jsonb subscripting assignment performance

Hi,

commit 676887a3 added support for jsonb subscripting.

Many thanks for working on this. I really like the improved syntax.

I was also hoping for some performance benefits,
but my testing shows that

jsonb_value['existing_key'] = new_value;

takes just as long time as

jsonb_value := jsonb_set(jsonb_value, ARRAY['existing_key'], new_value);

which is a bit surprising to me. Shouldn't subscripting be a lot faster, since it could modify the existing data structure in-place? What am I missing here?

I came to think of the this new functionality when trying to optimize some
PL/pgSQL code where the bottle-neck turned out to be lots of calls to jsonb_set() for large jsonb objects.

Here is the output from attached bench:

n=10000
00:00:00.002628 jsonb := jsonb_set(jsonb, ARRAY[existing key], value);
00:00:00.002778 jsonb := jsonb_set(jsonb, ARRAY[new key], value);
00:00:00.002332 jsonb[existing key] := value;
00:00:00.002794 jsonb[new key] := value;
n=100000
00:00:00.042843 jsonb := jsonb_set(jsonb, ARRAY[existing key], value);
00:00:00.046515 jsonb := jsonb_set(jsonb, ARRAY[new key], value);
00:00:00.044974 jsonb[existing key] := value;
00:00:00.075429 jsonb[new key] := value;
n=1000000
00:00:00.420808 jsonb := jsonb_set(jsonb, ARRAY[existing key], value);
00:00:00.449622 jsonb := jsonb_set(jsonb, ARRAY[new key], value);
00:00:00.31834 jsonb[existing key] := value;
00:00:00.527904 jsonb[new key] := value;

Many thanks for clarifying.

Best regards,

Joel