[
https://issues.apache.org/jira/browse/CASSANDRA-18060?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17642564#comment-17642564
]
Andres de la Peña commented on CASSANDRA-18060:
-----------------------------------------------
Sure, let's see an example of how the previously existing aggregate functions
and the new collection functions are different:
{code:java}
> CREATE TABLE t (k int PRIMARY KEY, l list<int>);
> INSERT INTO t(k, l) VALUES (1, [1, 2, 3]);
> INSERT INTO t(k, l) VALUES (2, [4, 5, 6]);
> SELECT count(l), max(l) FROM t;
system.count(l) | system.max(l)
-----------------+---------------
2 | [4, 5, 6]
> SELECT collection_count(l), collection_max(l) FROM t;
system.collection_count(l) | system.collection_max(l)
----------------------------+--------------------------
3 | 3
3 | 6
{code}
We need different function names to determine what type of functions we want to
run.
Note that this is similar for example to the difference between MySQL's [MAX
function|https://www.w3schools.com/sql/func_mysql_max.asp] and
[GREATEST|https://www.w3schools.com/sql/func_mysql_greatest.asp], although here
we work with collections. Having used the nice name for aggregates, we need to
figure out a more convoluted name for the row-per-row functions.
> Add aggregation scalar functions on collections
> -----------------------------------------------
>
> Key: CASSANDRA-18060
> URL: https://issues.apache.org/jira/browse/CASSANDRA-18060
> Project: Cassandra
> Issue Type: New Feature
> Components: CQL/Semantics
> Reporter: Andres de la Peña
> Assignee: Andres de la Peña
> Priority: Normal
> Fix For: 4.2
>
> Time Spent: 1h 40m
> Remaining Estimate: 0h
>
> The new mechanism for dynamically building native functions introduced by
> CASSANDRA-17811 can be used to provide within-collection aggregation
> functions. We can use that mechanism to add new CQL functions to get:
> * The number of items in a collection.
> * The max/min items of a collection.
> * The sum/avg of the items of a numeric collection.
> * The keys or the values of a map.
> For example:
> {code:java}
> CREATE TABLE k.t (k int PRIMARY KEY, l list<int>, m map<int, int>);
> INSERT INTO t(k, l, m) VALUES (0, [1, 2, 3], {1:10, 2:20, 3:30});
> > SELECT map_keys(m), map_values(m) FROM t;
> system.map_keys(m) | system.map_values(m)
> --------------------+----------------------
> {1, 2, 3} | [10, 20, 30]
> > SELECT collection_count(m), collection_count(l) FROM t;
> system.collection_count(m) | system.collection_count(l)
> ----------------------------+----------------------------
> 3 | 3
> > SELECT collection_min(l), collection_max(l) FROM t;
> system.collection_min(l) | system.collection_max(l)
> --------------------------+--------------------------
> 1 | 3
> > SELECT collection_sum(l), collection_avg(l) FROM t;
> system.collection_sum(l) | system.collection_avg(l)
> --------------------------+--------------------------
> 6 | 2
> {code}
> Note that this type of aggregation is different from the kind of aggregation
> provided by {{min}}, {{max}}, {{sum}} and {{avg}}, which aggregate entire
> collections across rows. Here we only aggregate the items of a collection row
> per row.
--
This message was sent by Atlassian Jira
(v8.20.10#820010)
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
