Sylvain Lebresne created CASSANDRA-7059:
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Summary: Range query with strict bound on clustering column can
return less results than required for compact tables
Key: CASSANDRA-7059
URL: https://issues.apache.org/jira/browse/CASSANDRA-7059
Project: Cassandra
Issue Type: Bug
Reporter: Sylvain Lebresne
What's wrong:
{noformat}
CREATE TABLE test (
k int,
v int,
PRIMARY KEY (k, v)
) WITH COMPACT STORAGE;
INSERT INTO test(k, v) VALUES (0, 0);
INSERT INTO test(k, v) VALUES (0, 1);
INSERT INTO test(k, v) VALUES (1, 0);
INSERT INTO test(k, v) VALUES (1, 1);
INSERT INTO test(k, v) VALUES (2, 0);
INSERT INTO test(k, v) VALUES (2, 1);
SELECT * FROM test WHERE v > 0 LIMIT 3 ALLOW FILTERING;
k | v
---+---
1 | 1
0 | 1
{noformat}
That last query should return 3 results.
The problem lies into how we deal with 'strict greater than' ({{>}}) for "wide"
compact storage table. Namely, for those tables, we internally only support
inclusive bounds (for CQL3 tables this is not a problem as we deal with this
using the 'end-of-component' of the CompositeType encoding). So we "compensate"
by asking one more result than asked by the user, and we trim afterwards if
that was unnecessary. This works fine for per-partition queries, but don't for
"range" queries since we potentially would have to ask for {{X}} more results
where {{X}} is the number of partition fetched, but we don't know {{X}}
beforehand.
I'll note that:
* this has always be there
* this only (potentially) affect compact tables
* this only affect range queries that have a strict bound on the clustering
column (this means only {{ALLOW FILTERING}}) queries in particular.
* this only matters if a {{LIMIT}} is set on the query.
As for fixes, it's not entirely trivial. The "right" fix would probably be to
start supporting non-inclusive bound internally, but that's far from a small
fix and is "at best" a 2.1 fix (since we'll have to make a messaging protocol
change to ship some additional info for SliceQueryFilter). Also, this might be
a lot of work for something that only affect some {{ALLOW FILTERING}} queries
on compact tables.
Another (somewhat simpler) solution might be to detect when we have this kind
of queries and use a pager with no limit. We would then query a first page
using the user limit (plus some smudge factor to avoid being inefficient too
often) and would continue paging unless either we've exhausted all results or
we can prove that post-processing we do have enough results to satisfy the user
limit. This does mean in some case we might do 2 or more internal queries, but
in practice we can probably make that case very rare, and since the query is an
{{ALLOW FILTERING}} one, the user is somewhat warned that the query may not be
terribly efficient.
Lastly, we could always start by disallowing the kind of query that is
potentially problematic (until we have a proper fix), knowing that users can
work around that by either using non-strict bounds or removing the {{LIMIT}},
whichever makes the most sense in their case. In 1.2 in particular, we don't
have the query pagers, so the previous solution I describe would be a bit of a
mess to implement.
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