<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div>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 "<span style="background-color: rgba(255, 255, 255, 0);">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. ..."</span><br><br><br></div><div><br>On 05 Jul 2015, at 21:11, Jesper Louis Andersen <<a href="mailto:jesper.louis.andersen@gmail.com">jesper.louis.andersen@gmail.com</a>> wrote:<br><br></div><blockquote type="cite"><div><div dir="ltr"><div class="gmail_extra"><br><div class="gmail_quote">On Sat, Jul 4, 2015 at 10:56 AM, Benoit Chesneau <span dir="ltr"><<a href="mailto:bchesneau@gmail.com" target="_blank">bchesneau@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Also how does it works internally, the doc let me think it is traversing all the keys but not sure about it.</blockquote></div><br>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.</div><div class="gmail_extra"><br></div><div class="gmail_extra">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. <br><br clear="all"><div><br></div>-- <br><div class="gmail_signature">J.</div>
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