailbox.org.invalid> >>>> <jank...@mailbox.org.invalid> wrote: >>>> >>>> Hi all, >>>> >>>> I would like to reemphasize the purpose of the refresh-state for >>>> materialized views. The purpose is to determine if the precomputed data is >>>> fresh, stale or invalid. For that the current snapshot-id of every table in >>>> the query tree has to be fetched from the catalog by using its full >>>> identifier and ref. Additionally the refresh state stores the snapshot-id >>>> of the last refresh. >>>> >>>> To summarize: *To determine the freshness of the precomputed data we >>>> require the full identifier + ref and snapshot-id of the last refresh for >>>> every table in the fully expanded query tree* >>>> >>>> This is a requirement from how the catalog works and independent from >>>> how we design the lineage/refresh state. Additionally we previously agreed >>>> that we should be able to obtain the full list of identifiers without >>>> needing to parse the SQL definition. >>>> >>>> Now we are having a discussion in how to store and obtain the fully >>>> expanded list of table identifiers and snapshot-ids. To move the discussion >>>> forward I think it would be valuable to answer the following 3 questions: >>>> >>>> 1. Should we move the identifiers out of the refresh-state into a new >>>> lineage record that is stored as part of the view metadata? >>>> >>>> 2. If yes, should the lineage in the view be fully expanded? >>>> >>>> 3. What should be used as an identifier in the lineage to reference >>>> entries in the refresh-state? >>>> >>>> 1. Question: >>>> >>>> We already agreed that this would be a good idea because we wouldn't >>>> introduce the identifier concept to the table metadata. However, looking at >>>> the complexity that comes with the alternatives, I would like to keep this >>>> question open. >>>> >>>> 2. Question: >>>> >>>> I'm against using a not fully expanded lineage in the view struct. To >>>> recall we require every identifier in the fully expanded query tree to >>>> determine the freshness. Not storing all identifiers in the lineage would >>>> mean to recursively call the catalog and expand the query tree at read >>>> time. This can lead to a large overhead for determining the refresh state >>>> compared to expanding the query tree once at creation time and then storing >>>> the fully expanded lineage. >>>> >>>> 3. Question: >>>> >>>> This depends on Question 2. >>>> >>>> For a not fully expanded lineage, the only options would be uuids or >>>> catalog identifiers. >>>> >>>> For a fully expanded lineage the question isn't all that relevant. The >>>> current design specifies that the lineage is a map from an identifier to an >>>> id and the refresh-state is a map from such id to a snapshot-id. For this >>>> to work we don't have to specify which kind of identifier has to be used. >>>> One query engine could use uuids, the other engine sequence-ids. The >>>> important assumption we are making is that every id that is used in the >>>> refresh-state has to be defined in the lineage. >>>> So the question about using uuids is rather, can the query engine trust >>>> that the id defined in the lineage is the uuid of the table. >>>> >>>> >>>> Regarding the complexity that comes from introducing the lineage in the >>>> view I would like to revisit question 1. Introducing the lineage in the >>>> view metadata opens up the question of when should the lineage be fully >>>> expanded. We see that we have 3 options: >>>> >>>> 1. Not fully expanded lineage -> Expansion at read time >>>> >>>> 2. Fully expanded lineage -> Expansion at creation time >>>> >>>> 3. No lineage (use identifiers in refresh-state) -> Expansion at >>>> refresh time >>>> >>>> As reading is expected to be the most frequent operation I see option 1 >>>> as not favorable. As the query engine has to fully expand the query tree >>>> for a refresh anyway, I see option 3 as the most natural. For a refresh >>>> operation the query engine must understand the SQL dialects of all views in >>>> the query tree and therefore is guaranteed to successfully expand the >>>> lineage. This might not be the case at creation time, which makes option 2 >>>> less favorable. >>>> >>>> As can be seen, I'm in favor of just storing the refresh-state as a map >>>> from identifier to snapshot-id and not using the lineage. I know that this >>>> introduces the concept of a catalog identifiers to the table metadata spec, >>>> but in my opinion it is by far the simplest option. >>>> >>>> I'm interested in your opinions. >>>> >>>> Best wishes, >>>> >>>> Jan >>>> On 14.08.24 22:24, Walaa Eldin Moustafa wrote: >>>> >>>> Thanks Benny. For refs, I am +1 to represent them as UUID + optional >>>> ref, although we can iterate ohe exact JSON structure (e.g., another option >>>> is splitting for (UUID) state from (UUID + ref) state into two separate >>>> higher-level fields). >>>> >>>> Generally agree on REFRESH VIEW strategy could be up to the engine, but >>>> it seems like an area where Iceberg could have an opinion/spec on. I will >>>> start a separate thread for that. >>>> >>>> Thanks, >>>> Walaa. >>>> >>>> >>>>
