Samphel Norden created CASSANDRA-7494:
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Summary: CQL support to return first column of each row
Key: CASSANDRA-7494
URL: https://issues.apache.org/jira/browse/CASSANDRA-7494
Project: Cassandra
Issue Type: Improvement
Components: Core
Environment: fedora 64bit
Reporter: Samphel Norden
This jira is a request to support a query like
select first 5 columns of each row where <whereclause>
Currently in CQL, if we put a limit clause it applies over all rows. Not a per
partition key limit.
More details below
IF we create a table as follows
CREATE TABLE xy (
a int,
b int,
c int,
d int,
value int,
PRIMARY KEY ((a, b), c, d)
) WITH CLUSTERING ORDER BY (c DESC, d ASC)
with data =
a | b | c | d | value
--------------
1 | 2 | 2007 | 307 | 950
1 | 2 | 2006 | 305 | 900
1 | 1 | 1006 | 205 | 800
1 | 1 | 1005 | 105 | 700
The rows are sorted by c descending where assuming c is a timestamp, the idea
is to store the latest timestamp first. Hence if we pull a single column from
each row given a set of rows, we want that to be the latest 'c' for each row.
In other words:
select first 1 value from xy where a=1 and b in (1,2)
should return a single "value" for each rowkey
a | b | c | d | value
--------------
1 | 1 | 1006 | 205 | 800
1 | 2 | 2007 | 307 | 950
I realize that if we do individual queries such as
select a,b,c,value from xy where a=1 and b =1 limit 1;
a | b | c | value
-------+----
1 | 1 | 1006 | 800
(1 rows)
cqlsh:> select a,b,c,e from xy where a=1 and b =2 limit 1;
a | b | c | value
-------+----
1 | 2 | 2007 | 950
We get the desired result.However this is highly inefficient since we would
need to fire a separate query per row. If we can have a construct change to
allow getting a single column for a given row that would be very helpful
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