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.
