Hi,

I'm sorry,if this message is duplicated previous this one, but I'm not sure that the previous message is sent correctly. I sent it from email address a.rybakina@postgrespro.ru and I couldn't send this one email from those address.

I like idea to create patch for logging query plan. After reviewing this code and notice some moments and I'd rather ask you some questions.

Firstly, I suggest some editing in the comment of commit. I think, it is turned out the more laconic and the same clear. I wrote it below since I can't think of any other way to add it.

```
Currently, we have to wait for finishing of the query execution to check its plan.
This is not so convenient in investigation long-running queries on production
environments where we cannot use debuggers.

To improve this situation there is proposed the patch containing the pg_log_query_plan()
function which request to log plan of the specified backend process.

By default, only superusers are allowed to request log of the plan otherwise
allowing any users to issue this request could create cause lots of log messages
and it can lead to denial of service.

At the next requesting CHECK_FOR_INTERRUPTS(), the target backend logs its plan at
LOG_SERVER_ONLY level and therefore this plan will appear in the server log only,
not to be sent to the client.
```

Secondly, I have question about deleting USE_ASSERT_CHECKING in lock.h?
It supposed to have been checked in another placed of the code by matching values. I am worry about skipping errors due to untesting with assert option in the places where it (GetLockMethodLocalHash) participates and we won't able to get core file in segfault cases. I might not understand something, then can you please explain to me?

Thirdly, I have incomprehension of the point why save_ActiveQueryDesc is declared in the pquery.h? I am seemed to save_ActiveQueryDesc to be used in an once time in the ExecutorRun function and its declaration superfluous. I added it in the attached patch.

Fourthly, it seems to me there are not enough explanatory comments in the code. I also added them in the attached patch.

Lastly, I have incomprehension about handling signals since have been unused it before. Could another signal disabled calling this signal to log query plan? I noticed this signal to be checked the latest in procsignal_sigusr1_handler function.