If the functionality truly has never actually worked, then throwing an error that MAX is not supported for collections seems reasonable.
But we should throw an error, I do not think we should have functions that aggregate across rows and functions that operate within a row use the same name. My expectation as a user would be that MAX either always aggregates across rows, so results in a single row of output or always operates within a row, so returns the full set of rows matching the query. So if we want a max that aggregates across rows that works for collections we could change it to return the aggregated max across all rows. Or we just leave it as an error and if someone wants the max across all rows they would ask for MAX(COLLECTION_MAX(column)). Yes I still agree COLLECTION_MAX may be a bad name. > On Dec 6, 2022, at 11:55 AM, Benedict <bened...@apache.org> wrote: > > As far as I am aware it has never worked in a release, and so deprecating it > is probably not as challenging as you think. Only folk that have been able to > parse the raw bytes of the collection in storage format would be affected - > which we can probably treat as zero. > > >> On 6 Dec 2022, at 17:31, Jeremiah D Jordan <jeremiah.jor...@gmail.com> wrote: >> >> >>> >>> 1. I think it is a mistake to offer a function MAX that operates over rows >>> containing collections, returning the collection with the most elements. >>> This is just a nonsensical operation to support IMO. We should decide as a >>> community whether we “fix” this aggregation, or remove it. >> >> The current MAX function does not work this way afaik? It returns the row >> with the column that has the highest value in clustering order sense, like >> if the collection was used as a clustering key. While that also may have >> limited use, I don’t think it worth while to deprecate such use and all the >> headache that comes with doing so. >> >>> 2. I think “collection_" prefixed methods are non-intuitive for discovery, >>> and all-else equal it would be better to use MAX,MIN, etc, same as for >>> aggregations. >> >> If we actually wanted to move towards using the existing names with new >> meanings, then I think that would take us multiple major releases. First >> deprecate existing use in current releases. Then make it an error in the >> next major release X. Then change the behavior in major release X+1. Just >> switching the behavior without having a major where such queries error out >> would make a bunch of user queries start returning “wrong” data. >> Also I don’t think those functions being cross row aggregations for some >> column types, but within row collection operations for other types, is any >> more intuitive, and actually would be more confusing. So I am -1 on using >> the same names. >> >>> 3. I think it is peculiar to permit methods named collection_ to operate >>> over non-collection types when they are explicitly collection variants. >> >> While I could see some point to this, I do not think it would be confusing >> for something named collection_XXX to treat a non-collection as a collection >> of 1. But maybe there is a better name for these function. Rather than >> seeing them as collection variants, we should see them as variants that >> operate on the data in a single row, rather than aggregating across multiple >> rows. But even with that perspective I don’t know what the best name would >> be. >> >>>> On Dec 6, 2022, at 7:30 AM, Benedict <bened...@apache.org> wrote: >>> >>> Thanks Andres, I think community input on direction here will be >>> invaluable. There’s a bunch of interrelated tickets, and my opinions are as >>> follows: >>> >>> 1. I think it is a mistake to offer a function MAX that operates over rows >>> containing collections, returning the collection with the most elements. >>> This is just a nonsensical operation to support IMO. We should decide as a >>> community whether we “fix” this aggregation, or remove it. >>> 2. I think “collection_" prefixed methods are non-intuitive for discovery, >>> and all-else equal it would be better to use MAX,MIN, etc, same as for >>> aggregations. >>> 3. I think it is peculiar to permit methods named collection_ to operate >>> over non-collection types when they are explicitly collection variants. >>> >>> Given (1), (2) becomes simple except for COUNT which remains ambiguous, but >>> this could be solved by either providing a separate method for collections >>> (e.g. SIZE) which seems fine to me, or by offering a precedence order for >>> matching and a keyword for overriding the precedence order (e.g. >>> COUNT(collection AS COLLECTION)). >>> >>> Given (2), (3) is a little more difficult. However, I think this can be >>> solved several ways. >>> - We could permit explicit casts to collection types, that for a collection >>> type would be a no-op, and for a single value would create a collection >>> - With precedence orders, by always selecting the scalar function last >>> - By permitting WRITETIME to accept a binary operator reduce function to >>> resolve multiple values >>> >>> These decisions all imply trade-offs on each other, and affect the >>> evolution of CQL, so I think community input would be helpful. >>> >>>>> On 6 Dec 2022, at 12:44, Andrés de la Peña <adelap...