Hi. I've got a nice little POC using PostgreSQL to implement a REST API server.
This uses json_agg(t) to generate the JSON of tables (or subqueries in general),
which means I always get back a single row (and column, before I added the count(t.*)).
But I'd like to get statistics on the number of rows aggregated (easy, count(*)),
but also the number of columns of those rows! And I'm stuck for the latter...
Is there a (hopefully efficient) way to get back the cardinality of a select-clause basically?
Obviously programmatically I can get the row and column count from the result-set,
but I see the result of json_agg() myself, while I want the value prior to json_agg().
Is there a way to achieve this?
Thanks, --DD
PS: In the example below, would return 1 for the 1st query, and 2 for the 2nd.
```
migrated=> create table t1 (id integer);
CREATE TABLE
migrated=> insert into t1 values (1), (2);
INSERT 0 2
migrated=> create table t2 (id integer, name text);
CREATE TABLE
migrated=> insert into t2 values (1, 'one'), (2, 'two');
INSERT 0 2
migrated=> select count(t.*), json_agg(t) from t1 t;
count | json_agg
-------+-------------
2 | [{"id":1}, +
| {"id":2}]
(1 row)
migrated=> select count(t.*), json_agg(t) from t2 t;
count | json_agg
-------+--------------------------
2 | [{"id":1,"name":"one"}, +
| {"id":2,"name":"two"}]
(1 row)
```