[jira] [Commented] (CASSANDRA-7396) Allow selecting Map key, List index

["Robert Stupp (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-7396?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14127503#comment-14127503 ] 

Robert Stupp commented on CASSANDRA-7396:
-----------------------------------------

Proposal for {{SELECT}} ("map" used for map columns, "set" for set columns, "list" for list columns):
* {{map\[key]}} to return single map value or {{null}}
* {{map?\[key]}} tests whether single map key exists ({{true}} or {{false}})
* {{map\[key1..key2]}} to return sliced map
* {{map?\[key1..key2]}} to return set with keys that are present in the map
* {{map\[..key2]}} to return sliced map
* {{map?\[..key2]}} to return set with keys that are present in the map
* {{map\[key1..]}} to return sliced map
* {{map?\[key1..]}} to return set with keys that are present in the map
* {{set\[key]}} returns {{true}} or {{false}} whether a single element exists
* {{set\[key1..key2]}} to return sliced set (check values of multiple elements)
* {{set\[key1..]}} to return sliced set (check values of multiple elements)
* {{set\[..key2]}} to return sliced set (check values of multiple elements)
* {{list\[idx]}} to return single list element
* {{list?\[idx]}} tests whether single list element exists ({{true}} or {{false}})
* {{list\[idx1..idx2]}} to return sliced list - list has length {{idx2-idx1+1}}
* {{list?\[idx1..idx2]}} to return sliced list with {{true}} or {{false}} indicating whether an element exists (is no tnull)
* {{sizeof(map)}}, {{sizeof(set)}}, {{sizeof(list)}} to return size of map/set/list

Restriction for {{key1..key2}} : {{key1}} must be less than {{key2}}

Using these collection operations on parts of primary key columns would require special handling.
Primary key columns are stored in their serialized byte representation - so it would need deserialzation, handling, 2nd serialization.

All would be added to {{unaliasedSelector}} in {{Cql.g}}.
Other thoughts?

I've built a POC for slicing (except {{sizeof}} functions) availabile at github at https://github.com/snazy/cassandra/tree/7396-coll-slice to illustrate functionality. 

That POC does not use column slicing to read only the requested values. In fact I do not know how to read only a map key „foobar“ from a table with clustering keys. AFAIK will the "internal column name“ something like "clusteringKey:mapColumnName:mapColumnKey“. Such a read optimization could be done for tables that not not have a clustering key.

But there’s room for optimization to prevent unnecessary internal serialization-deserialization-serialization sequences for the above collection operations by using only those {{Cell}}s that will be returned (stuff around {{SelectStatement#addValue}}).

> Allow selecting Map key, List index
> -----------------------------------
>
>                 Key: CASSANDRA-7396
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7396
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: API
>            Reporter: Jonathan Ellis
>              Labels: cql
>             Fix For: 3.0
>
>
> Allow "SELECT map['key]" and "SELECT list[index]."  (Selecting a UDT subfield is already supported.)



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