Re: Since '2001-09-09 01:46:40'::timestamp microseconds are lost when extracting epoch

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Petr Fedorov <petr(dot)fedorov(at)phystech(dot)edu>
Cc: pgsql-bugs(at)postgresql(dot)org
Subject: Re: Since '2001-09-09 01:46:40'::timestamp microseconds are lost when extracting epoch
Date: 2019-11-30 15:21:01
Message-ID: 17991.1575127261@sss.pgh.pa.us
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Petr Fedorov <petr(dot)fedorov(at)phystech(dot)edu> writes:
> select extract(epoch from '2001-09-09 01:46:40.000021'::timestamp)
> returns 1000000000.00002 - 1 microsecond is truncated.
> Obviously, it is due to the fact that extract epoch returns double
> precision which in turn has 15 decimal digits precision.

I can't get very excited about this. However, it might be worth
noting that v12 and HEAD print "1000000000.000021" as expected,
thanks to the Ryu float output code. You can get that from older
branches as well if you set extra_float_digits = 1.

By my arithmetic, IEEE float8 ought to be able to represent
microseconds accurately out to about 285 years either way from the
1970 epoch, so for practical purposes it'll be fine for a long time.

regards, tom lane

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