Jeremy Hanna created CASSANDRA-15803:
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Summary: Separate out allow filtering scanning through a partition
versus scanning over the table
Key: CASSANDRA-15803
URL: https://issues.apache.org/jira/browse/CASSANDRA-15803
Project: Cassandra
Issue Type: Improvement
Components: CQL/Syntax
Reporter: Jeremy Hanna
Currently allow filtering can mean two things in the spirit of "avoid
operations that don't seek to a specific row or sequential rows of data."
First, it can mean scanning across the entire table to meet the criteria of the
query. That's almost always a bad thing and should be discouraged or disabled
(see CASSANDRA-8303). Second, it can mean filtering within a specific
partition. For example, in a query you could specify the full partition key
and if you specify a criterion on a non-key field, it requires allow filtering.
The second reason to require allow filtering is significantly less work to scan
through a partition. It is still extra work over seeking to a specific row and
getting N sequential rows though. So while an application developer and/or
operator needs to be cautious about this second type, it's not necessarily a
bad thing, depending on the table and the use case.
I propose that we separate the way to specify allow filtering across an entire
table (involving a scatter gather) from specifying allow filtering across a
partition in a backwards compatible way. One idea that was brought up in Slack
in the cassandra-dev room was to have allow filtering mean the superset -
scanning across the table. Then if you want to specify that you *only* want to
scan within a partition. So it will succeed if you specify non-key criteria
within a single partition, but fail with a message to say it requires the full
allow filtering. One way would be to have it be
{{ALLOW FILTERING [WITHIN PARTITION]}}
This would allow for a backwards compatible full allow filtering while allowing
a user to specify that they want to just scan within a partition, but error out
if trying to scan a full table.
This is potentially also related to the capability limitation framework by
which operators could more granularly specify what features are allowed or
disallowed per user, discussed in CASSANDRA-8303. This way an operator could
disallow the more general allow filtering while allowing the partition scan (or
disallow them both at their discretion).
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