erlang - 我如何可以匹配元组内容与qlc和mnesia？ [英] erlang - how can I match tuple contents with qlc and mnesia?

问题描述

  -record（peer，{
 peer_key，%% key是元组{FileId，PeerId} 
 last_seen，
 last_event，
 uploaded = 0，
已下载= 0，
 left = 0，
 ip_port ，
 key 
}）。 
  

Peer_key是一个元组{FileId，ClientId}，现在我需要从所有对等端提取ip_port字段具有特定的FileId。

我想出了一个可行的解决方案，但我不知道这是否是一个好办法：

  qlc：q（[IpPort || #peer {peer_key = {FileId，_}，ip_port = IpPort}<  -  mnesia：table（peer），FileId =：= RequiredFileId] 
  

谢谢。

解决方案

使用带有诸如{FileId，PeerId}的元组主键的ordered_set表类型，然后部分地绑定元组的前缀，如{RequiredFileId，_}将非常有效，因为只有具有该前缀的键范围将被检查，而不是全表扫描。您可以使用qlc：info / 1来检查查询计划，并确保发生的任何选择都绑定了密钥前缀。

I have a mnesia table for this record.

-record(peer, {
    peer_key,   %% key is the tuple {FileId, PeerId}
    last_seen,
    last_event,
    uploaded = 0,
    downloaded = 0,
    left = 0,
    ip_port,
    key
}).

Peer_key is a tuple {FileId, ClientId}, now I need to extract the ip_port field from all peers that have a specific FileId.

I came up with a workable solution, but I'm not sure if this is a good approach:

qlc:q([IpPort || #peer{peer_key={FileId,_}, ip_port=IpPort} <- mnesia:table(peer), FileId=:=RequiredFileId])

Thanks.

解决方案

Using on ordered_set table type with a tuple primary key like { FileId, PeerId } and then partially binding a prefix of the tuple like { RequiredFileId, _ } will be very efficient as only the range of keys with that prefix will be examined, not a full table scan. You can use qlc:info/1 to examine the query plan and ensure that any selects that are occurring are binding the key prefix.

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