[erlang-questions] Why is Mnesia read-locking an entire table?

[Jim]

Is there an alternative that would avoid this in my two uses of select below? Would qlc avoid the table lock?

Sent from my iPhone

On Nov 3, 2012, at 1:49 PM, Dan Gudmundsson <dangud@REDACTED> wrote:

> Mnesia grabs locks before the actual operation is performed,
> a select call thus requires a table lock since mnesia does not analyse
> the select expression.
> 
> /Dan
> 
> On Sat, Nov 3, 2012 at 2:32 PM, Jim <jrosenblum@REDACTED> wrote:
>> Dear List
>> 
>> I am having trouble understanding why Mnesia is grabbing a read-lock on an
>> entire table. Can someone give me any insight?
>> 
>> Here are some facts (beliefs ;)
>> 
>> * The table is of type ordered_set
>> * My key is a two-tuple,  {unix-style-milliseconds-since-1970, Key} as in,
>> {1351872742126278,"BI13626"}},
>> * There are two additional indexed columns
>> *  I am frequently, in the same transaction, deleting and inserting two
>> records which differ *only* in the time-stamp portion of the key – I am
>> replacing an older version of the record with a newer version
>> * All access is done within a function that is passed to
>> mnesia:transaction/1
>> * All access to the table in question is via:
>>    o   mnesia:index_read/3 where all three variables being passed in are
>> *always* bound,  or
>>    o   mnesia:select/2 as shown below
>> 
>> 
>> lookup_since(Since) ->
>>    M = [{#key_to_value{update_map={'$1', '_'}, _='_'},
>>    [{'>', '$1', Since}], ['$_']}]
>>   {ok, mnesia:select(key_to_value, M)}.
>> 
>> or
>> 
>> lookup_keylist(Keys, Since) ->
>>   M = [{#key_to_value{update_map={'$1', K}, _='_'},
>>          [{'>', '$1', Since}],  ['$_']} || K <- Keys],
>>    {ok, mnesia:select(key_to_value, M)}.
>> 
>> Mnesia:info reports the following
>> 
>> 
>> 
>> Lock: {{key_to_value,'______WHOLETABLE_____'},read,{tid,247,<0.1717.0>}}
>> 
>> Lock:
>> {{key_to_value,{1351872742126278,"BI13626"}},write,{tid,247,<0.1717.0>}}
>> 
>> Lock:
>> {{key_to_value,{1351872742130724,"BI13623"}},write,{tid,247,<0.1717.0>}}
>> 
>> Lock:
>> {{key_to_value,{1351872742136170,"BI13621"}},write,{tid,247,<0.1717.0>}}
>> 
>> Lock:
>> {{key_to_value,{1351872742140957,"BI13616"}},write,{tid,247,<0.1717.0>}}
>> 
>> Lock:
>> {{key_to_value,{1351872746638841,"BI13628"}},write,{tid,247,<0.1717.0>}}
>> 
>> Lock:
>> {{key_to_value,{1351872746640989,"BI13626"}},write,{tid,247,<0.1717.0>}}
>> 
>> Lock:
>> {{key_to_value,{1351872746642481,"BI13623"}},write,{tid,247,<0.1717.0>}}
>> 
>> Lock:
>> {{key_to_value,{1351872746643406,"BI13621"}},write,{tid,247,<0.1717.0>}}
>> 
>> Lock:
>> {{key_to_value,{1351872746644468,"BI13616"}},write,{tid,247,<0.1717.0>}}
>> 
>> Lock: {{scope_to_keys,"6B-PED"},write,{tid,247,<0.1717.0>}}
>> 
>> Lock: {{scope_to_keys,"InPatient"},write,{tid,247,<0.1717.0>}}
>> 
>> Lock: {{type_to_keys,"b_index"},write,{tid,247,<0.1717.0>}}
>> 
>> ---> Processes waiting for locks <---
>> 
>> ---> Participant transactions <---
>> 
>> ---> Coordinator transactions <---
>> 
>> Tid: 247 (owned by <0.1717.0>)
>> 
>> ---> Uncertain transactions <---
>> 
>> 
>> 
>> 
>> 
>> Here is a second example of the mnesia:info() output
>> 
>> 
>> 
>> ---> Processes holding locks <---
>> 
>> Lock: {{key_to_value,'______WHOLETABLE_____'},read,{tid,707,<0.1876.0>}}
>> 
>> Lock:
>> {{key_to_value,{1351872861465784,"VI14543"}},write,{tid,707,<0.1876.0>}}
>> 
>> Lock:
>> {{key_to_value,{1351872868375564,"VI14543"}},write,{tid,707,<0.1876.0>}}
>> 
>> ---> Processes waiting for locks <---
>> 
>> ---> Participant transactions <---
>> 
>> ---> Coordinator transactions <---
>> 
>> Tid: 707 (owned by <0.1876.0>)
>> 
>> ---> Uncertain transactions <---
>> 
>> Any help would be greatly appreciated,
>> Jr0
>> 
>> 
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