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Jonathan Ellis commented on CASSANDRA-7970:
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bq. With SELECT, the number and type of selected columns changes, which is
somewhat quirky.
I guess I don't see how this is more quirky than number and type of selected
columns changing in a non-json select.
bq. It's not possible to select both normal columns and a JSON document that
contains some of the columns.
True, but that seems like a reasonable constraint for the design goals here.
bq. Functions cannot be applied to the JSON document. (For example, concat(),
if we consider aggregation functions.)
Also a reasonable constraint for the design goals.
Here's my problem with the row_to_json approach: it inherently requires special
casing the function machinery, which is Bad.
What I mean by this is, all other function calls should have the same behavior
whether called on a literal or a column with the same value. Thus, sin(2) and
sin(a) where a=2 are semantically equivalent. But row_to_json(a) has to cheat
and generate {code}{'a': 2}{code}.
(Unless you are saying that row_to_json(a) would generate just the literal 2,
in which case I have to say that having to spell out all the json fields is
insufficiently usable to be a viable candidate.)
Postgresql gets around this with subselects and the concept of a row
constructor:
http://hashrocket.com/blog/posts/faster-json-generation-with-postgresql
I'm okay with the postgresql approach in a lot of respects, but it introduces a
ton of complexity that I don't really want to block for. Nor am I sure that we
want to open up the subquery can of worms for this, which will inevitably lead
to people pushing for them to be generalized.
> JSON support for CQL
> --------------------
>
> Key: CASSANDRA-7970
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7970
> Project: Cassandra
> Issue Type: Bug
> Components: API
> Reporter: Jonathan Ellis
> Assignee: Tyler Hobbs
> Fix For: 3.0
>
>
> JSON is popular enough that not supporting it is becoming a competitive
> weakness. We can add JSON support in a way that is compatible with our
> performance goals by *mapping* JSON to an existing schema: one JSON documents
> maps to one CQL row.
> Thus, it is NOT a goal to support schemaless documents, which is a misfeature
> [1] [2] [3]. Rather, it is to allow a convenient way to easily turn a JSON
> document from a service or a user into a CQL row, with all the validation
> that entails.
> Since we are not looking to support schemaless documents, we will not be
> adding a JSON data type (CASSANDRA-6833) a la postgresql. Rather, we will
> map the JSON to UDT, collections, and primitive CQL types.
> Here's how this might look:
> {code}
> CREATE TYPE address (
> street text,
> city text,
> zip_code int,
> phones set<text>
> );
> CREATE TABLE users (
> id uuid PRIMARY KEY,
> name text,
> addresses map<text, address>
> );
> INSERT INTO users JSON
> {‘id’: 4b856557-7153,
> ‘name’: ‘jbellis’,
> ‘address’: {“home”: {“street”: “123 Cassandra Dr”,
> “city”: “Austin”,
> “zip_code”: 78747,
> “phones”: [2101234567]}}};
> SELECT JSON id, address FROM users;
> {code}
> (We would also want to_json and from_json functions to allow mapping a single
> column's worth of data. These would not require extra syntax.)
> [1] http://rustyrazorblade.com/2014/07/the-myth-of-schema-less/
> [2] https://blog.compose.io/schema-less-is-usually-a-lie/
> [3] http://dl.acm.org/citation.cfm?id=2481247
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