IMO it's wrong to change an aggregate's meaning from "aggregate across
GROUPs or entire SELECT" to "aggregate within column". Aggregation is
long established in SQL and it will just confuse experienced database
users.
PostgresQL maintains the meaning of max:
CREATE TABLE tab (
x int[]
);
INSERT INTO tab(x) VALUES ( '{1, 2}' );
INSERT INTO tab(x) VALUES ( '{3, 4}' );
SELECT max(x) FROM tab;
max
[3,4]
One option is to treat the collection as a tiny table:
SELECT (SELECT max(key) FROM a_set_column) AS m1, (SELECT max(value)
FROM a_map_column) FROM tab;
Though it's better to look for existing practice and emulate it than to
exercise creativity here, IMO.
On Tue, 2022-12-06 at 13:30 +0000, Benedict 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?
