Re: jsonb access operators inefficiency

On 05/30/2014 11:08 AM, Andrew Dunstan wrote:
>
> On 05/30/2014 09:35 AM, Teodor Sigaev wrote:
>> Hi!
>>
>> jsonb operators ->text, ->>text,->int, ->>int use inefficient methods 
>> to access to needed field, proportional O(N/2). Attached patch 
>> suggests for text operators O(log(N)) and for integer - O(1). The 
>> fuctions with fast access already are existed in current code and are 
>> used in contains operation, for example. Attached patch uses that 
>> functions instead of sequentual loop over object/array.
>
> Teodor, thanks for this.
>
> My only worry is this sort of code, which in a quick search I didn't 
> find used elsewhere
>
>    -                   (void) JsonbToCString(jtext, &tjb->root , -1);
>    -                   result = cstring_to_text_with_len(jtext->data,
>    jtext->len);
>    +                   appendStringInfoSpaces(jtext, VARHDRSZ);
>    +                   (void) JsonbToCString(jtext, v->val.binary.data,
>    -1);
>    +
>    +                   result = (text*)jtext->data;
>    +                   SET_VARSIZE(result, jtext->len);
>
>
> If we're going to construct varlena objects inside a StringInfo, maybe 
> we need a proper API for it. Isn't there a danger that data member of 
> the StringInfo won't be properly aligned to allow us to do this? In 
> any case, we should get most of the benefit of your patch without this 
> "optimization".


I see that palloc.h says:
   The result of palloc() is always word-aligned


so maybe my alignment fear is misplaced. So my remaining question is 
whether this is OK stylistically.

cheers

andrew