Re: How to stop array_to_json from interpolating column names that weren't there

On Wed, Jul 19, 2017 at 8:53 PM, Guyren Howe <guyren(at)gmail(dot)com> wrote:

> Thanks. Seeking greater understanding, why is json_agg(*) not equivalent?
>

​Are you referring to the fact that ​this provokes an error?

"select json_agg(*) from schemata;"

The json_agg(expression) function has an arity of 1 (i.e., one input
argument only). If you write:

select schemata from schemata;

you get one column in the output, while:

select * from schemata

results in an output relation having three columns - the "*" expands the
composite type in the FROM clause into its component columns in the
select-list

json_agg(*) fails since it is not expecting 3 columns (though oddly the
error I see in 9.6 indicates its actually looking for a 0-arity function...
"function json_agg() does not exist" - which implies that generally one
cannot place a * in a function call, even if it would result in a single
column. This may even be documented but I haven't the desire to look right
now...)

David J.