Sam Tunnicliffe created CASSANDRA-10217:
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Summary: Support custom query expressions in SELECT
Key: CASSANDRA-10217
URL: https://issues.apache.org/jira/browse/CASSANDRA-10217
Project: Cassandra
Issue Type: New Feature
Reporter: Sam Tunnicliffe
Assignee: Sam Tunnicliffe
Fix For: 3.0 beta 2
(Broken out of CASSANDRA-10124)
Custom index implementations often support query expressions which do not fit
the structure of CQL. To support these, it has been necessary to add a fake
column to the base table and query that using the custom syntax. Taking an
example from the [Stratio
docs|https://github.com/Stratio/cassandra-lucene-index]:
{code}
SELECT * FROM tweets WHERE lucene='{
filter : {type:"range", field:"time", lower:"2014/04/25",
upper:"2014/05/1"},
query : {type:"phrase", field:"body", value:"big data gives
organizations", slop:1}
}'
{code}
The {{lucene}} field is a dummy column that has to be added to the table in
order to associate the pre-3.0 row-based index with the {{tweets}} table. We
could rewrite this query as:
{code}
SELECT * FROM tweets
WHERE expr(lucene, '{filter : {type:"range", field:"time", lower:"2014/04/25",
upper:"2014/05/1"},
query : {type:"phrase", field:"body", value:"big data gives
organizations", slop:1}}');
{code}
In this version the {{expr}} function takes 2 arguments: the first is the name
of the index being targetted, {{lucene}} and the second is the query string
itself.
Parsing and validation of those expressions would be delegated to the custom
index implementations which support them.
One thing to consider is index selection. If a query contains custom
expressions, but the target index is not selected, C* has no way to use the
custom expressions as a post-query filter, like it does with standard
expressions & {{ALLOW FILTERING}}. To compensate for that, index selection
should be weighted in favour of indexes targetted by custom expressions. At
least in the first instance, we should also restrict queries to targetting a
single index via custom expressions, i.e. disallow queries like {{SELECT * FROM
t WHERE expr(index1, 'foo') AND expr(index2, 'bar')}}
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