Re: cache type info in json_agg and friends

> Currently, json_agg, jsonb_agg, json_object_agg and jsonb_object_agg do
> type classification on their arguments on each call to the transition
> function. This is quite unnecessary, as the argument types won't change.
> This patch remedies the defect by caching the necessary values in the
> aggregate state object.
+1

>
> While this doesn't change the performance much, since these functions
> are essentially dominated by other bits of the processing, I think it is
> nevertheless worth doing.
Agree

After quick observation of your patch, why don't you use FmgrInfo 
instead of  JsonAggState.val_output_func/JsonAggState.key_category? 
FmgrInfo could be filled by fmgr_info_cxt() in aggcontext memory 
context. Suppose, direct usage of FmgrInfo with FunctionCall a bit 
faster than OidFunctionCall.
-- 
Teodor Sigaev                      E-mail: teodor@sigaev.ru                                      WWW: http://www.sigaev.ru/