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

[Jim]

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

[Dan Gudmundsson]

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

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[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