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Improve documentation of timestamp internal representation.

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Be more clear that we represent timestamps in microseconds when
integer timestamps are used, and in fractional seconds when
floating-point timestamps are used.

Discussion: http://postgr.es/m/20161212135045.GB15488@e733.localdomain

Report by Alexander Alekseev.  Wording by me with a suggestion
from Tom Lane.
This commit is contained in:
Robert Haas 2017-01-04 16:30:16 -05:00
parent 3633b3f656
commit 44f7afba79
1 changed files with 8 additions and 7 deletions
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doc/src/sgml/datatype.sgml
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@ -1651,13 +1651,14 @@ SELECT E'\\xDEADBEEF';
<para> <para>
When <type>timestamp</> values are stored as eight-byte integers When <type>timestamp</> values are stored as eight-byte integers
(currently the default), microsecond precision is available over (currently the default), microsecond precision is available over
the full range of values. When <type>timestamp</> values are the full range of values. In this case, the internal representation
stored as double precision floating-point numbers instead (a is the number of microseconds before or after midnight 2000-01-01.
deprecated compile-time option), the effective limit of precision When <type>timestamp</> values are stored as double precision
might be less than 6. <type>timestamp</type> values are stored as floating-point numbers (a deprecated compile-time option), the
seconds before or after midnight 2000-01-01. When internal representation is the number of seconds before or after
<type>timestamp</type> values are implemented using floating-point midnight 2000-01-01. With this representation, the effective limit
numbers, microsecond precision is achieved for dates within a few of precision might be less than 6; in practice,
microsecond precision is achieved for dates within a few
years of 2000-01-01, but the precision degrades for dates further years of 2000-01-01, but the precision degrades for dates further
away. Note that using floating-point datetimes allows a larger away. Note that using floating-point datetimes allows a larger
range of <type>timestamp</type> values to be represented than range of <type>timestamp</type> values to be represented than