Re: jsonb subscripting assignment performance

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От Pavel Stehule
Тема Re: jsonb subscripting assignment performance
Дата
Msg-id CAFj8pRCThLQuYyJwHqDoXjy-QhWCQVE8U50v-k+=M4_-QZpdiA@mail.gmail.com
обсуждение исходный текст
Ответ на jsonb subscripting assignment performance  ("Joel Jacobson" <joel@compiler.org>)
Ответы Re: jsonb subscripting assignment performance
Re: jsonb subscripting assignment performance
Список pgsql-hackers
Дерево обсуждения


st 14. 4. 2021 v 7:39 odesílatel Joel Jacobson <joel@compiler.org> napsal:
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?

no - it doesn't support in-place modification. Only arrays and records support it.


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.

sure - there is big room for optimization. But this patch was big enough without its optimization. And it was not clean, if I will be committed or not (it waited in commitfest application for 4 years). So I accepted implemented behaviour (without inplace update). Now, this patch is in core, and anybody can work on others possible optimizations.

Regards

Pavel
 

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

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