Re: On-demand running query plans using auto_explain and signals

On Sep 3, 2015 10:14 PM, "Pavel Stehule" <pavel.stehule@gmail.com> wrote:
>>>
>>> Please find attached a v3.
>>>
>>> It uses a shared memory queue and also has the ability to capture plans nested deeply in the call stack.  Not sure about using the executor hook, since this is not an extension...
>>>
>>> The LWLock is used around initializing/cleaning the shared struct and the message queue, the IO synchronization is handled by the message queue itself.
>>
>> I am not pretty happy from this design. Only one EXPLAIN PID/GET STATUS in one time can be executed per server - I remember lot of queries that doesn't handle CANCEL well ~ doesn't handle interrupt well, and this can be unfriendly. Cannot to say if it is good enough for first iteration. This is functionality that can be used for diagnostic when you have overloaded server and this risk looks too high (for me). The idea of receive slot can to solve this risk well (and can be used elsewhere). The difference from this code should not be too big - although it is not trivial - needs work with PGPROC. The opinion of our multiprocess experts can be interesting. Maybe I am too careful.

Sorry, but I still don't see how the slots help this issue - could you please elaborate?

>> Other smaller issues:
>>
>> * probably sending line by line is useless - shm_mq_send can pass bigger data when nowait = false

I'm not sending it like that because of the message size - I just find it more convenient. If you think it can be problematic, its easy to do this as before, by splitting lines on the receiving side.

>> * pg_usleep(1000L); - it is related to single point resource

But not a highly concurrent one.

-
Alex