@apache.org> wrote: >>>> >>>> >>>> This will require some long introduction for context: >>>> >>>> The MAX/MIN functions aggregate rows to get the row with min/max column >>>> value according to their comparator. For collections, the comparison is on >>>> the lexicographical order of the collection elements. That's the very same >>>> comparator that is used when collections are used as clustering keys and >>>> for ORDER BY. >>>> >>>> However, a bug in the MIN/MAX aggregate functions used to make that the >>>> results were presented in their unserialized form, although the row >>>> selection was correct. That bug was recently solved by CASSANDRA-17811. >>>> During that ticket it was also considered the option of simply disabling >>>> MIN/MAX on collection since applying those functions to collections, since >>>> they don't seem super useful. However, that option was quickly discarded >>>> and the operation was fixed so the MIN/MAX functions correctly work for >>>> every data type. >>>> >>>> As a byproduct of the internal improvements of that fix, CASSANDRA-8877 >>>> introduced a new set of functions that can perform aggregations of the >>>> elements of a collection. Those where named "map_keys", "map_values", >>>> "collection_min", "collection_max", "collection_sum", and >>>> "collection_count". Those are the names mentioned on the mail list thread >>>> about function naming conventions. Despite doing a kind of >>>> within-collection aggregation, these functions are not what we usually >>>> call aggregate functions, since they don't aggregate multiple rows >>>> together. >>>> >>>> On a different line of work, CASSANDRA-17425 added to trunk a MAXWRITETIME >>>> function to get the max timestamp of a multi-cell column. However, the new >>>> collection functions can be used in combination with the WRITETIME and TTL >>>> functions to retrieve the min/max/sum/avg timestamp or ttl of a multi-cell >>>> column. Since the new functions give a generic way of aggreagting >>>> timestamps ant TTLs of multi-cell columns, CASSANDRA-18078 proposed to >>>> remove that MAXWRITETIME function. >>>> >>>> Yifan Cai, author of the MAXWRITETIME function, agreed to remove that >>>> function in favour of the new generic collection functions. However, the >>>> MAXWRITETIME function can work on both single-cell and multi-cell columns, >>>> whereas "COLLECTION_MAX(WRITETIME(column))" would only work on multi-cell >>>> columns, That's because MAXWRITETIME of a not-multicell column doesn't >>>> return a collection, and one should simply use "WRITETIME(column)" >>>> instead. So it was proposed in CASSANDRA-18037 that collections functions >>>> applied to a not-collection value consider that value as the only element >>>> of a singleton collection. So, for example, COLLECTION_MAX(7) = >>>> COLLECTION_MAX([7]) = 7. That ticket has already been reviewed and it's >>>> mostly ready to commit. >>>> >>>> Now we can go straight to the point: >>>> >>>> Recently Benedict brought back the idea of deprecating aggregate functions >>>> applied to collections, the very same idea that was mentioned on >>>> CASSANDRA-17811 description almost four months ago. That way we could >>>> rename the new collection functions MIN/MAX/SUM/AVG, same as the classic >>>> aggregate functions. That way MIN/MAX/SUM/AVG would be an aggregate >>>> function when applied to not-collection columns, and a scalar function >>>> when applied to collection. We can't do that with COUNT because there >>>> would be an ambiguity, so the proposal for that case is renaming >>>> COLLECTION_COUNT to SIZE. Benedict, please correct me if I'm not correctly >>>> exposing the proposal. >>>> >>>> I however would prefer to keep aggregate functions working on collections, >>>> and keep the names of the new collection functions as "COLLECTION_*". >>>> Reasons are: >>>> >>>> 1 - Making aggregate functions not work on collections might be cosidered >>>> as breaking backward compatibility and require a deprecation plan. >>>> 2 - Keeping aggregate functions working on collections might not look >>>> superuseful, but they make the set of aggregate functions consistent and >>>> applicable to every column type. >>>> 3 - Using the "COLLECTION_" prefix on collection functions establishes a >>>> clear distinction between row aggregations and collection aggregations, >>>> while at the same time exposing the analogy between each pair of functions. >>>> 4 - Not using the "COLLECTION_" prefix forces us to search for workarounds >>>> such as using the column type when possible, or trying to figure out >>>> synonyms like in the case of COUNT/SIZE. Even if that works for this case, >>>> future functions can find more trouble when trying to figure out >>>> workarounds to avoid clashing with existing function names. For example, >>>> we might want to add a SIZE function that gets the size in bytes of any >>>> column, or we might want to add a MAX function that gets the maximum of a >>>> set of columns, etc. And example of the synonym-based approach that comes >>>> to mind is MySQL's MAX and GREATEST functions, where MAX is for row >>>> aggregation and GREATEST is for column aggregation. >>>> 5 - If MIN/MAX function selection is based on the column type, we can't >>>> implement Yifan's proposal of making COLLECTION_MAX(7) = >>>> COLLECTION_MAX([7]) = 7, which would be very useful for combining >>>> collection functions with time functions. >>>> >>>> What do others think? What should we do with aggregate functions on >>>> collections, collection functions and MAXWRITETIME? >>
