Jeff Janes <jeff.janes@gmail.com> writes:
> The docs for creating aggregates for 9.6 and beyond say:
> "For aggregate functions whose state_data_type is internal, the combinefunc
> must not be strict. In this case the combinefunc must ensure that null
> states are handled correctly and that the state being returned is properly
> stored in the aggregate memory context."
> Since combinefunc with an internal type is only useful when serialfunc and
> deserialfunc are also defined, why can't the built-in machinery just do the
> right thing when faced with a strict combinefunc?
The issue is how to initialize the state value to begin with.
I suppose you're imagining that we could fill pg_aggregate.agginitval
with something that the serialfunc could read. But that would place
serious constraints on the serialization format, like that it be
machine-independent and valid as text.
regards, tom lane