On 09/14/2015 04:24 PM, Andrew Dunstan wrote:

On 09/14/2015 03:42 PM, Teodor Sigaev wrote:
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.

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.

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.

Well, we need the category to help data_to_json(b) do its work. Nevertheless, it might be doable to pass an FmgrInfo* to datum_to_json. I'll see what I can do.

The real problem about this is that in the most important cases to improve (composite_to_json(b) and the array processing functions) we'll still end up calling fmgr_info for every attribute of every record and for every array, although not for every array element, which is what OidOutputFunctionCall does for us anyway.

It's not obvious to me how to fix that, and before I put lots of effort into it I want to do some profiling to see where the time is actually being spent - I don't want to add a whole lot of code for a very marginal improvement.

One thought I did have that might be worth testing is that in the case of jsonb all the micro operations might be killing us, and that it might well be faster to generate a JSON string in the aggregates and then parse that into jsonb in the final function.



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