Re: Freezing localtimestamp and other time function on some value

On 04/13/2016 04:16 AM, Alex Ignatov wrote:

On 12.04.2016 20:50, Tom Lane wrote:

Alex Ignatov <a.ignatov@postgrespro.ru> writes:

You always should keep in mind that your application may run in test
mode (future/past time) and maintain this code. While with my proposal
you  can always use some time function(now or localtimestamp or
whatever)  which you can  freeze at anytime on DB level, not operation
system(using some 3rd libs) or application(using wrappers and other
hacks).

We're not really in the business of being Oracle --- which in this
particular context means not trying to duplicate tens of thousands of
bizarre little features with very narrow use-cases.  If there's a
reasonable way for users to provide corner-case functionality for
themselves (and I'd say a wrapper function is a perfectly reasonable
way for this) then we don't really want to embed it in Postgres.

This particular feature seems like a seriously-poorly-thought-out
one, too.  Time stops advancing across the whole DB?  Really?

1. That would break all manner of stuff, for example the timestamps
in automatically-taken dumps, if you've got background jobs running
pg_dump.  Just about everything except the session running the test
case would be unhappy, AFAICS.

2. Would this extend to, say, preventing autovacuum from running?
Or changing the timestamps of messages in the postmaster log, or
timestamps appearing in places like pg_stat_activity?  Or causing
pg_sleep() to wait forever, because time isn't passing?  If your
answer is "yes" across the board, that makes problem #1 an order
of magnitude worse, while if you want to be selective then you
have a bunch of nitty-gritty (and rather arbitrary) decisions to
make about what's frozen and what's not.  And you've weakened the
argument that your test is actually valid, since potentially the
app would see some of the non-frozen values and misbehave.

3. While I can see the point of wanting to, say, test weekend behavior
on a weekday, I do not see how a value of now() that doesn't advance
between transactions would represent a realistic test environment for
an app with time-dependent behavior.  As an example, you might
accidentally write code that expects two successive transactions to
see identical values of now(), and such a testbed wouldn't detect
the problem.

            regards, tom lane

1. background jobs in pg?? cron you mean or  may be  EnterpriseDB vesion?
2. All i need is to freeze some(or may be one ) function for example
now() or smth else =). I dont want to freeze time for the whole
postmaster process!

That was not obvious:

http://www.postgresql.org/message-id/570CD2E3.4030400@postgrespro.ru

"In oracle there is alter system set fixed_date command. Have Postgres
this functionality?"

https://appsfromrajiv.wordpress.com/2011/06/14/oracle-fixed_date-parameter-helpful-during-testing/

"This parameter is useful primarily for testing. The value can be in the format shown above or in the default Oracle date format, without a time. Setting this parameter to a specified timestamp will make the time constant for the database engine (the clock will not tick) "

http://www.postgresql.org/message-id/570CE996.30301@postgrespro.ru

"Hi!
It is not about localtimestamp in transactions. It is about global
localtimestamp value  for all session  new and existed no matter inside
transaction or outside."

3. In multithreaded applications it is possible that two transactions
from different sessions  started at the same time and to resolve this
issue some sort of unique id(say serial) is used while inserting some
value in some table ;)

>> "This parameter is useful primarily for testing. The value can be in the format shown above or in the default Oracle date format, without a time. Setting this parameter to a specified timestamp will make the time constant for the database engine (the clock will not tick) "

And if we use TL;DR tag on your link  we'll see ;) 
"This parameter did help us in testing future and in past but we had our own share of issues also for application testing."

Did help us + issues = Did help us and ≠  issue ;)

Say if we don't need this feature- we dont use it, but if we need it  but  we have nothing it makes us sad.  I think that have feature > have not =)..



 

-- 
Alex Ignatov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company