> > 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> >>>>> 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. >>>>>> >>>>>>
