[erlang-questions] Fetching several ets keys

Ahmed Omar <>
Thu Feb 9 12:42:46 CET 2012


Thanks for the tip Sverker.

2012/2/9 Sverker Eriksson <>

> ets:select will not do a full scan if the key is bound:
>
> ets:select(Tid, [{{Key,'_'},[],['$_']} || Key <- ["A","B","C"]]).
>
> But you should specify the key in the first part of the matchspec tuple,
> not use a $-variable and '=='.
>
> /Sverker, Erlang/OTP Ericsson
>
> Ahmed Omar wrote:
>
>> Hynek,
>> Yes you are right, if the table is big full scan is not a good idea.
>>
>> 2012/2/9 ori brost <>
>>
>>
>>
>>> The question is whether there is any way that I can avoid the
>>> "erlang-to-c call" cost and do these 4 lookups (i.e. with one ets call
>>> do lookup for all 4).
>>>
>>> On Thu, Feb 9, 2012 at 1:00 PM, Hynek Vychodil <>
>>> wrote:
>>>
>>>
>>>> This code will cause table full scan which usually will be far slower
>>>>
>>>>
>>> than just
>>>
>>>
>>>> [ V || K <- ["A","B","C","D"], V <- ets:lookup(Tid, K) ]
>>>> except very small tables full scan will not be good idea.
>>>>
>>>> On Thu, Feb 9, 2012 at 11:49 AM, Ahmed Omar <>
>>>>
>>>>
>>> wrote:
>>>
>>>
>>>> ets:select(Tid, [{{'$1','$2'},[{'==','$1',X}],**['$_']}|| X<-
>>>>> ["A","B","C","D"]]).
>>>>>
>>>>>
>>>>> On Thu, Feb 9, 2012 at 11:11 AM, dmitry kolesnikov <
>>>>>
>>>>>
>>>> >
>>>
>>>
>>>> wrote:
>>>>>
>>>>>
>>>>>> You could try
>>>>>> ets:select or qlc interface.
>>>>>>
>>>>>> It table is not big enough then qlc and custom fold function would
>>>>>>
>>>>>>
>>>>> work:
>>>
>>>
>>>> Q=qlc:q([ X || X <- ets:table(mytab)]),
>>>>>> qlc:fold(FoldFun, [], Q)
>>>>>>
>>>>>> Best Regards,
>>>>>> Dmitry >-|-|-*>
>>>>>>
>>>>>>
>>>>>> On 9.2.2012, at 11.40, ori brost <> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>> Is there any way in ets to fetch values for several keys in 1 call?
>>>>>>> i.e. something like
>>>>>>>
>>>>>>> ets:multi_fetch(Tid, ["A","B","C","D"]) % will fetch all entries with
>>>>>>> keys A, B, C, D
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>>>>>
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>>>>>
>>>>
>>>> --
>>>> Hynek Vychodil
>>>> BI consultant
>>>>
>>>> GoodData
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>>>>
>>>>
>>>
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