`duration` is a new native type that was introduced in Cassandra 3.10 [1].
Support for parsing and the internal representation of the type was added in
8fa47b74e8.
Important note: The version of cqlsh distributed with Scylla does not have
support for durations included (it was added to Cassandra in [2]). To test this
change, you can use cqlsh distributed with Cassandra.
Duration types are useful when working with time-series tables, because they can
be used to manipulate date-time values in relative terms.
Two interesting applications are:
- Aggregation by time intervals [3]:
`SELECT * FROM my_table GROUP BY floor(time, 3h)`
- Querying on changes in date-times:
`SELECT ... WHERE last_heartbeat_time < now() - 3h`
(Note: neither of these is currently supported, though columns with duration
values are.)
Internally, durations are represented as three signed counters: one for months,
for days, and for nanoseconds. Each of these counters is serialized using a
variable-length encoding which is described in version 5 of the CQL native
protocol specification.
The representation of a duration as three counters means that a semantic
ordering on durations doesn't exist: Is `1mo` greater than `1mo1d`? We cannot
know, because some months have more days than others. Durations can only have a
concrete absolute value when they are "attached" to absolute date-time
references. For example, `2015-04-31 at 12:00:00 + 1mo`.
That duration values are not comparable presents some difficulties for the
implementation, because most CQL types are. Like in Cassandra's implementation
[2], I adopted a similar strategy to the way restrictions on the `counter` type
are checked. A type "references" a duration if it is either a duration or it
contains a duration (like a `tuple<..., duration, ...>`, or a UDT with a
duration member).
The following restrictions apply on durations. Note that some of these contexts
are either experimental features (materialized views), or not currently
supported at run-time (though support exists in the parser and code, so it is
prudent to add the restrictions now):
- Durations cannot appear in any part of a primary key, either for tables or
materialized views.
- Durations cannot be directly used as the element type of a `set`, nor can they
be used as the key type of a `map`. Because internal ordering on durations is
based on a byte-level comparison, this property of Cassandra was intended to
help avoid user confusion around ordering of collection elements.
- Secondary indexes on durations are not supported.
- "Slice" relations (<=, <, >=, >) are not supported on durations with `WHERE`
restrictions (like `SELECT ... WHERE span <= 3d`). Multi-column restrictions
only work with clustering columns, which cannot be `duration` due to the
first rule.
- "Slice" relations are not supported on durations with query conditions (like
`UPDATE my_table ... IF span > 5us`).
Backwards incompatibility note:
As described in the documentation [4], duration literals take one of two
forms: either ISO 8601 formats (there are three), or a "standard" format. The ISO
8601 formats start with "P" (like "P5W"). Therefore, identifiers that have this
form are no longer supported.
Fixes#2240.
[1] https://issues.apache.org/jira/browse/CASSANDRA-11873
[2] bfd57d13b7
[3] https://issues.apache.org/jira/browse/CASSANDRA-11871
[4] http://cassandra.apache.org/doc/latest/cql/types.html#working-with-durations
To prepare a user-defined type, we need to look up its name in the keyspace.
While we get the keyspace name as an argument to prepare(), it is useless
without the database instance.
Fix the problem by passing a database reference along with the keyspace.
This precolates through the class structure, so most cql3 raw types end up
receiving this treatment.
Origin gets along without it by using a singleton. We can't do this due
to sharding (we could use a thread-local instance, but that's ugly too).
Hopefully the transition to a visitor will clean this up.
