[erlang-questions] How performant is ets:select ?

[Alexander Gunin]

Is true that ets optimizes composite key scan(like {8,'_'}) for select? I can't find it in documentation. Only this from ets:match documentation "If the key is specified in the pattern, the match is very efficient. If the key is not specified, i.e. if it is a variable or an underscore, the entire table must be searched. ..."



> On 05 Jul 2015, at 21:11, Jesper Louis Andersen <jesper.louis.andersen@REDACTED> wrote:
> 
> 
>> On Sat, Jul 4, 2015 at 10:56 AM, Benoit Chesneau <bchesneau@REDACTED> wrote:
>> Also how does it works internally, the doc let me think it is traversing all the keys but not sure about it.
> 
> IIRC select makes the optimizations it can. If the key is present in the selection it can narrow the amount of work it has to do by a lookup on the key. If, on the other hand, the key is not present, then it has to scan the full table. Jachym notes the special case on ordered_set tables: If the key is a composite tuple, and you are searching a prefix of that tuple, then the system can use the ordering to narrow down the keys to search. For example, searching for a key {8, '_'} on an ordered_set only has to look at the range {8, X} for all X, and this is a O(lg n) operation to find the bound and then a linear scan over the ordered set.
> 
> select that has to do a full table scan are best limited to small'ish tables (for some notion of small mostly defined by your memory bandwidth) or rarely executed work that can run in the background. 
> 
> 
> -- 
> J.
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